rcu: Cleanup RCU tree initialization

PATCH Superseded Alexander Gordeev 2 participants 21 messages Mar 9, 2015 – Mar 10, 2015 RCU

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Alexander Gordeev

author

2015-03-09 08:34 UTC #0 lore raw

cover rcu: Cleanup RCU tree initialization

Hi Paul,

Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.

The series successfully passes kernel build test with CONFIG_RCU_FANOUT and CONFIG_RCU_FANOUT_LEAF equal to 5.

Cc "Paul E. McKenney" <[email protected]>

Alexander Gordeev (10):
rcu: Panic if RCU tree can not accommodate all CPUs
rcu: Remove superfluous local variable in rcu_init_geometry()
rcu: Cleanup rcu_init_geometry() code and arithmetics
rcu: Simplify rcu_init_geometry() capacity arithmetics
rcu: Limit rcu_state::levelcnt[] to RCU_NUM_LVLS items
rcu: Limit rcu_capacity[] size to RCU_NUM_LVLS items
rcu: Remove unnecessary fields from rcu_state structure
rcu: Limit count of static data to the number of RCU levels
rcu: Simplify arithmetic to calculate number of RCU nodes
rcu: Remove unnecessary grpnum field from rcu_node structure

 kernel/rcu/tree.c        | 102 ++++++++++++++++++++---------------------------
 kernel/rcu/tree.h        |  35 ++++++++--------
 kernel/rcu/tree_plugin.h |   4 +-
 3 files changed, 63 insertions(+), 78 deletions(-)

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 08:36 UTC #1 lore raw

PATCH 1/10 rcu: Panic if RCU tree can not accommodate all CPUs

Currently a condition when RCU tree is unable to accommodate the configured number of CPUs is not permitted and causes a fall back to compile-time values. However, the code has no means to exceed the RCU tree capacity neither at compile-time nor in run-time. Therefore, if the condition is met in run- time then it indicates a serios problem elsewhere and should be handled with a panic.

Cc "Paul E. McKenney" <[email protected]>

Signed-off-by Alexander Gordeev <[email protected]>

---
 kernel/rcu/tree.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

kernel/rcu/tree.c +9 -6

index cc2e9be..9308ec9 100644
@@ -4043,16 +4043,19 @@ static void __init rcu_init_geometry(void)
 		rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;
 
 	/*
+	 * The tree must be able to accommodate the configured number of CPUs.
+	 * If this limit is exceeded than we have a serious problem elsewhere.
+	 *
 	 * The boot-time rcu_fanout_leaf parameter is only permitted
 	 * to increase the leaf-level fanout, not decrease it.  Of course,
 	 * the leaf-level fanout cannot exceed the number of bits in
-	 * the rcu_node masks.  Finally, the tree must be able to accommodate
-	 * the configured number of CPUs.  Complain and fall back to the
-	 * compile-time values if these limits are exceeded.
+	 * the rcu_node masks.  Complain and fall back to the compile-
+	 * time values if these limits are exceeded.
 	 */
-	if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
-	    rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
-	    n > rcu_capacity[MAX_RCU_LVLS]) {
+	if (n > rcu_capacity[MAX_RCU_LVLS])
+		panic("rcu_init_geometry: rcu_capacity[] is too small");
+	else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
+		 rcu_fanout_leaf > sizeof(unsigned long) * 8) {
 		WARN_ON(1);
 		return;
 	}

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 08:36 UTC #2 lore raw

PATCH 2/10 rcu: Remove superfluous local variable in rcu_init_geometry()

In reply to Alexander Gordeev

Local variable 'n' mimics 'nr_cpu_ids' while the both are used within one function. There is no reason for 'n' to exist whatsoever.

Cc "Paul E. McKenney" <[email protected]>

Signed-off-by Alexander Gordeev <[email protected]>

---
 kernel/rcu/tree.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

kernel/rcu/tree.c +7 -7

index 9308ec9..4045163 100644
@@ -4009,7 +4009,6 @@ static void __init rcu_init_geometry(void)
 	ulong d;
 	int i;
 	int j;
-	int n = nr_cpu_ids;
 	int rcu_capacity[MAX_RCU_LVLS + 1];
 
 	/*

@@ -4052,7 +4051,7 @@ static void __init rcu_init_geometry(void)
 	 * the rcu_node masks.  Complain and fall back to the compile-
 	 * time values if these limits are exceeded.
 	 */
-	if (n > rcu_capacity[MAX_RCU_LVLS])
+	if (nr_cpu_ids > rcu_capacity[MAX_RCU_LVLS])
 		panic("rcu_init_geometry: rcu_capacity[] is too small");
 	else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
 		 rcu_fanout_leaf > sizeof(unsigned long) * 8) {

@@ -4062,10 +4061,11 @@ static void __init rcu_init_geometry(void)
 
 	/* Calculate the number of rcu_nodes at each level of the tree. */
 	for (i = 1; i <= MAX_RCU_LVLS; i++)
-		if (n <= rcu_capacity[i]) {
-			for (j = 0; j <= i; j++)
-				num_rcu_lvl[j] =
-					DIV_ROUND_UP(n, rcu_capacity[i - j]);
+		if (nr_cpu_ids <= rcu_capacity[i]) {
+			for (j = 0; j <= i; j++) {
+				int cap = rcu_capacity[i - j];
+				num_rcu_lvl[j] = DIV_ROUND_UP(nr_cpu_ids, cap);
+			}
 			rcu_num_lvls = i;
 			for (j = i + 1; j <= MAX_RCU_LVLS; j++)
 				num_rcu_lvl[j] = 0;

@@ -4076,7 +4076,7 @@ static void __init rcu_init_geometry(void)
 	rcu_num_nodes = 0;
 	for (i = 0; i <= MAX_RCU_LVLS; i++)
 		rcu_num_nodes += num_rcu_lvl[i];
-	rcu_num_nodes -= n;
+	rcu_num_nodes -= nr_cpu_ids;
 }
 
 void __init rcu_init(void)

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 08:36 UTC #3 lore raw

PATCH 3/10 rcu: Cleanup rcu_init_geometry() code and arithmetics

In reply to Alexander Gordeev

This update simplifies rcu_init_geometry() code flow and makes calculation of the total number of rcu_node structures more easy to read.

The update relies on the fact num_rcu_lvl[] is never accessed beyond rcu_num_lvls index by the rest of the code. Therefore, there is no need initialize the whole num_rcu_lvl[].

Cc "Paul E. McKenney" <[email protected]>

Signed-off-by Alexander Gordeev <[email protected]>

---
 kernel/rcu/tree.c | 24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

kernel/rcu/tree.c +10 ~~-14~~

index 4045163..c7ad0a9 100644
@@ -4008,7 +4008,6 @@ static void __init rcu_init_geometry(void)
 {
 	ulong d;
 	int i;
-	int j;
 	int rcu_capacity[MAX_RCU_LVLS + 1];
 
 	/*

@@ -4059,24 +4058,21 @@ static void __init rcu_init_geometry(void)
 		return;
 	}
 
+	/* Calculate the number of levels in the tree. */
+	for (i = 0; nr_cpu_ids > rcu_capacity[i]; i++) {
+	}
+	rcu_num_lvls = i;
+
 	/* Calculate the number of rcu_nodes at each level of the tree. */
-	for (i = 1; i <= MAX_RCU_LVLS; i++)
-		if (nr_cpu_ids <= rcu_capacity[i]) {
-			for (j = 0; j <= i; j++) {
-				int cap = rcu_capacity[i - j];
-				num_rcu_lvl[j] = DIV_ROUND_UP(nr_cpu_ids, cap);
-			}
-			rcu_num_lvls = i;
-			for (j = i + 1; j <= MAX_RCU_LVLS; j++)
-				num_rcu_lvl[j] = 0;
-			break;
-		}
+	for (i = 0; i < rcu_num_lvls; i++) {
+		int cap = rcu_capacity[rcu_num_lvls - i];
+		num_rcu_lvl[i] = DIV_ROUND_UP(nr_cpu_ids, cap);
+	}
 
 	/* Calculate the total number of rcu_node structures. */
 	rcu_num_nodes = 0;
-	for (i = 0; i <= MAX_RCU_LVLS; i++)
+	for (i = 0; i < rcu_num_lvls; i++)
 		rcu_num_nodes += num_rcu_lvl[i];
-	rcu_num_nodes -= nr_cpu_ids;
 }
 
 void __init rcu_init(void)

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 08:34 UTC #4 lore raw

PATCH 4/10 rcu: Simplify rcu_init_geometry() capacity arithmetics

In reply to Alexander Gordeev

Current code suggests that introducing the extra level to rcu_capacity[] array makes some of the arithmetic easier. Well, in fact it appears rather confusing and unnecessary.

Cc "Paul E. McKenney" <[email protected]>

Signed-off-by Alexander Gordeev <[email protected]>

---
 kernel/rcu/tree.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

kernel/rcu/tree.c +7 -9

index c7ad0a9..98e36c3 100644
@@ -4008,7 +4008,7 @@ static void __init rcu_init_geometry(void)
 {
 	ulong d;
 	int i;
-	int rcu_capacity[MAX_RCU_LVLS + 1];
+	int rcu_capacity[MAX_RCU_LVLS];
 
 	/*
 	 * Initialize any unspecified boot parameters.

@@ -4032,12 +4032,10 @@ static void __init rcu_init_geometry(void)
 
 	/*
 	 * Compute number of nodes that can be handled an rcu_node tree
-	 * with the given number of levels.  Setting rcu_capacity[0] makes
-	 * some of the arithmetic easier.
+	 * with the given number of levels.
 	 */
-	rcu_capacity[0] = 1;
-	rcu_capacity[1] = rcu_fanout_leaf;
-	for (i = 2; i <= MAX_RCU_LVLS; i++)
+	rcu_capacity[0] = rcu_fanout_leaf;
+	for (i = 1; i < MAX_RCU_LVLS; i++)
 		rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;
 
 	/*

@@ -4050,7 +4048,7 @@ static void __init rcu_init_geometry(void)
 	 * the rcu_node masks.  Complain and fall back to the compile-
 	 * time values if these limits are exceeded.
 	 */
-	if (nr_cpu_ids > rcu_capacity[MAX_RCU_LVLS])
+	if (nr_cpu_ids > rcu_capacity[MAX_RCU_LVLS - 1])
 		panic("rcu_init_geometry: rcu_capacity[] is too small");
 	else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
 		 rcu_fanout_leaf > sizeof(unsigned long) * 8) {

@@ -4061,11 +4059,11 @@ static void __init rcu_init_geometry(void)
 	/* Calculate the number of levels in the tree. */
 	for (i = 0; nr_cpu_ids > rcu_capacity[i]; i++) {
 	}
-	rcu_num_lvls = i;
+	rcu_num_lvls = i + 1;
 
 	/* Calculate the number of rcu_nodes at each level of the tree. */
 	for (i = 0; i < rcu_num_lvls; i++) {
-		int cap = rcu_capacity[rcu_num_lvls - i];
+		int cap = rcu_capacity[(rcu_num_lvls - 1) - i];
 		num_rcu_lvl[i] = DIV_ROUND_UP(nr_cpu_ids, cap);
 	}

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 08:36 UTC #5 lore raw

PATCH 5/10 rcu: Limit rcu_state::levelcnt[] to RCU_NUM_LVLS items

In reply to Alexander Gordeev

Variable rcu_num_lvls is limited by RCU_NUM_LVLS macro. In turn, rcu_state::levelcnt[] array is never accessed beyond rcu_num_lvls. Thus, rcu_state::levelcnt[] is safe to limit to RCU_NUM_LVLS items.

Since rcu_num_lvls could be changed during boot (as result of rcutree.rcu_fanout_leaf kernel parameter update) one might assume a new value could overflow the value of RCU_NUM_LVLS. However, that is not the case, since leaf-level fanout is only permitted to increase, resulting in rcu_num_lvls possibly to decrease.

Cc "Paul E. McKenney" <[email protected]>

Signed-off-by Alexander Gordeev <[email protected]>

---
 kernel/rcu/tree.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

kernel/rcu/tree.h +1 -1

index dd5ce40..f358a83 100644
@@ -421,7 +421,7 @@ do {									\
 struct rcu_state {
 	struct rcu_node node[NUM_RCU_NODES];	/* Hierarchy. */
 	struct rcu_node *level[RCU_NUM_LVLS];	/* Hierarchy levels. */
-	u32 levelcnt[MAX_RCU_LVLS + 1];		/* # nodes in each level. */
+	u32 levelcnt[RCU_NUM_LVLS];		/* # nodes in each level. */
 	u8 levelspread[RCU_NUM_LVLS];		/* kids/node in each level. */
 	u8 flavor_mask;				/* bit in flavor mask. */
 	struct rcu_data __percpu *rda;		/* pointer of percu rcu_data. */

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 08:34 UTC #6 lore raw

PATCH 6/10 rcu: Limit rcu_capacity[] size to RCU_NUM_LVLS items

In reply to Alexander Gordeev

Number of items in rcu_capacity[] array is defined by macro MAX_RCU_LVLS. However, that array is never accessed beyond RCU_NUM_LVLS index. Therefore, we can limit the array to RCU_NUM_LVLS items and eliminate MAX_RCU_LVLS. As result, in most cases the memory is conserved.

Cc "Paul E. McKenney" <[email protected]>

Signed-off-by Alexander Gordeev <[email protected]>

---
 kernel/rcu/tree.c | 12 ++++++------
 kernel/rcu/tree.h |  1 -
 2 files changed, 6 insertions(+), 7 deletions(-)

kernel/rcu/tree.c +6 -6

index 98e36c3..dbcec66 100644
@@ -3924,19 +3924,19 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 		"rcu_node_0",
 		"rcu_node_1",
 		"rcu_node_2",
-		"rcu_node_3" };  /* Match MAX_RCU_LVLS */
+		"rcu_node_3" };
 	static const char * const fqs[] = {
 		"rcu_node_fqs_0",
 		"rcu_node_fqs_1",
 		"rcu_node_fqs_2",
-		"rcu_node_fqs_3" };  /* Match MAX_RCU_LVLS */
+		"rcu_node_fqs_3" };
 	static u8 fl_mask = 0x1;
 	int cpustride = 1;
 	int i;
 	int j;
 	struct rcu_node *rnp;
 
-	BUILD_BUG_ON(MAX_RCU_LVLS > ARRAY_SIZE(buf));  /* Fix buf[] init! */
+	BUILD_BUG_ON(RCU_NUM_LVLS > ARRAY_SIZE(buf));  /* Fix buf[] init! */
 
 	/* Silence gcc 4.8 warning about array index out of range. */
 	if (rcu_num_lvls > RCU_NUM_LVLS)

@@ -4008,7 +4008,7 @@ static void __init rcu_init_geometry(void)
 {
 	ulong d;
 	int i;
-	int rcu_capacity[MAX_RCU_LVLS];
+	int rcu_capacity[RCU_NUM_LVLS];
 
 	/*
 	 * Initialize any unspecified boot parameters.

@@ -4035,7 +4035,7 @@ static void __init rcu_init_geometry(void)
 	 * with the given number of levels.
 	 */
 	rcu_capacity[0] = rcu_fanout_leaf;
-	for (i = 1; i < MAX_RCU_LVLS; i++)
+	for (i = 1; i < RCU_NUM_LVLS; i++)
 		rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;
 
 	/*

@@ -4048,7 +4048,7 @@ static void __init rcu_init_geometry(void)
 	 * the rcu_node masks.  Complain and fall back to the compile-
 	 * time values if these limits are exceeded.
 	 */
-	if (nr_cpu_ids > rcu_capacity[MAX_RCU_LVLS - 1])
+	if (nr_cpu_ids > rcu_capacity[RCU_NUM_LVLS - 1])
 		panic("rcu_init_geometry: rcu_capacity[] is too small");
 	else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
 		 rcu_fanout_leaf > sizeof(unsigned long) * 8) {

kernel/rcu/tree.h +0 -1

index f358a83..661d2d4 100644
@@ -35,7 +35,6 @@
  * In practice, this did work well going from three levels to four.
  * Of course, your mileage may vary.
  */
-#define MAX_RCU_LVLS 4
 #define RCU_FANOUT_1	      (CONFIG_RCU_FANOUT_LEAF)
 #define RCU_FANOUT_2	      (RCU_FANOUT_1 * CONFIG_RCU_FANOUT)
 #define RCU_FANOUT_3	      (RCU_FANOUT_2 * CONFIG_RCU_FANOUT)

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 08:35 UTC #7 lore raw

PATCH 7/10 rcu: Remove unnecessary fields from rcu_state structure

In reply to Alexander Gordeev

Members rcu_state::levelcnt[] and rcu_state::levelspread[] are only used at init. There is no reason to keep them afterwards.

Cc "Paul E. McKenney" <[email protected]>

Signed-off-by Alexander Gordeev <[email protected]>

---
 kernel/rcu/tree.c | 27 +++++++++++++++------------
 kernel/rcu/tree.h |  2 --
 2 files changed, 15 insertions(+), 14 deletions(-)

kernel/rcu/tree.c +15 ~~-12~~

index dbcec66..cde1323 100644
@@ -3893,22 +3893,22 @@ void rcu_scheduler_starting(void)
  * Compute the per-level fanout, either using the exact fanout specified
  * or balancing the tree, depending on CONFIG_RCU_FANOUT_EXACT.
  */
-static void __init rcu_init_levelspread(struct rcu_state *rsp)
+static void __init rcu_init_levelspread(int *levelspread, const int *levelcnt)
 {
 	int i;
 
 	if (IS_ENABLED(CONFIG_RCU_FANOUT_EXACT)) {
-		rsp->levelspread[rcu_num_lvls - 1] = rcu_fanout_leaf;
+		levelspread[rcu_num_lvls - 1] = rcu_fanout_leaf;
 		for (i = rcu_num_lvls - 2; i >= 0; i--)
-			rsp->levelspread[i] = CONFIG_RCU_FANOUT;
+			levelspread[i] = CONFIG_RCU_FANOUT;
 	} else {
 		int ccur;
 		int cprv;
 
 		cprv = nr_cpu_ids;
 		for (i = rcu_num_lvls - 1; i >= 0; i--) {
-			ccur = rsp->levelcnt[i];
-			rsp->levelspread[i] = (cprv + ccur - 1) / ccur;
+			ccur = levelcnt[i];
+			levelspread[i] = (cprv + ccur - 1) / ccur;
 			cprv = ccur;
 		}
 	}

@@ -3931,6 +3931,9 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 		"rcu_node_fqs_2",
 		"rcu_node_fqs_3" };
 	static u8 fl_mask = 0x1;
+
+	int levelcnt[RCU_NUM_LVLS];		/* # nodes in each level. */
+	int levelspread[RCU_NUM_LVLS];		/* kids/node in each level. */
 	int cpustride = 1;
 	int i;
 	int j;

@@ -3945,19 +3948,19 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 	/* Initialize the level-tracking arrays. */
 
 	for (i = 0; i < rcu_num_lvls; i++)
-		rsp->levelcnt[i] = num_rcu_lvl[i];
+		levelcnt[i] = num_rcu_lvl[i];
 	for (i = 1; i < rcu_num_lvls; i++)
-		rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
-	rcu_init_levelspread(rsp);
+		rsp->level[i] = rsp->level[i - 1] + levelcnt[i - 1];
+	rcu_init_levelspread(levelspread, levelcnt);
 	rsp->flavor_mask = fl_mask;
 	fl_mask <<= 1;
 
 	/* Initialize the elements themselves, starting from the leaves. */
 
 	for (i = rcu_num_lvls - 1; i >= 0; i--) {
-		cpustride *= rsp->levelspread[i];
+		cpustride *= levelspread[i];
 		rnp = rsp->level[i];
-		for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
+		for (j = 0; j < levelcnt[i]; j++, rnp++) {
 			raw_spin_lock_init(&rnp->lock);
 			lockdep_set_class_and_name(&rnp->lock,
 						   &rcu_node_class[i], buf[i]);

@@ -3977,10 +3980,10 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 				rnp->grpmask = 0;
 				rnp->parent = NULL;
 			} else {
-				rnp->grpnum = j % rsp->levelspread[i - 1];
+				rnp->grpnum = j % levelspread[i - 1];
 				rnp->grpmask = 1UL << rnp->grpnum;
 				rnp->parent = rsp->level[i - 1] +
-					      j / rsp->levelspread[i - 1];
+					      j / levelspread[i - 1];
 			}
 			rnp->level = i;
 			INIT_LIST_HEAD(&rnp->blkd_tasks);

kernel/rcu/tree.h +0 -2

index 661d2d4..55886d2 100644
@@ -420,8 +420,6 @@ do {									\
 struct rcu_state {
 	struct rcu_node node[NUM_RCU_NODES];	/* Hierarchy. */
 	struct rcu_node *level[RCU_NUM_LVLS];	/* Hierarchy levels. */
-	u32 levelcnt[RCU_NUM_LVLS];		/* # nodes in each level. */
-	u8 levelspread[RCU_NUM_LVLS];		/* kids/node in each level. */
 	u8 flavor_mask;				/* bit in flavor mask. */
 	struct rcu_data __percpu *rda;		/* pointer of percu rcu_data. */
 	void (*call)(struct rcu_head *head,	/* call_rcu() flavor. */

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 08:35 UTC #8 lore raw

PATCH 8/10 rcu: Limit count of static data to the number of RCU levels

In reply to Alexander Gordeev

Although a number of RCU levels may be less than the current maximum of four, some static data associated with each level are allocated for all four levels. As result, the extra data never get accessed and just wast memory. This update limits count of allocated items to the number of used RCU levels.

Cc "Paul E. McKenney" <[email protected]>

Signed-off-by Alexander Gordeev <[email protected]>

---
 kernel/rcu/tree.c | 21 ++++-----------------
 kernel/rcu/tree.h | 12 ++++++++++++
 2 files changed, 16 insertions(+), 17 deletions(-)

kernel/rcu/tree.c +4 ~~-17~~

index cde1323..1cc1286 100644
@@ -118,13 +118,8 @@ LIST_HEAD(rcu_struct_flavors);
 static int rcu_fanout_leaf = CONFIG_RCU_FANOUT_LEAF;
 module_param(rcu_fanout_leaf, int, 0444);
 int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
-static int num_rcu_lvl[] = {  /* Number of rcu_nodes at specified level. */
-	NUM_RCU_LVL_0,
-	NUM_RCU_LVL_1,
-	NUM_RCU_LVL_2,
-	NUM_RCU_LVL_3,
-	NUM_RCU_LVL_4,
-};
+/* Number of rcu_nodes at specified level. */
+static int num_rcu_lvl[] = NUM_RCU_LVL_INIT;
 int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
 
 /*

@@ -3920,16 +3915,8 @@ static void __init rcu_init_levelspread(int *levelspread, const int *levelcnt)
 static void __init rcu_init_one(struct rcu_state *rsp,
 		struct rcu_data __percpu *rda)
 {
-	static const char * const buf[] = {
-		"rcu_node_0",
-		"rcu_node_1",
-		"rcu_node_2",
-		"rcu_node_3" };
-	static const char * const fqs[] = {
-		"rcu_node_fqs_0",
-		"rcu_node_fqs_1",
-		"rcu_node_fqs_2",
-		"rcu_node_fqs_3" };
+	static const char * const buf[] = RCU_NODE_NAME_INIT;
+	static const char * const fqs[] = RCU_FQS_NAME_INIT;
 	static u8 fl_mask = 0x1;
 
 	int levelcnt[RCU_NUM_LVLS];		/* # nodes in each level. */

kernel/rcu/tree.h +12 -0

index 55886d2..cdb98e5 100644
@@ -47,6 +47,9 @@
 #  define NUM_RCU_LVL_2	      0
 #  define NUM_RCU_LVL_3	      0
 #  define NUM_RCU_LVL_4	      0
+#  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0 }
+#  define RCU_NODE_NAME_INIT  { "rcu_node_0" }
+#  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0" }
 #elif NR_CPUS <= RCU_FANOUT_2
 #  define RCU_NUM_LVLS	      2
 #  define NUM_RCU_LVL_0	      1

@@ -54,6 +57,9 @@
 #  define NUM_RCU_LVL_2	      (NR_CPUS)
 #  define NUM_RCU_LVL_3	      0
 #  define NUM_RCU_LVL_4	      0
+#  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
+#  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1" }
+#  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1" }
 #elif NR_CPUS <= RCU_FANOUT_3
 #  define RCU_NUM_LVLS	      3
 #  define NUM_RCU_LVL_0	      1

@@ -61,6 +67,9 @@
 #  define NUM_RCU_LVL_2	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
 #  define NUM_RCU_LVL_3	      (NR_CPUS)
 #  define NUM_RCU_LVL_4	      0
+#  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
+#  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
+#  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
 #elif NR_CPUS <= RCU_FANOUT_4
 #  define RCU_NUM_LVLS	      4
 #  define NUM_RCU_LVL_0	      1

@@ -68,6 +77,9 @@
 #  define NUM_RCU_LVL_2	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
 #  define NUM_RCU_LVL_3	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
 #  define NUM_RCU_LVL_4	      (NR_CPUS)
+#  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
+#  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
+#  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
 #else
 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
 #endif /* #if (NR_CPUS) <= RCU_FANOUT_1 */

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 08:34 UTC #9 lore raw

PATCH 9/10 rcu: Simplify arithmetic to calculate number of RCU nodes

In reply to Alexander Gordeev

This update makes arithmetic to calculate number of RCU nodes more straight and easy to read.

Cc "Paul E. McKenney" <[email protected]>

Signed-off-by Alexander Gordeev <[email protected]>

---
 kernel/rcu/tree.h        | 17 ++++-------------
 kernel/rcu/tree_plugin.h |  4 ++--
 2 files changed, 6 insertions(+), 15 deletions(-)

kernel/rcu/tree.h +4 ~~-13~~

index cdb98e5..f12fe83 100644
@@ -43,10 +43,7 @@
 #if NR_CPUS <= RCU_FANOUT_1
 #  define RCU_NUM_LVLS	      1
 #  define NUM_RCU_LVL_0	      1
-#  define NUM_RCU_LVL_1	      (NR_CPUS)
-#  define NUM_RCU_LVL_2	      0
-#  define NUM_RCU_LVL_3	      0
-#  define NUM_RCU_LVL_4	      0
+#  define NUM_RCU_NODES	      NUM_RCU_LVL_0
 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0 }
 #  define RCU_NODE_NAME_INIT  { "rcu_node_0" }
 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0" }

@@ -54,9 +51,7 @@
 #  define RCU_NUM_LVLS	      2
 #  define NUM_RCU_LVL_0	      1
 #  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
-#  define NUM_RCU_LVL_2	      (NR_CPUS)
-#  define NUM_RCU_LVL_3	      0
-#  define NUM_RCU_LVL_4	      0
+#  define NUM_RCU_NODES	      (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1" }
 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1" }

@@ -65,8 +60,7 @@
 #  define NUM_RCU_LVL_0	      1
 #  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
 #  define NUM_RCU_LVL_2	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
-#  define NUM_RCU_LVL_3	      (NR_CPUS)
-#  define NUM_RCU_LVL_4	      0
+#  define NUM_RCU_NODES	      (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }

@@ -76,7 +70,7 @@
 #  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
 #  define NUM_RCU_LVL_2	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
 #  define NUM_RCU_LVL_3	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
-#  define NUM_RCU_LVL_4	      (NR_CPUS)
+#  define NUM_RCU_NODES	      (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }

@@ -84,9 +78,6 @@
 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
 #endif /* #if (NR_CPUS) <= RCU_FANOUT_1 */
 
-#define RCU_SUM (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3 + NUM_RCU_LVL_4)
-#define NUM_RCU_NODES (RCU_SUM - NR_CPUS)
-
 extern int rcu_num_lvls;
 extern int rcu_num_nodes;

kernel/rcu/tree_plugin.h +2 -2

index 2549f4b..cf1410f 100644
@@ -84,8 +84,8 @@ static void __init rcu_bootup_announce_oddness(void)
 		pr_info("\tRCU torture testing starts during boot.\n");
 	if (IS_ENABLED(CONFIG_RCU_CPU_STALL_INFO))
 		pr_info("\tAdditional per-CPU info printed with stalls.\n");
-	if (NUM_RCU_LVL_4 != 0)
-		pr_info("\tFour-level hierarchy is enabled.\n");
+	if (RCU_NUM_LVLS >= 4)
+		pr_info("\tFour(or more)-level hierarchy is enabled.\n");
 	if (CONFIG_RCU_FANOUT_LEAF != 16)
 		pr_info("\tBuild-time adjustment of leaf fanout to %d.\n",
 			CONFIG_RCU_FANOUT_LEAF);

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 08:34 UTC #10 lore raw

PATCH 10/10 rcu: Remove unnecessary grpnum field from rcu_node structure

In reply to Alexander Gordeev

Field rcu_node::grpnum is used to set rcu_node::grpmask at init and never accessed afterwards. There is no reason to have it.

Cc "Paul E. McKenney" <[email protected]>

Signed-off-by Alexander Gordeev <[email protected]>

---
 kernel/rcu/tree.c | 5 ++---
 kernel/rcu/tree.h | 3 +--
 2 files changed, 3 insertions(+), 5 deletions(-)

kernel/rcu/tree.c +2 -3

index 1cc1286..6afcf8f 100644
@@ -3963,12 +3963,11 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 			if (rnp->grphi >= nr_cpu_ids)
 				rnp->grphi = nr_cpu_ids - 1;
 			if (i == 0) {
-				rnp->grpnum = 0;
 				rnp->grpmask = 0;
 				rnp->parent = NULL;
 			} else {
-				rnp->grpnum = j % levelspread[i - 1];
-				rnp->grpmask = 1UL << rnp->grpnum;
+				int grpnum = j % levelspread[i - 1];
+				rnp->grpmask = 1UL << grpnum;
 				rnp->parent = rsp->level[i - 1] +
 					      j / levelspread[i - 1];
 			}

kernel/rcu/tree.h +1 -2

index f12fe83..bd66e85 100644
@@ -151,8 +151,7 @@ struct rcu_node {
 				/*  Only one bit will be set in this mask. */
 	int	grplo;		/* lowest-numbered CPU or group here. */
 	int	grphi;		/* highest-numbered CPU or group here. */
-	u8	grpnum;		/* CPU/group number for next level up. */
-	u8	level;		/* root is at level 0. */
+	int	level;		/* root is at level 0. */
 	bool	wait_blkd_tasks;/* Necessary to wait for blocked tasks to */
 				/*  exit RCU read-side critical sections */
 				/*  before propagating offline up the */

Signature · 1 line

1.8.3.1

Alexander Gordeev

author

2015-03-09 09:23 UTC #11 lore raw

In reply to Alexander Gordeev

Quoted 5 lines · Alexander Gordeev

Hi Paul,
Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.

Paul,

Please, ignore patch #10 for now. I missed to notice rcu_node::grpnum is used in tracing, so the patch is incomplete. I am not sure why trailing
spaces in seq_printf(m, "%lx/%lx->%lx %c%c>%c %d:%d ^%d ", ....) are
needed for, so not sure if "^%d" part should be removed (possibly with the traling spaces) or replaced with three spaces.

Thanks!

Signature · 3 lines

Regards,
Alexander Gordeev
[email protected]

Paul E. McKenney

2015-03-09 21:35 UTC #12 lore raw

Quoted 7 lines · Alexander Gordeev

Quoted 5 lines · Alexander Gordeev
Hi Paul,
Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.

Quoted 7 lines

Paul,
Please, ignore patch #10 for now. I missed to notice rcu_node::grpnum is used in tracing, so the patch is incomplete. I am not sure why trailing
spaces in seq_printf(m, "%lx/%lx->%lx %c%c>%c %d:%d ^%d ", ....) are
needed for, so not sure if "^%d" part should be removed (possibly with the traling spaces) or replaced with three spaces.

OK, dropping this one for the moment.

The original use of ->grpnum was for manual debugging purposes. Yes, you can get the same information out of ->grpmask, but the number is easier to read. And on the debugfs trace information, ->grpnum is printed, but ->grpmask is not.

The trailing spaces on the seq_printf() allow the rcu_node data to be printed on a single line, while still allowing the eye to pick out where one rcu_node structure's data ends and the next one begins.

So here are the choices, as far as I can see:

1. Leave ->grpnum as is.

2. Remove ->grpnum, but regenerate it in print_one_rcu_state(),

	for example, by counting the number of rcu_node structures
	since the last ->level change.

3. Drop ->grpnum and also remove it from the debugfs tracing.
The reader can rely on the ->grplo and ->grphi fields to work out where this rcu_node structure fits in, but we lose the visual indication of any bugs in computing these quantities.

4. Drop ->grpnum and replace it with ->grpmask. This seems a
bit obtuse to me.

5. Redesign print_one_rcu_state()'s output from scratch.

#1 has certain advantages from a laziness viewpoint. #2 would open up some space in the rcu_node structure, but space really isn't an issue for that structure given that huge systems have only 257 of them and the really small systems use Tiny RCU instead. #3 might be OK, but I am not really convinced. #4 seems a bit ugly. I am not signing up for #5, in part because not all that many people use RCU's debugfs output, so I don't see the point in investing the time.

But what did you have in mind?

Thanx, Paul

Alexander Gordeev

author

2015-03-10 14:20 UTC #13 lore raw

Quoted 54 lines · Paul E. McKenney

Quoted 7 lines · Alexander Gordeev
Quoted 5 lines · Alexander Gordeev
Hi Paul,
Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.
Quoted 7 lines
Paul,
Please, ignore patch #10 for now. I missed to notice rcu_node::grpnum is used in tracing, so the patch is incomplete. I am not sure why trailing
spaces in seq_printf(m, "%lx/%lx->%lx %c%c>%c %d:%d ^%d ", ....) are
needed for, so not sure if "^%d" part should be removed (possibly with the traling spaces) or replaced with three spaces.
OK, dropping this one for the moment.
The original use of ->grpnum was for manual debugging purposes. Yes, you can get the same information out of ->grpmask, but the number is easier to read. And on the debugfs trace information, ->grpnum is printed, but ->grpmask is not.
The trailing spaces on the seq_printf() allow the rcu_node data to be printed on a single line, while still allowing the eye to pick out where one rcu_node structure's data ends and the next one begins.
So here are the choices, as far as I can see:
1. Leave ->grpnum as is.
2. Remove ->grpnum, but regenerate it in print_one_rcu_state(),
	for example, by counting the number of rcu_node structures
	since the last ->level change.
3. Drop ->grpnum and also remove it from the debugfs tracing.
The reader can rely on the ->grplo and ->grphi fields to work out where this rcu_node structure fits in, but we lose the visual indication of any bugs in computing these quantities.
4. Drop ->grpnum and replace it with ->grpmask. This seems a
bit obtuse to me.
5. Redesign print_one_rcu_state()'s output from scratch.
#1 has certain advantages from a laziness viewpoint. #2 would open up some space in the rcu_node structure, but space really isn't an issue for that structure given that huge systems have only 257 of them and the really small systems use Tiny RCU instead. #3 might be OK, but I am not really convinced. #4 seems a bit ugly. I am not signing up for #5, in part because not all that many people use RCU's debugfs output, so I don't see the point in investing the time.

Quoted 1 line

But what did you have in mind?

I probably should have marked this patch as an RFC. Given your summary #1 seems as the best choice.

However, I have something else in mind, indeed. What is the reason to have 'grpnum' and 'level' as u8 while, say 'grplo' and 'grphi' - as int? IOW, do we conserve on memory for this structure or not?

Thanks!

Quoted 2 lines

Thanx, Paul

Signature · 3 lines

Regards,
Alexander Gordeev
[email protected]

Paul E. McKenney

2015-03-10 14:57 UTC #14 lore raw

Quoted 55 lines · Alexander Gordeev

Quoted 54 lines · Paul E. McKenney
Quoted 7 lines · Alexander Gordeev
Quoted 5 lines · Alexander Gordeev
Hi Paul,
Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.
Quoted 7 lines
Paul,
Please, ignore patch #10 for now. I missed to notice rcu_node::grpnum is used in tracing, so the patch is incomplete. I am not sure why trailing
spaces in seq_printf(m, "%lx/%lx->%lx %c%c>%c %d:%d ^%d ", ....) are
needed for, so not sure if "^%d" part should be removed (possibly with the traling spaces) or replaced with three spaces.
OK, dropping this one for the moment.
The original use of ->grpnum was for manual debugging purposes. Yes, you can get the same information out of ->grpmask, but the number is easier to read. And on the debugfs trace information, ->grpnum is printed, but ->grpmask is not.
The trailing spaces on the seq_printf() allow the rcu_node data to be printed on a single line, while still allowing the eye to pick out where one rcu_node structure's data ends and the next one begins.
So here are the choices, as far as I can see:
1. Leave ->grpnum as is.
2. Remove ->grpnum, but regenerate it in print_one_rcu_state(),
	for example, by counting the number of rcu_node structures
	since the last ->level change.
3. Drop ->grpnum and also remove it from the debugfs tracing.
The reader can rely on the ->grplo and ->grphi fields to work out where this rcu_node structure fits in, but we lose the visual indication of any bugs in computing these quantities.
4. Drop ->grpnum and replace it with ->grpmask. This seems a
bit obtuse to me.
5. Redesign print_one_rcu_state()'s output from scratch.
#1 has certain advantages from a laziness viewpoint. #2 would open up some space in the rcu_node structure, but space really isn't an issue for that structure given that huge systems have only 257 of them and the really small systems use Tiny RCU instead. #3 might be OK, but I am not really convinced. #4 seems a bit ugly. I am not signing up for #5, in part because not all that many people use RCU's debugfs output, so I don't see the point in investing the time.

Quoted 8 lines

Quoted 1 line
But what did you have in mind?
I probably should have marked this patch as an RFC. Given your summary #1 seems as the best choice.
However, I have something else in mind, indeed. What is the reason to have 'grpnum' and 'level' as u8 while, say 'grplo' and 'grphi' - as int? IOW, do we conserve on memory for this structure or not?

The ->grplo and ->grphi fields must hold a CPU number. Since CPU numbers are int elsewhere, they are int here. I considered making them a short, but there are systems uncomfortably close to the limit. There have been 4096-CPU systems for quite some time, and I have heard rumors of 16384-CPU systems. A limit of 32768 seems uncomfortably tight, especially given that memory footprint is at best a minor requirement for Tree RCU. Tiny RCU is of course another story -- memory savings is Job One there.

And yes, I do owe the community a writeup of the requirements on RCU.

Thanx, Paul

Paul E. McKenney

2015-03-09 21:40 UTC #15 lore raw

In reply to Alexander Gordeev

Quoted 8 lines · Alexander Gordeev

Hi Paul,
Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.
The series successfully passes kernel build test with CONFIG_RCU_FANOUT and CONFIG_RCU_FANOUT_LEAF equal to 5.

I queued up 1-9, as discussed and have started testing. I will let you know how it goes.

Thanx, Paul

Quoted 23 lines

Cc "Paul E. McKenney" <[email protected]>
Alexander Gordeev (10):
rcu: Panic if RCU tree can not accommodate all CPUs
rcu: Remove superfluous local variable in rcu_init_geometry()
rcu: Cleanup rcu_init_geometry() code and arithmetics
rcu: Simplify rcu_init_geometry() capacity arithmetics
rcu: Limit rcu_state::levelcnt[] to RCU_NUM_LVLS items
rcu: Limit rcu_capacity[] size to RCU_NUM_LVLS items
rcu: Remove unnecessary fields from rcu_state structure
rcu: Limit count of static data to the number of RCU levels
rcu: Simplify arithmetic to calculate number of RCU nodes
rcu: Remove unnecessary grpnum field from rcu_node structure
 kernel/rcu/tree.c        | 102 ++++++++++++++++++++---------------------------
 kernel/rcu/tree.h        |  35 ++++++++--------
 kernel/rcu/tree_plugin.h |   4 +-
 3 files changed, 63 insertions(+), 78 deletions(-)
Signature · 1 line
1.8.3.1

Paul E. McKenney

2015-03-09 23:39 UTC #16 lore raw

Quoted 7 lines · Paul E. McKenney

Quoted 6 lines · Alexander Gordeev
Hi Paul,
Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.

Quoted 5 lines

Quoted 2 lines
The series successfully passes kernel build test with CONFIG_RCU_FANOUT and CONFIG_RCU_FANOUT_LEAF equal to 5.
I queued up 1-9, as discussed and have started testing. I will let you know how it goes.

Initial testing went well except for the following warning:

/home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c: In function ‘rcu_init_one.isra.63’:
/home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c:3961:3: warning: ‘levelcnt[0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
for (j = 0; j < levelcnt[i]; j++, rnp++) {

This warning looks like a false positive to me, given that the loop near the beginning of the function initializes levelcnt[0]. Am I missing something here, and either way, what is the best way to shut this warning up?

I am using gcc version 4.8.2.

This warning only appears for some configurations.

SRCU-N:  Four CPUs, single rcu_node structure.
SRCU-P:  Eight CPUs, single rcu_node structure.
TASKS01: Two CPUs, single rcu_node structure.
TASKS03: Two CPUs, single rcu_node structure.
TREE09:  Single CPU, single rcu_node structure.

All the other TREE0N configurations have multiple rcu_node structures.

TASKS02 didn't warn either despite having only one CPU, but that is because it uses Tiny RCU instead of Tree RCU.

So this warning shows up only in the case where the compiler knows that the rcu_node tree consists of only one rcu_node structure.

Thanx, Paul

Quoted 23 lines

Quoted 23 lines
Cc "Paul E. McKenney" <[email protected]>
Alexander Gordeev (10):
rcu: Panic if RCU tree can not accommodate all CPUs
rcu: Remove superfluous local variable in rcu_init_geometry()
rcu: Cleanup rcu_init_geometry() code and arithmetics
rcu: Simplify rcu_init_geometry() capacity arithmetics
rcu: Limit rcu_state::levelcnt[] to RCU_NUM_LVLS items
rcu: Limit rcu_capacity[] size to RCU_NUM_LVLS items
rcu: Remove unnecessary fields from rcu_state structure
rcu: Limit count of static data to the number of RCU levels
rcu: Simplify arithmetic to calculate number of RCU nodes
rcu: Remove unnecessary grpnum field from rcu_node structure
 kernel/rcu/tree.c        | 102 ++++++++++++++++++++---------------------------
 kernel/rcu/tree.h        |  35 ++++++++--------
 kernel/rcu/tree_plugin.h |   4 +-
 3 files changed, 63 insertions(+), 78 deletions(-)
Signature · 1 line
1.8.3.1

Paul E. McKenney

2015-03-09 23:49 UTC #17 lore raw

Quoted 16 lines · Paul E. McKenney

Quoted 7 lines · Paul E. McKenney
Quoted 6 lines · Alexander Gordeev
Hi Paul,
Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.
Quoted 5 lines
Quoted 2 lines
The series successfully passes kernel build test with CONFIG_RCU_FANOUT and CONFIG_RCU_FANOUT_LEAF equal to 5.
I queued up 1-9, as discussed and have started testing. I will let you know how it goes.
Initial testing went well except for the following warning:

Quoted 8 lines

/home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c: In function ‘rcu_init_one.isra.63’:
/home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c:3961:3: warning: ‘levelcnt[0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
for (j = 0; j < levelcnt[i]; j++, rnp++) {
This warning looks like a false positive to me, given that the loop near the beginning of the function initializes levelcnt[0]. Am I missing something here, and either way, what is the best way to shut this warning up?

My suggestion is the following:

	if (rcu_num_lvls <= 0)
		panic("rcu_init_one: rcu_num_lvls underflow");

Just following the other panic() in rcu_init_one().

Thanx, Paul

Quoted 42 lines

I am using gcc version 4.8.2.
This warning only appears for some configurations.
SRCU-N:  Four CPUs, single rcu_node structure.
SRCU-P:  Eight CPUs, single rcu_node structure.
TASKS01: Two CPUs, single rcu_node structure.
TASKS03: Two CPUs, single rcu_node structure.
TREE09:  Single CPU, single rcu_node structure.
All the other TREE0N configurations have multiple rcu_node structures.
TASKS02 didn't warn either despite having only one CPU, but that is because it uses Tiny RCU instead of Tree RCU.
So this warning shows up only in the case where the compiler knows that the rcu_node tree consists of only one rcu_node structure.
Thanx, Paul
Quoted 23 lines
Quoted 23 lines
Cc "Paul E. McKenney" <[email protected]>
Alexander Gordeev (10):
rcu: Panic if RCU tree can not accommodate all CPUs
rcu: Remove superfluous local variable in rcu_init_geometry()
rcu: Cleanup rcu_init_geometry() code and arithmetics
rcu: Simplify rcu_init_geometry() capacity arithmetics
rcu: Limit rcu_state::levelcnt[] to RCU_NUM_LVLS items
rcu: Limit rcu_capacity[] size to RCU_NUM_LVLS items
rcu: Remove unnecessary fields from rcu_state structure
rcu: Limit count of static data to the number of RCU levels
rcu: Simplify arithmetic to calculate number of RCU nodes
rcu: Remove unnecessary grpnum field from rcu_node structure
 kernel/rcu/tree.c        | 102 ++++++++++++++++++++---------------------------
 kernel/rcu/tree.h        |  35 ++++++++--------
 kernel/rcu/tree_plugin.h |   4 +-
 3 files changed, 63 insertions(+), 78 deletions(-)
Signature · 1 line
1.8.3.1

Paul E. McKenney

2015-03-10 04:43 UTC #18 lore raw

Quoted 26 lines · Paul E. McKenney

Quoted 16 lines · Paul E. McKenney
Quoted 7 lines · Paul E. McKenney
Quoted 6 lines · Alexander Gordeev
Hi Paul,
Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.
Quoted 5 lines
Quoted 2 lines
The series successfully passes kernel build test with CONFIG_RCU_FANOUT and CONFIG_RCU_FANOUT_LEAF equal to 5.
I queued up 1-9, as discussed and have started testing. I will let you know how it goes.
Initial testing went well except for the following warning:
Quoted 8 lines
/home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c: In function ‘rcu_init_one.isra.63’:
/home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c:3961:3: warning: ‘levelcnt[0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
for (j = 0; j < levelcnt[i]; j++, rnp++) {
This warning looks like a false positive to me, given that the loop near the beginning of the function initializes levelcnt[0]. Am I missing something here, and either way, what is the best way to shut this warning up?

Quoted 6 lines

My suggestion is the following:
	if (rcu_num_lvls <= 0)
		panic("rcu_init_one: rcu_num_lvls underflow");
Just following the other panic() in rcu_init_one().

As in the following patch.

Thanx, Paul

------------------------------------------------------------------------

rcu: Shut up spurious gcc uninitialized-variable warning

Because gcc doesn't realize that rcu_num_lvls must be strictly greater than zero, some versions give a spurious warning about levelcnt[0] being uninitialized in rcu_init_one(). This commit adds a panic() in that case in order to educate gcc on this point.

Signed-off-by Paul E. McKenney <[email protected]>

kernel/rcu/tree.c +2 -0

index bd5a9a1db048..5b42d94335e3 100644
@@ -3942,6 +3942,8 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 	/* Silence gcc 4.8 warning about array index out of range. */
 	if (rcu_num_lvls > RCU_NUM_LVLS)
 		panic("rcu_init_one: rcu_num_lvls overflow");
+	if (rcu_num_lvls <= 0)
+		panic("rcu_init_one: rcu_num_lvls underflow");
 
 	/* Initialize the level-tracking arrays. */

Alexander Gordeev

author

2015-03-10 14:26 UTC #19 lore raw

Quoted 54 lines · Paul E. McKenney

Quoted 26 lines · Paul E. McKenney
Quoted 16 lines · Paul E. McKenney
Quoted 7 lines · Paul E. McKenney
Quoted 6 lines · Alexander Gordeev
Hi Paul,
Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.
Quoted 5 lines
Quoted 2 lines
The series successfully passes kernel build test with CONFIG_RCU_FANOUT and CONFIG_RCU_FANOUT_LEAF equal to 5.
I queued up 1-9, as discussed and have started testing. I will let you know how it goes.
Initial testing went well except for the following warning:
Quoted 8 lines
/home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c: In function ‘rcu_init_one.isra.63’:
/home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c:3961:3: warning: ‘levelcnt[0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
for (j = 0; j < levelcnt[i]; j++, rnp++) {
This warning looks like a false positive to me, given that the loop near the beginning of the function initializes levelcnt[0]. Am I missing something here, and either way, what is the best way to shut this warning up?
Quoted 6 lines
My suggestion is the following:
	if (rcu_num_lvls <= 0)
		panic("rcu_init_one: rcu_num_lvls underflow");
Just following the other panic() in rcu_init_one().
As in the following patch.
Thanx, Paul
------------------------------------------------------------------------
rcu: Shut up spurious gcc uninitialized-variable warning
Because gcc doesn't realize that rcu_num_lvls must be strictly greater than zero, some versions give a spurious warning about levelcnt[0] being uninitialized in rcu_init_one(). This commit adds a panic() in that case in order to educate gcc on this point.
Signed-off-by Paul E. McKenney <[email protected]>
kernel/rcu/tree.c +0 -0
index bd5a9a1db048..5b42d94335e3 100644

kernel/rcu/tree.c static void __init rcu_init_one(struct rcu_state *rsp, · line 3946

Quoted 5 lines

kernel/rcu/tree.c +2 -0

@@ -3942,6 +3942,8 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 	/* Silence gcc 4.8 warning about array index out of range. */
 	if (rcu_num_lvls > RCU_NUM_LVLS)
 		panic("rcu_init_one: rcu_num_lvls overflow");
+	if (rcu_num_lvls <= 0)
+		panic("rcu_init_one: rcu_num_lvls underflow");

I believe '... else if (rcu_num_lvls <= 0)' is more appropriate here.

But do you think keeping two static strings just to shut the compiler worth it? May be a single complain would be enough?

  	if (rcu_num_lvls <= 0 || rcu_num_lvls > RCU_NUM_LVLS)
  		panic("rcu_init_one: rcu_num_lvls is out of range");

Quoted 3 lines

kernel/rcu/tree.c +0 -0

 	/* Initialize the level-tracking arrays. */

Signature · 3 lines

Regards,
Alexander Gordeev
[email protected]

Paul E. McKenney

2015-03-10 14:52 UTC #20 lore raw

Quoted 50 lines · Alexander Gordeev

Quoted 49 lines · Paul E. McKenney
Quoted 26 lines · Paul E. McKenney
Quoted 16 lines · Paul E. McKenney
Quoted 7 lines · Paul E. McKenney
Quoted 6 lines · Alexander Gordeev
Hi Paul,
Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next repo, as you requested. Please, note an extra patch #10 that was not present in the first post.
Quoted 5 lines
Quoted 2 lines
The series successfully passes kernel build test with CONFIG_RCU_FANOUT and CONFIG_RCU_FANOUT_LEAF equal to 5.
I queued up 1-9, as discussed and have started testing. I will let you know how it goes.
Initial testing went well except for the following warning:
Quoted 8 lines
/home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c: In function ‘rcu_init_one.isra.63’:
/home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c:3961:3: warning: ‘levelcnt[0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
for (j = 0; j < levelcnt[i]; j++, rnp++) {
This warning looks like a false positive to me, given that the loop near the beginning of the function initializes levelcnt[0]. Am I missing something here, and either way, what is the best way to shut this warning up?
Quoted 6 lines
My suggestion is the following:
	if (rcu_num_lvls <= 0)
		panic("rcu_init_one: rcu_num_lvls underflow");
Just following the other panic() in rcu_init_one().
As in the following patch.
Thanx, Paul
------------------------------------------------------------------------
rcu: Shut up spurious gcc uninitialized-variable warning
Because gcc doesn't realize that rcu_num_lvls must be strictly greater than zero, some versions give a spurious warning about levelcnt[0] being uninitialized in rcu_init_one(). This commit adds a panic() in that case in order to educate gcc on this point.
Signed-off-by Paul E. McKenney <[email protected]>

Quoted 12 lines

kernel/rcu/tree.c static void __init rcu_init_one(struct rcu_state *rsp, · line 3946
Quoted 10 lines
kernel/rcu/tree.c +2 -0
index bd5a9a1db048..5b42d94335e3 100644
@@ -3942,6 +3942,8 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 	/* Silence gcc 4.8 warning about array index out of range. */
 	if (rcu_num_lvls > RCU_NUM_LVLS)
 		panic("rcu_init_one: rcu_num_lvls overflow");
+	if (rcu_num_lvls <= 0)
+		panic("rcu_init_one: rcu_num_lvls underflow");
I believe '... else if (rcu_num_lvls <= 0)' is more appropriate here.

Good point.

Quoted 2 lines

But do you think keeping two static strings just to shut the compiler worth it? May be a single complain would be enough?

As in the following?

	if (rcu_num_lvls <= 0 || rcu_num_lvls > RCU_NUM_LVLS)
		panic("rcu_init_one: rcu_num_lvls out of range");

For Tree RCU, I doubt that the memory size matters, but I do like having two lines of code instead of four lines. I took this approach.

Thanx, Paul

Quoted 12 lines

  	if (rcu_num_lvls <= 0 || rcu_num_lvls > RCU_NUM_LVLS)
  		panic("rcu_init_one: rcu_num_lvls is out of range");

Quoted 3 lines

kernel/rcu/tree.c +0 -0

 	/* Initialize the level-tracking arrays. */

Signature · 3 lines

Regards,
Alexander Gordeev
[email protected]