Skip to contentSkip to conversation

sched/debug: more schedstats fixes

PATCH In review Josh Poimboeuf 5 participants 15 messages Jun 7, 2016 – Jun 14, 2016

Subsystems (2)
Tracing Scheduler
lore
Share & tools
lore

Josh Poimboeuf

author
#0 lore raw

cover sched/debug: more schedstats fixes

Fixes for two more bugs I found in the schedstats code, in addition to to the other patches I sent out:

https://lkml.kernel.org/r/[email protected]

Josh Poimboeuf (2):
sched/debug: fix 'schedstats=enable' cmdline option sched/debug: fix deadlock when enabling sched events

 include/trace/events/sched.h | 30 +++++++++++++++---------------
 kernel/sched/core.c          | 26 +++++++++++++++++++++-----
 kernel/sched/fair.c          | 29 +++++++----------------------
 3 files changed, 43 insertions(+), 42 deletions(-)
Signature · 1 line
2.4.11
author
#1 lore raw

PATCH 1/2 sched/debug: fix 'schedstats=enable' cmdline option

The 'schedstats=enable' option doesn't work, and also produces the following warning during boot:

  WARNING: CPU: 0 PID: 0 at /home/jpoimboe/git/linux/kernel/jump_label.c:61 static_key_slow_inc+0x8c/0xa0
  static_key_slow_inc used before call to jump_label_init
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper Not tainted 4.7.0-rc1+ #25
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
   0000000000000086 3ae3475a4bea95d4 ffffffff81e03da8 ffffffff8143fc83
   ffffffff81e03df8 0000000000000000 ffffffff81e03de8 ffffffff810b1ffb
   0000003d00000096 ffffffff823514d0 ffff88007ff197c8 0000000000000000
  Call Trace:
   [<ffffffff8143fc83>] dump_stack+0x85/0xc2
   [<ffffffff810b1ffb>] __warn+0xcb/0xf0
   [<ffffffff810b207f>] warn_slowpath_fmt+0x5f/0x80
   [<ffffffff811e9c0c>] static_key_slow_inc+0x8c/0xa0
   [<ffffffff810e07c6>] static_key_enable+0x16/0x40
   [<ffffffff8216d633>] setup_schedstats+0x29/0x94
   [<ffffffff82148a05>] unknown_bootoption+0x89/0x191
   [<ffffffff810d8617>] parse_args+0x297/0x4b0
   [<ffffffff82148d61>] start_kernel+0x1d8/0x4a9
   [<ffffffff8214897c>] ? set_init_arg+0x55/0x55
   [<ffffffff82148120>] ? early_idt_handler_array+0x120/0x120
   [<ffffffff821482db>] x86_64_start_reservations+0x2f/0x31
   [<ffffffff82148427>] x86_64_start_kernel+0x14a/0x16d

The problem is that it tries to update the 'sched_schedstats' static key before jump labels have been initialized.

Changing jump_label_init() to be called earlier before parse_early_param() wouldn't fix it: it would still fail trying to poke_text() because mm isn't yet initialized.

Instead, just create a temporary '__sched_schedstats' variable which can be copied to the static key later during sched_init() after jump labels have been initialized.

Fixes cb2517653fcc ("sched/debug: Make schedstats a runtime tunable that is disabled by default")
Cc [email protected]
Signed-off-by Josh Poimboeuf <[email protected]>
---
 kernel/sched/core.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)
kernel/sched/core.c +21 -5
index 7f2cae4..385c947 100644
@@ -2253,9 +2253,11 @@ int sysctl_numa_balancing(struct ctl_table *table, int write,
 #endif
 #endif
 
+#ifdef CONFIG_SCHEDSTATS
+
 DEFINE_STATIC_KEY_FALSE(sched_schedstats);
+static bool __initdata __sched_schedstats = false;
 
-#ifdef CONFIG_SCHEDSTATS
 static void set_schedstats(bool enabled)
 {
 	if (enabled)

@@ -2278,11 +2280,16 @@ static int __init setup_schedstats(char *str)
 	if (!str)
 		goto out;
 
+	/*
+	 * This code is called before jump labels have been set up, so we can't
+	 * change the static branch directly just yet.  Instead set a temporary
+	 * variable so init_schedstats() can do it later.
+	 */
 	if (!strcmp(str, "enable")) {
-		set_schedstats(true);
+		__sched_schedstats = true;
 		ret = 1;
 	} else if (!strcmp(str, "disable")) {
-		set_schedstats(false);
+		__sched_schedstats = false;
 		ret = 1;
 	}
 out:

@@ -2293,6 +2300,11 @@ out:
 }
 __setup("schedstats=", setup_schedstats);
 
+static void __init init_schedstats(void)
+{
+	set_schedstats(__sched_schedstats);
+}
+
 #ifdef CONFIG_PROC_SYSCTL
 int sysctl_schedstats(struct ctl_table *table, int write,
 			 void __user *buffer, size_t *lenp, loff_t *ppos)

@@ -2313,8 +2325,10 @@ int sysctl_schedstats(struct ctl_table *table, int write,
 		set_schedstats(state);
 	return err;
 }
-#endif
-#endif
+#endif /* CONFIG_PROC_SYSCTL */
+#else  /* !CONFIG_SCHEDSTATS */
+static inline void init_schedstats(void) {}
+#endif /* CONFIG_SCHEDSTATS */
 
 /*
  * fork()/clone()-time setup:

@@ -7487,6 +7501,8 @@ void __init sched_init(void)
 #endif
 	init_sched_fair_class();
 
+	init_schedstats();
+
 	scheduler_running = 1;
 }
Signature · 1 line
2.4.11
Acked-by on v1 1/2
#2 lore raw

Re: [PATCH 1/2] sched/debug: fix 'schedstats=enable' cmdline option

Quoted 26 lines · Josh Poimboeuf

The 'schedstats=enable' option doesn't work, and also produces the following warning during boot:

  WARNING: CPU: 0 PID: 0 at /home/jpoimboe/git/linux/kernel/jump_label.c:61 static_key_slow_inc+0x8c/0xa0
  static_key_slow_inc used before call to jump_label_init
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper Not tainted 4.7.0-rc1+ #25
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
   0000000000000086 3ae3475a4bea95d4 ffffffff81e03da8 ffffffff8143fc83
   ffffffff81e03df8 0000000000000000 ffffffff81e03de8 ffffffff810b1ffb
   0000003d00000096 ffffffff823514d0 ffff88007ff197c8 0000000000000000
  Call Trace:
   [<ffffffff8143fc83>] dump_stack+0x85/0xc2
   [<ffffffff810b1ffb>] __warn+0xcb/0xf0
   [<ffffffff810b207f>] warn_slowpath_fmt+0x5f/0x80
   [<ffffffff811e9c0c>] static_key_slow_inc+0x8c/0xa0
   [<ffffffff810e07c6>] static_key_enable+0x16/0x40
   [<ffffffff8216d633>] setup_schedstats+0x29/0x94
   [<ffffffff82148a05>] unknown_bootoption+0x89/0x191
   [<ffffffff810d8617>] parse_args+0x297/0x4b0
   [<ffffffff82148d61>] start_kernel+0x1d8/0x4a9
   [<ffffffff8214897c>] ? set_init_arg+0x55/0x55
   [<ffffffff82148120>] ? early_idt_handler_array+0x120/0x120
   [<ffffffff821482db>] x86_64_start_reservations+0x2f/0x31
   [<ffffffff82148427>] x86_64_start_kernel+0x14a/0x16d
Quoted 3 lines

The problem is that it tries to update the 'sched_schedstats' static key before jump labels have been initialized.

Odd, I thought that particular one had been addressed. At least I saw errors like that in other areas before and they got fixed later by ftrace.

Quoted 20 lines

Changing jump_label_init() to be called earlier before parse_early_param() wouldn't fix it: it would still fail trying to poke_text() because mm isn't yet initialized.

Instead, just create a temporary '__sched_schedstats' variable which can be copied to the static key later during sched_init() after jump labels have been initialized.

Fixes cb2517653fcc ("sched/debug: Make schedstats a runtime tunable that is disabled by default")
Cc [email protected]
Signed-off-by Josh Poimboeuf <[email protected]>
---
 kernel/sched/core.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)
kernel/sched/core.c +0 -0
index 7f2cae4..385c947 100644

kernel/sched/core.c int sysctl_numa_balancing(struct ctl_table *table, int write, · line 2260

Quoted 8 lines
kernel/sched/core.c +3 -0
@@ -2253,9 +2253,11 @@ int sysctl_numa_balancing(struct ctl_table *table, int write,
 #endif
 #endif
 
+#ifdef CONFIG_SCHEDSTATS
+
 DEFINE_STATIC_KEY_FALSE(sched_schedstats);
+static bool __initdata __sched_schedstats = false;

Initialisation is unnecessary. Otherwise;

Acked-by Mel Gorman <[email protected]>
Signature · 2 lines
Mel Gorman
SUSE Labs
merge bot
#3 lore raw

[tip:sched/core] sched/debug: Fix 'schedstats=enable' cmdline option

In reply to Josh Poimboeuf
sched/debug: Fix 'schedstats=enable' cmdline option
Commit-ID:  4698f88c06b893f2acc0b443004a53bf490fde7c
Gitweb:     http://git.kernel.org/tip/4698f88c06b893f2acc0b443004a53bf490fde7c
Author:     Josh Poimboeuf <[email protected]>
AuthorDate: Tue, 7 Jun 2016 14:43:16 -0500
Committer:  Ingo Molnar <[email protected]>
CommitDate: Wed, 8 Jun 2016 14:33:05 +0200

sched/debug: Fix 'schedstats=enable' cmdline option

The 'schedstats=enable' option doesn't work, and also produces the following warning during boot:

  WARNING: CPU: 0 PID: 0 at /home/jpoimboe/git/linux/kernel/jump_label.c:61 static_key_slow_inc+0x8c/0xa0
  static_key_slow_inc used before call to jump_label_init
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper Not tainted 4.7.0-rc1+ #25
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
   0000000000000086 3ae3475a4bea95d4 ffffffff81e03da8 ffffffff8143fc83
   ffffffff81e03df8 0000000000000000 ffffffff81e03de8 ffffffff810b1ffb
   0000003d00000096 ffffffff823514d0 ffff88007ff197c8 0000000000000000
  Call Trace:
   [<ffffffff8143fc83>] dump_stack+0x85/0xc2
   [<ffffffff810b1ffb>] __warn+0xcb/0xf0
   [<ffffffff810b207f>] warn_slowpath_fmt+0x5f/0x80
   [<ffffffff811e9c0c>] static_key_slow_inc+0x8c/0xa0
   [<ffffffff810e07c6>] static_key_enable+0x16/0x40
   [<ffffffff8216d633>] setup_schedstats+0x29/0x94
   [<ffffffff82148a05>] unknown_bootoption+0x89/0x191
   [<ffffffff810d8617>] parse_args+0x297/0x4b0
   [<ffffffff82148d61>] start_kernel+0x1d8/0x4a9
   [<ffffffff8214897c>] ? set_init_arg+0x55/0x55
   [<ffffffff82148120>] ? early_idt_handler_array+0x120/0x120
   [<ffffffff821482db>] x86_64_start_reservations+0x2f/0x31
   [<ffffffff82148427>] x86_64_start_kernel+0x14a/0x16d

The problem is that it tries to update the 'sched_schedstats' static key before jump labels have been initialized.

Changing jump_label_init() to be called earlier before parse_early_param() wouldn't fix it: it would still fail trying to poke_text() because mm isn't yet initialized.

Instead, just create a temporary '__sched_schedstats' variable which can be copied to the static key later during sched_init() after jump labels have been initialized.

Signed-off-by Josh Poimboeuf <[email protected]>
Signed-off-by Peter Zijlstra (Intel) <[email protected]>
Cc Linus Torvalds <[email protected]>
Cc Matt Fleming <[email protected]>
Cc Mel Gorman <[email protected]>
Cc Peter Zijlstra <[email protected]>
Cc Srikar Dronamraju <[email protected]>
Cc Thomas Gleixner <[email protected]>
Fixes cb2517653fcc ("sched/debug: Make schedstats a runtime tunable that is disabled by default")
Link http://lkml.kernel.org/r/453775fe3433bed65731a583e228ccea806d18cd.1465322027.git.jpoimboe@redhat.com open here
Signed-off-by Ingo Molnar <[email protected]>
---
 kernel/sched/core.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)
kernel/sched/core.c +21 -5
index 7f2cae4..385c947 100644
@@ -2253,9 +2253,11 @@ int sysctl_numa_balancing(struct ctl_table *table, int write,
 #endif
 #endif
 
+#ifdef CONFIG_SCHEDSTATS
+
 DEFINE_STATIC_KEY_FALSE(sched_schedstats);
+static bool __initdata __sched_schedstats = false;
 
-#ifdef CONFIG_SCHEDSTATS
 static void set_schedstats(bool enabled)
 {
 	if (enabled)

@@ -2278,11 +2280,16 @@ static int __init setup_schedstats(char *str)
 	if (!str)
 		goto out;
 
+	/*
+	 * This code is called before jump labels have been set up, so we can't
+	 * change the static branch directly just yet.  Instead set a temporary
+	 * variable so init_schedstats() can do it later.
+	 */
 	if (!strcmp(str, "enable")) {
-		set_schedstats(true);
+		__sched_schedstats = true;
 		ret = 1;
 	} else if (!strcmp(str, "disable")) {
-		set_schedstats(false);
+		__sched_schedstats = false;
 		ret = 1;
 	}
 out:

@@ -2293,6 +2300,11 @@ out:
 }
 __setup("schedstats=", setup_schedstats);
 
+static void __init init_schedstats(void)
+{
+	set_schedstats(__sched_schedstats);
+}
+
 #ifdef CONFIG_PROC_SYSCTL
 int sysctl_schedstats(struct ctl_table *table, int write,
 			 void __user *buffer, size_t *lenp, loff_t *ppos)

@@ -2313,8 +2325,10 @@ int sysctl_schedstats(struct ctl_table *table, int write,
 		set_schedstats(state);
 	return err;
 }
-#endif
-#endif
+#endif /* CONFIG_PROC_SYSCTL */
+#else  /* !CONFIG_SCHEDSTATS */
+static inline void init_schedstats(void) {}
+#endif /* CONFIG_SCHEDSTATS */
 
 /*
  * fork()/clone()-time setup:

@@ -7487,6 +7501,8 @@ void __init sched_init(void)
 #endif
 	init_sched_fair_class();
 
+	init_schedstats();
+
 	scheduler_running = 1;
 }
author
#4 lore raw

PATCH 2/2 sched/debug: fix deadlock when enabling sched events

In reply to Josh Poimboeuf

When enabling sched trace events via:

echo 1 > /sys/kernel/debug/tracing/events/sched/enable

I see a hang, with the following BUG in the printk buffer:

  BUG: spinlock recursion on CPU#1, swapper/1/0
   lock: 0xffff88007d5d8c00, .magic: dead4ead, .owner: swapper/1/0, .owner_cpu: 1
  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.7.0-rc2+ #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
   0000000000000086 3a8e170cf69ad23f ffff88007d403898 ffffffff8143d663
   ffff88007c535480 ffff88007d5d8c00 ffff88007d4038b8 ffffffff81115948
   ffff88007d5d8c00 ffff88007d5d8c00 ffff88007d4038e8 ffffffff81115aea
  Call Trace:
   <IRQ>  [<ffffffff8143d663>] dump_stack+0x85/0xc2
   [<ffffffff81115948>] spin_dump+0x78/0xc0
   [<ffffffff81115aea>] do_raw_spin_lock+0x11a/0x150
   [<ffffffff81891471>] _raw_spin_lock+0x61/0x80
   [<ffffffff810e5466>] ? try_to_wake_up+0x256/0x4e0
   [<ffffffff810e5466>] try_to_wake_up+0x256/0x4e0
   [<ffffffff81891a0a>] ? _raw_spin_unlock_irqrestore+0x4a/0x80
   [<ffffffff810e5705>] wake_up_process+0x15/0x20
   [<ffffffff810cebb4>] insert_work+0x84/0xc0
   [<ffffffff810ced7f>] __queue_work+0x18f/0x660
   [<ffffffff810cf9a6>] queue_work_on+0x46/0x90
   [<ffffffffa00cd95b>] drm_fb_helper_dirty.isra.11+0xcb/0xe0 [drm_kms_helper]
   [<ffffffffa00cdac0>] drm_fb_helper_sys_imageblit+0x30/0x40 [drm_kms_helper]
   [<ffffffff814babcd>] soft_cursor+0x1ad/0x230
   [<ffffffff814ba379>] bit_cursor+0x649/0x680
   [<ffffffff814b9d30>] ? update_attr.isra.2+0x90/0x90
   [<ffffffff814b5e6a>] fbcon_cursor+0x14a/0x1c0
   [<ffffffff81555ef8>] hide_cursor+0x28/0x90
   [<ffffffff81558b6f>] vt_console_print+0x3bf/0x3f0
   [<ffffffff81122c63>] call_console_drivers.constprop.24+0x183/0x200
   [<ffffffff811241f4>] console_unlock+0x3d4/0x610
   [<ffffffff811247f5>] vprintk_emit+0x3c5/0x610
   [<ffffffff81124bc9>] vprintk_default+0x29/0x40
   [<ffffffff811e965b>] printk+0x57/0x73
   [<ffffffff810f7a9e>] enqueue_entity+0xc2e/0xc70
   [<ffffffff810f7b39>] enqueue_task_fair+0x59/0xab0
   [<ffffffff8106dcd9>] ? kvm_sched_clock_read+0x9/0x20
   [<ffffffff8103fb39>] ? sched_clock+0x9/0x10
   [<ffffffff810e3fcc>] activate_task+0x5c/0xa0
   [<ffffffff810e4514>] ttwu_do_activate+0x54/0xb0
   [<ffffffff810e5cea>] sched_ttwu_pending+0x7a/0xb0
   [<ffffffff810e5e51>] scheduler_ipi+0x61/0x170
   [<ffffffff81059e7f>] smp_trace_reschedule_interrupt+0x4f/0x2a0
   [<ffffffff81893ba6>] trace_reschedule_interrupt+0x96/0xa0
   <EOI>  [<ffffffff8106e0d6>] ? native_safe_halt+0x6/0x10
   [<ffffffff8110fb1d>] ? trace_hardirqs_on+0xd/0x10
   [<ffffffff81040ac0>] default_idle+0x20/0x1a0
   [<ffffffff8104147f>] arch_cpu_idle+0xf/0x20
   [<ffffffff81102f8f>] default_idle_call+0x2f/0x50
   [<ffffffff8110332e>] cpu_startup_entry+0x37e/0x450
   [<ffffffff8105af70>] start_secondary+0x160/0x1a0

The bug is caused by a deadlock where the task is trying to grab the rq lock twice because printk()'s aren't safe in sched code.

The offending printk() is the pr_warn_once() in check_schedstat_required() which warns the user that their 'trace_sched_stat_*' tracepoints aren't going to work because schedstats is disabled:

	/* Force schedstat enabled if a dependent tracepoint is active */
	if (trace_sched_stat_wait_enabled()    ||
			trace_sched_stat_sleep_enabled()   ||
			trace_sched_stat_iowait_enabled()  ||
			trace_sched_stat_blocked_enabled() ||
			trace_sched_stat_runtime_enabled())  {
		pr_warn_once("Scheduler tracepoints stat_sleep, stat_iowait, "
			     "stat_blocked and stat_runtime require the "
			     "kernel parameter schedstats=enabled or "
			     "kernel.sched_schedstats=1\n");
	}

In addition to the deadlock, I think this code has other issues:

1. Instead of just warning and allowing the tracepoints to be broken, I'd argue that it would be better to make them work by forcing schedstats enabled and printing a warning about that, which is what's already being done in other similar cases (latencytop and profiling). Otherwise things like "perf sched record" won't have the intended effect. In fact the comment in the above code snippet seems to agree with me, so maybe that was the original intent.

2. It's called in the scheduler hot path from enqueue_entity().

So change the warning to a call to force_schedstat_enabled(), and do it only when the tracepoint gets enabled.

Fixes cb2517653fcc ("sched/debug: Make schedstats a runtime tunable that is disabled by default")
Cc [email protected]
Signed-off-by Josh Poimboeuf <[email protected]>
---
 include/trace/events/sched.h | 30 +++++++++++++++---------------
 kernel/sched/fair.c          | 29 +++++++----------------------
 2 files changed, 22 insertions(+), 37 deletions(-)
include/trace/events/sched.h +15 -15
index 9b90c57..ce12e93 100644
@@ -341,37 +341,36 @@ DECLARE_EVENT_CLASS(sched_stat_template,
 			(unsigned long long)__entry->delay)
 );
 
+extern void trace_sched_stat_register(void);
+
+#define DEFINE_SCHED_STAT_EVENT(name)					\
+	DEFINE_EVENT_FN(sched_stat_template, name,			\
+			TP_PROTO(struct task_struct *tsk, u64 delay),	\
+			TP_ARGS(tsk, delay),				\
+			trace_sched_stat_register, NULL)
 
 /*
  * Tracepoint for accounting wait time (time the task is runnable
  * but not actually running due to scheduler contention).
  */
-DEFINE_EVENT(sched_stat_template, sched_stat_wait,
-	     TP_PROTO(struct task_struct *tsk, u64 delay),
-	     TP_ARGS(tsk, delay));
+DEFINE_SCHED_STAT_EVENT(sched_stat_wait);
 
 /*
  * Tracepoint for accounting sleep time (time the task is not runnable,
  * including iowait, see below).
  */
-DEFINE_EVENT(sched_stat_template, sched_stat_sleep,
-	     TP_PROTO(struct task_struct *tsk, u64 delay),
-	     TP_ARGS(tsk, delay));
+DEFINE_SCHED_STAT_EVENT(sched_stat_sleep);
 
 /*
  * Tracepoint for accounting iowait time (time the task is not runnable
  * due to waiting on IO to complete).
  */
-DEFINE_EVENT(sched_stat_template, sched_stat_iowait,
-	     TP_PROTO(struct task_struct *tsk, u64 delay),
-	     TP_ARGS(tsk, delay));
+DEFINE_SCHED_STAT_EVENT(sched_stat_iowait);
 
 /*
  * Tracepoint for accounting blocked time (time the task is in uninterruptible).
  */
-DEFINE_EVENT(sched_stat_template, sched_stat_blocked,
-	     TP_PROTO(struct task_struct *tsk, u64 delay),
-	     TP_ARGS(tsk, delay));
+DEFINE_SCHED_STAT_EVENT(sched_stat_blocked);
 
 /*
  * Tracepoint for accounting runtime (time the task is executing

@@ -403,9 +402,10 @@ DECLARE_EVENT_CLASS(sched_stat_runtime,
 			(unsigned long long)__entry->vruntime)
 );
 
-DEFINE_EVENT(sched_stat_runtime, sched_stat_runtime,
-	     TP_PROTO(struct task_struct *tsk, u64 runtime, u64 vruntime),
-	     TP_ARGS(tsk, runtime, vruntime));
+DEFINE_EVENT_FN(sched_stat_runtime, sched_stat_runtime,
+		TP_PROTO(struct task_struct *tsk, u64 runtime, u64 vruntime),
+		TP_ARGS(tsk, runtime, vruntime),
+		trace_sched_stat_register, NULL);
 
 /*
  * Tracepoint for showing priority inheritance modifying a tasks
kernel/sched/fair.c +7 -22
index c6dd8ba..b0cbcc0 100644
@@ -789,6 +789,13 @@ static void update_curr_fair(struct rq *rq)
 	update_curr(cfs_rq_of(&rq->curr->se));
 }
 
+void trace_sched_stat_register(void)
+{
+#ifdef CONFIG_SCHEDSTATS
+	force_schedstat_enabled();
+#endif
+}
+
 #ifdef CONFIG_SCHEDSTATS
 static inline void
 update_stats_wait_start(struct cfs_rq *cfs_rq, struct sched_entity *se)

@@ -3208,27 +3215,6 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 
 static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
 
-static inline void check_schedstat_required(void)
-{
-#ifdef CONFIG_SCHEDSTATS
-	if (schedstat_enabled())
-		return;
-
-	/* Force schedstat enabled if a dependent tracepoint is active */
-	if (trace_sched_stat_wait_enabled()    ||
-			trace_sched_stat_sleep_enabled()   ||
-			trace_sched_stat_iowait_enabled()  ||
-			trace_sched_stat_blocked_enabled() ||
-			trace_sched_stat_runtime_enabled())  {
-		pr_warn_once("Scheduler tracepoints stat_sleep, stat_iowait, "
-			     "stat_blocked and stat_runtime require the "
-			     "kernel parameter schedstats=enabled or "
-			     "kernel.sched_schedstats=1\n");
-	}
-#endif
-}
-
-
 /*
  * MIGRATION
  *

@@ -3293,7 +3279,6 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 			enqueue_sleeper(cfs_rq, se);
 	}
 
-	check_schedstat_required();
 	if (schedstat_enabled()) {
 		update_stats_enqueue(cfs_rq, se);
 		check_spread(cfs_rq, se);
Signature · 1 line
2.4.11
maintainer
#5 lore raw

Re: [PATCH 2/2] sched/debug: fix deadlock when enabling sched events

Quoted 3 lines · Josh Poimboeuf

1. Instead of just warning and allowing the tracepoints to be broken, I'd argue that it would be better to make them work by forcing schedstats enabled and printing a warning about that, which is what's

Forcing them enabled doesn't make them useful per se; some of these tracepoint rely on previously recoded state, which now wasn't previously recorded because back then it was disabled.

But yes, maybe this is better...

author
#6 lore raw

Re: [PATCH 2/2] sched/debug: fix deadlock when enabling sched events

Quoted 8 lines · Peter Zijlstra
Quoted 3 lines · Josh Poimboeuf

1. Instead of just warning and allowing the tracepoints to be broken, I'd argue that it would be better to make them work by forcing schedstats enabled and printing a warning about that, which is what's

Forcing them enabled doesn't make them useful per se; some of these tracepoint rely on previously recoded state, which now wasn't previously recorded because back then it was disabled.

Ah, that's true. Though this issue exists regardless of whether we force enable schedstats or the user manually enables them. It's a side effect of allowing schedstats to be enabled after booting.

Signature · 1 line
Josh
#7 lore raw

Re: [PATCH 2/2] sched/debug: fix deadlock when enabling sched events

In reply to Peter Zijlstra
Quoted 9 lines · Peter Zijlstra
Quoted 3 lines · Josh Poimboeuf

1. Instead of just warning and allowing the tracepoints to be broken, I'd argue that it would be better to make them work by forcing schedstats enabled and printing a warning about that, which is what's

Forcing them enabled doesn't make them useful per se; some of these tracepoint rely on previously recoded state, which now wasn't previously recorded because back then it was disabled.

This is unfortunately true. It means there is a window when the data is unreliable after the stats are enabled. It is assumed that either someone enabling the stats knows that or is willing to enable them on the command line.

Signature · 2 lines
Mel Gorman
SUSE Labs
author
#8 lore raw

Re: [PATCH 2/2] sched/debug: fix deadlock when enabling sched events

In reply to Josh Poimboeuf
Quoted 4 lines · Josh Poimboeuf
include/trace/events/sched.h +0 -0
@@ -403,9 +402,10 @@ DECLARE_EVENT_CLASS(sched_stat_runtime,
 			(unsigned long long)__entry->vruntime)
 );

include/trace/events/sched.h DECLARE_EVENT_CLASS(sched_stat_runtime, · line 411

Quoted 10 lines
include/trace/events/sched.h +4 -3
-DEFINE_EVENT(sched_stat_runtime, sched_stat_runtime,
-	     TP_PROTO(struct task_struct *tsk, u64 runtime, u64 vruntime),
-	     TP_ARGS(tsk, runtime, vruntime));
+DEFINE_EVENT_FN(sched_stat_runtime, sched_stat_runtime,
+		TP_PROTO(struct task_struct *tsk, u64 runtime, u64 vruntime),
+		TP_ARGS(tsk, runtime, vruntime),
+		trace_sched_stat_register, NULL);
 
 /*
  * Tracepoint for showing priority inheritance modifying a tasks

Looking closer at the tracepoints, this one doesn't actually seem to have a dependency on schedstats as far as I can tell. So this hunk can probably be removed.

Signature · 1 line
Josh
#9 lore raw

Re: [PATCH 2/2] sched/debug: fix deadlock when enabling sched events

In reply to Josh Poimboeuf

kernel/sched/fair.c static void update_curr_fair(struct rq *rq) · line 797

Quoted 12 lines · Josh Poimboeuf
kernel/sched/fair.c +6 -0
@@ -789,6 +789,13 @@ static void update_curr_fair(struct rq *rq)
 	update_curr(cfs_rq_of(&rq->curr->se));
 }
 
+void trace_sched_stat_register(void)
+{
+#ifdef CONFIG_SCHEDSTATS
+	force_schedstat_enabled();
+#endif
+}

I think it would be cleaner to provide an empty force_schedstat_enabled() definition in sched.h, on !CONFIG_SCHEDSTATS.

But it might make sense to further decouple schedstats from tracing?

Thanks,

Ingo

author
#10 lore raw

Re: [PATCH 2/2] sched/debug: fix deadlock when enabling sched events

Quoted 9 lines · Ingo Molnar
Quoted 6 lines · Josh Poimboeuf
kernel/sched/fair.c +0 -0
@@ -789,6 +789,13 @@ static void update_curr_fair(struct rq *rq)
 	update_curr(cfs_rq_of(&rq->curr->se));
 }
Quoted 9 lines

kernel/sched/fair.c static void update_curr_fair(struct rq *rq) · line 797

Quoted 6 lines
kernel/sched/fair.c +6 -0
+void trace_sched_stat_register(void)
+{
+#ifdef CONFIG_SCHEDSTATS
+	force_schedstat_enabled();
+#endif
+}

I think it would be cleaner to provide an empty force_schedstat_enabled() definition in sched.h, on !CONFIG_SCHEDSTATS.

Yes, agreed.

Quoted 1 line

But it might make sense to further decouple schedstats from tracing?

Looking at how we could do that:

- The sched_stat_wait tracepoint is dependent on the wait_start schedstat.

- The sched_stat_sleep tracepoint is dependent on the sleep_start schedstat.

- The sched_stat_iowait and sched_stat_blocked tracepoints are dependent on the block_start schedstat.

We could move those three schedstats values out of sched_statistics and into sched_entity, and then always update them regardless of CONFIG_SCHEDSTATS.

That would ensure these tracepoints always work. But then again it would add a little bit of runtime overhead.

I don't have a strong opinion either way. Thoughts?

Signature · 1 line
Josh
#11 lore raw

Re: [PATCH 2/2] sched/debug: fix deadlock when enabling sched events

Quoted 20 lines · Josh Poimboeuf
Quoted 9 lines · Ingo Molnar
Quoted 6 lines · Josh Poimboeuf
kernel/sched/fair.c +0 -0
@@ -789,6 +789,13 @@ static void update_curr_fair(struct rq *rq)
 	update_curr(cfs_rq_of(&rq->curr->se));
 }
Quoted 9 lines

kernel/sched/fair.c static void update_curr_fair(struct rq *rq) · line 797

Quoted 6 lines
kernel/sched/fair.c +6 -0
+void trace_sched_stat_register(void)
+{
+#ifdef CONFIG_SCHEDSTATS
+	force_schedstat_enabled();
+#endif
+}

I think it would be cleaner to provide an empty force_schedstat_enabled() definition in sched.h, on !CONFIG_SCHEDSTATS.

Quoted 7 lines

Yes, agreed.

Quoted 1 line

But it might make sense to further decouple schedstats from tracing?

Looking at how we could do that:

- The sched_stat_wait tracepoint is dependent on the wait_start schedstat.

Can we maintain wait_start unconditionally? Having config dependent tracepoints sucks.

Quoted 2 lines

- The sched_stat_sleep tracepoint is dependent on the sleep_start schedstat.

Ditto.

Quoted 2 lines

- The sched_stat_iowait and sched_stat_blocked tracepoints are dependent on the block_start schedstat.

Ditto.

Quoted 8 lines

We could move those three schedstats values out of sched_statistics and into sched_entity, and then always update them regardless of CONFIG_SCHEDSTATS.

That would ensure these tracepoints always work. But then again it would add a little bit of runtime overhead.

I don't have a strong opinion either way. Thoughts?

Please do a before/after 'size vmlinux' dump of a defconfig build with tracing disabled. I.e. how much code overhead does the updating of those variables add?

Thanks,

Ingo

author
#12 lore raw

PATCH v2 1/1 sched/debug: fix deadlock when enabling sched events

Quoted 48 lines · Ingo Molnar
Quoted 20 lines · Josh Poimboeuf
Quoted 9 lines · Ingo Molnar
Quoted 6 lines · Josh Poimboeuf
kernel/sched/fair.c +0 -0
@@ -789,6 +789,13 @@ static void update_curr_fair(struct rq *rq)
 	update_curr(cfs_rq_of(&rq->curr->se));
 }
Quoted 9 lines

kernel/sched/fair.c static void update_curr_fair(struct rq *rq) · line 797

Quoted 6 lines
kernel/sched/fair.c +6 -0
+void trace_sched_stat_register(void)
+{
+#ifdef CONFIG_SCHEDSTATS
+	force_schedstat_enabled();
+#endif
+}

I think it would be cleaner to provide an empty force_schedstat_enabled() definition in sched.h, on !CONFIG_SCHEDSTATS.

Quoted 7 lines

Yes, agreed.

Quoted 1 line

But it might make sense to further decouple schedstats from tracing?

Looking at how we could do that:

- The sched_stat_wait tracepoint is dependent on the wait_start schedstat.

Can we maintain wait_start unconditionally? Having config dependent tracepoints sucks.

Quoted 2 lines

- The sched_stat_sleep tracepoint is dependent on the sleep_start schedstat.

Ditto.

Quoted 2 lines

- The sched_stat_iowait and sched_stat_blocked tracepoints are dependent on the block_start schedstat.

Ditto.

Quoted 4 lines

We could move those three schedstats values out of sched_statistics and into sched_entity, and then always update them regardless of CONFIG_SCHEDSTATS.

Quoted 7 lines
Quoted 4 lines

That would ensure these tracepoints always work. But then again it would add a little bit of runtime overhead.

I don't have a strong opinion either way. Thoughts?

Please do a before/after 'size vmlinux' dump of a defconfig build with tracing disabled. I.e. how much code overhead does the updating of those variables add?

When trying to decouple the sched_stat_* tracepoints from schedstats, I see a significant (~12%) performance degradation in pipetest for the CONFIG_SCHEDSTATS case. Still looking into it...

In the meantime, here's a straightforward fix for the deadlock.

---

From: Josh Poimboeuf <[email protected]>
Subject: [PATCH v2] sched/debug: fix deadlock when enabling sched events

I see a hang when enabling sched events:

echo 1 > /sys/kernel/debug/tracing/events/sched/enable

The printk buffer shows:

  BUG: spinlock recursion on CPU#1, swapper/1/0
   lock: 0xffff88007d5d8c00, .magic: dead4ead, .owner: swapper/1/0, .owner_cpu: 1
  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.7.0-rc2+ #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
   0000000000000086 3a8e170cf69ad23f ffff88007d403898 ffffffff8143d663
   ffff88007c535480 ffff88007d5d8c00 ffff88007d4038b8 ffffffff81115948
   ffff88007d5d8c00 ffff88007d5d8c00 ffff88007d4038e8 ffffffff81115aea
  Call Trace:
   <IRQ>  [<ffffffff8143d663>] dump_stack+0x85/0xc2
   [<ffffffff81115948>] spin_dump+0x78/0xc0
   [<ffffffff81115aea>] do_raw_spin_lock+0x11a/0x150
   [<ffffffff81891471>] _raw_spin_lock+0x61/0x80
   [<ffffffff810e5466>] ? try_to_wake_up+0x256/0x4e0
   [<ffffffff810e5466>] try_to_wake_up+0x256/0x4e0
   [<ffffffff81891a0a>] ? _raw_spin_unlock_irqrestore+0x4a/0x80
   [<ffffffff810e5705>] wake_up_process+0x15/0x20
   [<ffffffff810cebb4>] insert_work+0x84/0xc0
   [<ffffffff810ced7f>] __queue_work+0x18f/0x660
   [<ffffffff810cf9a6>] queue_work_on+0x46/0x90
   [<ffffffffa00cd95b>] drm_fb_helper_dirty.isra.11+0xcb/0xe0 [drm_kms_helper]
   [<ffffffffa00cdac0>] drm_fb_helper_sys_imageblit+0x30/0x40 [drm_kms_helper]
   [<ffffffff814babcd>] soft_cursor+0x1ad/0x230
   [<ffffffff814ba379>] bit_cursor+0x649/0x680
   [<ffffffff814b9d30>] ? update_attr.isra.2+0x90/0x90
   [<ffffffff814b5e6a>] fbcon_cursor+0x14a/0x1c0
   [<ffffffff81555ef8>] hide_cursor+0x28/0x90
   [<ffffffff81558b6f>] vt_console_print+0x3bf/0x3f0
   [<ffffffff81122c63>] call_console_drivers.constprop.24+0x183/0x200
   [<ffffffff811241f4>] console_unlock+0x3d4/0x610
   [<ffffffff811247f5>] vprintk_emit+0x3c5/0x610
   [<ffffffff81124bc9>] vprintk_default+0x29/0x40
   [<ffffffff811e965b>] printk+0x57/0x73
   [<ffffffff810f7a9e>] enqueue_entity+0xc2e/0xc70
   [<ffffffff810f7b39>] enqueue_task_fair+0x59/0xab0
   [<ffffffff8106dcd9>] ? kvm_sched_clock_read+0x9/0x20
   [<ffffffff8103fb39>] ? sched_clock+0x9/0x10
   [<ffffffff810e3fcc>] activate_task+0x5c/0xa0
   [<ffffffff810e4514>] ttwu_do_activate+0x54/0xb0
   [<ffffffff810e5cea>] sched_ttwu_pending+0x7a/0xb0
   [<ffffffff810e5e51>] scheduler_ipi+0x61/0x170
   [<ffffffff81059e7f>] smp_trace_reschedule_interrupt+0x4f/0x2a0
   [<ffffffff81893ba6>] trace_reschedule_interrupt+0x96/0xa0
   <EOI>  [<ffffffff8106e0d6>] ? native_safe_halt+0x6/0x10
   [<ffffffff8110fb1d>] ? trace_hardirqs_on+0xd/0x10
   [<ffffffff81040ac0>] default_idle+0x20/0x1a0
   [<ffffffff8104147f>] arch_cpu_idle+0xf/0x20
   [<ffffffff81102f8f>] default_idle_call+0x2f/0x50
   [<ffffffff8110332e>] cpu_startup_entry+0x37e/0x450
   [<ffffffff8105af70>] start_secondary+0x160/0x1a0

Note the hang only occurs when echoing the above from a physical serial console, not from an ssh session.

The bug is caused by a deadlock where the task is trying to grab the rq lock twice because printk()'s aren't safe in sched code.

Fixes cb2517653fcc ("sched/debug: Make schedstats a runtime tunable that is disabled by default")
Cc [email protected]
Signed-off-by Josh Poimboeuf <[email protected]>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
kernel/sched/fair.c +1 -1
index c6dd8ba..52d9b37 100644
@@ -3220,7 +3220,7 @@ static inline void check_schedstat_required(void)
 			trace_sched_stat_iowait_enabled()  ||
 			trace_sched_stat_blocked_enabled() ||
 			trace_sched_stat_runtime_enabled())  {
-		pr_warn_once("Scheduler tracepoints stat_sleep, stat_iowait, "
+		printk_deferred_once("Scheduler tracepoints stat_sleep, stat_iowait, "
 			     "stat_blocked and stat_runtime require the "
 			     "kernel parameter schedstats=enabled or "
 			     "kernel.sched_schedstats=1\n");
Signature · 1 line
2.4.11
merge bot
#13 lore raw

[tip:sched/urgent] sched/debug: Fix deadlock when enabling sched events

sched/debug: Fix deadlock when enabling sched events
Commit-ID:  eda8dca519269c92a0771668b3d5678792de7b78
Gitweb:     http://git.kernel.org/tip/eda8dca519269c92a0771668b3d5678792de7b78
Author:     Josh Poimboeuf <[email protected]>
AuthorDate: Mon, 13 Jun 2016 02:32:09 -0500
Committer:  Ingo Molnar <[email protected]>
CommitDate: Tue, 14 Jun 2016 12:47:21 +0200

sched/debug: Fix deadlock when enabling sched events

I see a hang when enabling sched events:

echo 1 > /sys/kernel/debug/tracing/events/sched/enable

The printk buffer shows:

  BUG: spinlock recursion on CPU#1, swapper/1/0
   lock: 0xffff88007d5d8c00, .magic: dead4ead, .owner: swapper/1/0, .owner_cpu: 1
  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.7.0-rc2+ #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
  ...
  Call Trace:
   <IRQ>  [<ffffffff8143d663>] dump_stack+0x85/0xc2
   [<ffffffff81115948>] spin_dump+0x78/0xc0
   [<ffffffff81115aea>] do_raw_spin_lock+0x11a/0x150
   [<ffffffff81891471>] _raw_spin_lock+0x61/0x80
   [<ffffffff810e5466>] ? try_to_wake_up+0x256/0x4e0
   [<ffffffff810e5466>] try_to_wake_up+0x256/0x4e0
   [<ffffffff81891a0a>] ? _raw_spin_unlock_irqrestore+0x4a/0x80
   [<ffffffff810e5705>] wake_up_process+0x15/0x20
   [<ffffffff810cebb4>] insert_work+0x84/0xc0
   [<ffffffff810ced7f>] __queue_work+0x18f/0x660
   [<ffffffff810cf9a6>] queue_work_on+0x46/0x90
   [<ffffffffa00cd95b>] drm_fb_helper_dirty.isra.11+0xcb/0xe0 [drm_kms_helper]
   [<ffffffffa00cdac0>] drm_fb_helper_sys_imageblit+0x30/0x40 [drm_kms_helper]
   [<ffffffff814babcd>] soft_cursor+0x1ad/0x230
   [<ffffffff814ba379>] bit_cursor+0x649/0x680
   [<ffffffff814b9d30>] ? update_attr.isra.2+0x90/0x90
   [<ffffffff814b5e6a>] fbcon_cursor+0x14a/0x1c0
   [<ffffffff81555ef8>] hide_cursor+0x28/0x90
   [<ffffffff81558b6f>] vt_console_print+0x3bf/0x3f0
   [<ffffffff81122c63>] call_console_drivers.constprop.24+0x183/0x200
   [<ffffffff811241f4>] console_unlock+0x3d4/0x610
   [<ffffffff811247f5>] vprintk_emit+0x3c5/0x610
   [<ffffffff81124bc9>] vprintk_default+0x29/0x40
   [<ffffffff811e965b>] printk+0x57/0x73
   [<ffffffff810f7a9e>] enqueue_entity+0xc2e/0xc70
   [<ffffffff810f7b39>] enqueue_task_fair+0x59/0xab0
   [<ffffffff8106dcd9>] ? kvm_sched_clock_read+0x9/0x20
   [<ffffffff8103fb39>] ? sched_clock+0x9/0x10
   [<ffffffff810e3fcc>] activate_task+0x5c/0xa0
   [<ffffffff810e4514>] ttwu_do_activate+0x54/0xb0
   [<ffffffff810e5cea>] sched_ttwu_pending+0x7a/0xb0
   [<ffffffff810e5e51>] scheduler_ipi+0x61/0x170
   [<ffffffff81059e7f>] smp_trace_reschedule_interrupt+0x4f/0x2a0
   [<ffffffff81893ba6>] trace_reschedule_interrupt+0x96/0xa0
   <EOI>  [<ffffffff8106e0d6>] ? native_safe_halt+0x6/0x10
   [<ffffffff8110fb1d>] ? trace_hardirqs_on+0xd/0x10
   [<ffffffff81040ac0>] default_idle+0x20/0x1a0
   [<ffffffff8104147f>] arch_cpu_idle+0xf/0x20
   [<ffffffff81102f8f>] default_idle_call+0x2f/0x50
   [<ffffffff8110332e>] cpu_startup_entry+0x37e/0x450
   [<ffffffff8105af70>] start_secondary+0x160/0x1a0

Note the hang only occurs when echoing the above from a physical serial console, not from an ssh session.

The bug is caused by a deadlock where the task is trying to grab the rq lock twice because printk()'s aren't safe in sched code.

Signed-off-by Josh Poimboeuf <[email protected]>
Cc Linus Torvalds <[email protected]>
Cc Matt Fleming <[email protected]>
Cc Mel Gorman <[email protected]>
Cc Mike Galbraith <[email protected]>
Cc Peter Zijlstra <[email protected]>
Cc Srikar Dronamraju <[email protected]>
Cc Thomas Gleixner <[email protected]>
Cc [email protected]
Fixes cb2517653fcc ("sched/debug: Make schedstats a runtime tunable that is disabled by default")
Link http://lkml.kernel.org/r/20160613073209.gdvdybiruljbkn3p@treble open here
Signed-off-by Ingo Molnar <[email protected]>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
kernel/sched/fair.c +1 -1
index 4e33ad1..a2348de 100644
@@ -3246,7 +3246,7 @@ static inline void check_schedstat_required(void)
 			trace_sched_stat_iowait_enabled()  ||
 			trace_sched_stat_blocked_enabled() ||
 			trace_sched_stat_runtime_enabled())  {
-		pr_warn_once("Scheduler tracepoints stat_sleep, stat_iowait, "
+		printk_deferred_once("Scheduler tracepoints stat_sleep, stat_iowait, "
 			     "stat_blocked and stat_runtime require the "
 			     "kernel parameter schedstats=enabled or "
 			     "kernel.sched_schedstats=1\n");
Acked-by on v1 2/2
#14 lore raw

Re: [PATCH 2/2] sched/debug: fix deadlock when enabling sched events

In reply to Josh Poimboeuf
Quoted 6 lines · Josh Poimboeuf

When enabling sched trace events via:

echo 1 > /sys/kernel/debug/tracing/events/sched/enable

I see a hang, with the following BUG in the printk buffer:

I was certain I had tested this case because I was monitoring to make sure the stats were not updating until the tuning knob was used. It was because the context was unsafe that any printing happened because at the time trying to enable stats in that context blew up.

Quoted 10 lines

In addition to the deadlock, I think this code has other issues:

1. Instead of just warning and allowing the tracepoints to be broken, I'd argue that it would be better to make them work by forcing schedstats enabled and printing a warning about that, which is what's already being done in other similar cases (latencytop and profiling). Otherwise things like "perf sched record" won't have the intended effect. In fact the comment in the above code snippet seems to agree with me, so maybe that was the original intent.

Quoted 8 lines

2. It's called in the scheduler hot path from enqueue_entity().

So change the warning to a call to force_schedstat_enabled(), and do it only when the tracepoint gets enabled.

Fixes cb2517653fcc ("sched/debug: Make schedstats a runtime tunable that is disabled by default")
Cc [email protected]
Signed-off-by Josh Poimboeuf <[email protected]>

I like it! I wasn't aware there was an option to have a registration callback.

Acked-by Mel Gorman <[email protected]>
Signature · 2 lines
Mel Gorman
SUSE Labs