block: delete part_round_stats and switch to less precise counting

We want to convert to per-cpu in_flight counters.

The function part_round_stats needs the in_flight counter every jiffy, it would be too costly to sum all the percpu variables every jiffy, so it must be deleted. part_round_stats is used to calculate two counters - time_in_queue and io_ticks.

time_in_queue can be calculated without part_round_stats, by adding the duration of the I/O when the I/O ends (the value is almost as exact as the previously calculated value, except that time for in-progress I/Os is not counted).

io_ticks can be approximated by increasing the value when I/O is started or ended and the jiffies value has changed. If the I/Os take less than a jiffy, the value is as exact as the previously calculated value. If the I/Os take more than a jiffy, io_ticks can drift behind the previously calculated value.

Signed-off-by Mikulas Patocka <[email protected]>

---
 block/bio.c               |   26 ++++++++++++++++--
 block/blk-core.c          |   64 +++-------------------------------------------
 block/blk-merge.c         |    1 
 block/genhd.c             |    4 --
 block/partition-generic.c |    4 --
 include/linux/genhd.h     |    3 --
 6 files changed, 29 insertions(+), 73 deletions(-)

Index: linux-block/block/bio.c
===================================================================

block/bio.c +23 -3

@@ -1663,13 +1663,29 @@ defer:
 }
 EXPORT_SYMBOL_GPL(bio_check_pages_dirty);
 
+void update_io_ticks(int cpu, struct hd_struct *part, unsigned long now)
+{
+	unsigned long stamp;
+again:
+	stamp = READ_ONCE(part->stamp);
+	if (unlikely(stamp != now)) {
+		if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) {
+			__part_stat_add(cpu, part, io_ticks, 1);
+		}
+	}
+	if (part->partno) {
+		part = &part_to_disk(part)->part0;
+		goto again;
+	}
+}
+
 void generic_start_io_acct(struct request_queue *q, int op,
 			   unsigned long sectors, struct hd_struct *part)
 {
 	const int sgrp = op_stat_group(op);
 	int cpu = part_stat_lock();
 
-	part_round_stats(q, cpu, part);
+	update_io_ticks(cpu, part, jiffies);
 	part_stat_inc(cpu, part, ios[sgrp]);
 	part_stat_add(cpu, part, sectors[sgrp], sectors);
 	part_inc_in_flight(q, part, op_is_write(op));

@@ -1681,12 +1697,16 @@ EXPORT_SYMBOL(generic_start_io_acct);
 void generic_end_io_acct(struct request_queue *q, int req_op,
 			 struct hd_struct *part, unsigned long start_time)
 {
-	unsigned long duration = jiffies - start_time;
+	unsigned long now = jiffies;
+	unsigned long duration = now - start_time;
 	const int sgrp = op_stat_group(req_op);
 	int cpu = part_stat_lock();
 
+	update_io_ticks(cpu, part, now);
 	part_stat_add(cpu, part, nsecs[sgrp], jiffies_to_nsecs(duration));
-	part_round_stats(q, cpu, part);
+	part_stat_add(cpu, part, time_in_queue, duration);
+	if (part->partno)
+		part_stat_add(cpu, &part_to_disk(part)->part0, time_in_queue, duration);
 	part_dec_in_flight(q, part, op_is_write(req_op));
 
 	part_stat_unlock();

Index: linux-block/block/blk-core.c
===================================================================

block/blk-core.c +0 -57

@@ -583,63 +583,6 @@ struct request *blk_get_request(struct r
 }
 EXPORT_SYMBOL(blk_get_request);
 
-static void part_round_stats_single(struct request_queue *q, int cpu,
-				    struct hd_struct *part, unsigned long now,
-				    unsigned int inflight)
-{
-	if (inflight) {
-		__part_stat_add(cpu, part, time_in_queue,
-				inflight * (now - part->stamp));
-		__part_stat_add(cpu, part, io_ticks, (now - part->stamp));
-	}
-	part->stamp = now;
-}
-
-/**
- * part_round_stats() - Round off the performance stats on a struct disk_stats.
- * @q: target block queue
- * @cpu: cpu number for stats access
- * @part: target partition
- *
- * The average IO queue length and utilisation statistics are maintained
- * by observing the current state of the queue length and the amount of
- * time it has been in this state for.
- *
- * Normally, that accounting is done on IO completion, but that can result
- * in more than a second's worth of IO being accounted for within any one
- * second, leading to >100% utilisation.  To deal with that, we call this
- * function to do a round-off before returning the results when reading
- * /proc/diskstats.  This accounts immediately for all queue usage up to
- * the current jiffies and restarts the counters again.
- */
-void part_round_stats(struct request_queue *q, int cpu, struct hd_struct *part)
-{
-	struct hd_struct *part2 = NULL;
-	unsigned long now = jiffies;
-	unsigned int inflight[2];
-	int stats = 0;
-
-	if (part->stamp != now)
-		stats |= 1;
-
-	if (part->partno) {
-		part2 = &part_to_disk(part)->part0;
-		if (part2->stamp != now)
-			stats |= 2;
-	}
-
-	if (!stats)
-		return;
-
-	part_in_flight(q, part, inflight);
-
-	if (stats & 2)
-		part_round_stats_single(q, cpu, part2, now, inflight[1]);
-	if (stats & 1)
-		part_round_stats_single(q, cpu, part, now, inflight[0]);
-}
-EXPORT_SYMBOL_GPL(part_round_stats);
-
 void blk_put_request(struct request *req)
 {
 	blk_mq_free_request(req);

block/blk-core.c +1 -0

+			part_stat_add(cpu, &part_to_disk(part)->part0, time_in_queue, nsecs_to_jiffies64(now - req->start_time_ns));
 		part_dec_in_flight(req->q, part, rq_data_dir(req));
 
 		hd_struct_put(part);

Quoted 152 lines · Mikulas Patocka

We want to convert to per-cpu in_flight counters.

The function part_round_stats needs the in_flight counter every jiffy, it would be too costly to sum all the percpu variables every jiffy, so it must be deleted. part_round_stats is used to calculate two counters - time_in_queue and io_ticks.

time_in_queue can be calculated without part_round_stats, by adding the duration of the I/O when the I/O ends (the value is almost as exact as the previously calculated value, except that time for in-progress I/Os is not counted).

io_ticks can be approximated by increasing the value when I/O is started or ended and the jiffies value has changed. If the I/Os take less than a jiffy, the value is as exact as the previously calculated value. If the I/Os take more than a jiffy, io_ticks can drift behind the previously calculated value.

Signed-off-by Mikulas Patocka <[email protected]>

---
 block/bio.c               |   26 ++++++++++++++++--
 block/blk-core.c          |   64 +++-------------------------------------------
 block/blk-merge.c         |    1 
 block/genhd.c             |    4 --
 block/partition-generic.c |    4 --
 include/linux/genhd.h     |    3 --
 6 files changed, 29 insertions(+), 73 deletions(-)

Index: linux-block/block/bio.c
===================================================================

block/bio.c +23 -3

@@ -1663,13 +1663,29 @@ defer:
 }
 EXPORT_SYMBOL_GPL(bio_check_pages_dirty);
 
+void update_io_ticks(int cpu, struct hd_struct *part, unsigned long now)
+{
+	unsigned long stamp;
+again:
+	stamp = READ_ONCE(part->stamp);
+	if (unlikely(stamp != now)) {
+		if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) {
+			__part_stat_add(cpu, part, io_ticks, 1);
+		}
+	}
+	if (part->partno) {
+		part = &part_to_disk(part)->part0;
+		goto again;
+	}
+}
+
 void generic_start_io_acct(struct request_queue *q, int op,
 			   unsigned long sectors, struct hd_struct *part)
 {
 	const int sgrp = op_stat_group(op);
 	int cpu = part_stat_lock();
 
-	part_round_stats(q, cpu, part);
+	update_io_ticks(cpu, part, jiffies);
 	part_stat_inc(cpu, part, ios[sgrp]);
 	part_stat_add(cpu, part, sectors[sgrp], sectors);
 	part_inc_in_flight(q, part, op_is_write(op));

@@ -1681,12 +1697,16 @@ EXPORT_SYMBOL(generic_start_io_acct);
 void generic_end_io_acct(struct request_queue *q, int req_op,
 			 struct hd_struct *part, unsigned long start_time)
 {
-	unsigned long duration = jiffies - start_time;
+	unsigned long now = jiffies;
+	unsigned long duration = now - start_time;
 	const int sgrp = op_stat_group(req_op);
 	int cpu = part_stat_lock();
 
+	update_io_ticks(cpu, part, now);
 	part_stat_add(cpu, part, nsecs[sgrp], jiffies_to_nsecs(duration));
-	part_round_stats(q, cpu, part);
+	part_stat_add(cpu, part, time_in_queue, duration);
+	if (part->partno)
+		part_stat_add(cpu, &part_to_disk(part)->part0, time_in_queue, duration);
 	part_dec_in_flight(q, part, op_is_write(req_op));
 
 	part_stat_unlock();

Index: linux-block/block/blk-core.c
===================================================================

block/blk-core.c +0 -57

@@ -583,63 +583,6 @@ struct request *blk_get_request(struct r
 }
 EXPORT_SYMBOL(blk_get_request);
 
-static void part_round_stats_single(struct request_queue *q, int cpu,
-				    struct hd_struct *part, unsigned long now,
-				    unsigned int inflight)
-{
-	if (inflight) {
-		__part_stat_add(cpu, part, time_in_queue,
-				inflight * (now - part->stamp));
-		__part_stat_add(cpu, part, io_ticks, (now - part->stamp));
-	}
-	part->stamp = now;
-}
-
-/**
- * part_round_stats() - Round off the performance stats on a struct disk_stats.
- * @q: target block queue
- * @cpu: cpu number for stats access
- * @part: target partition
- *
- * The average IO queue length and utilisation statistics are maintained
- * by observing the current state of the queue length and the amount of
- * time it has been in this state for.
- *
- * Normally, that accounting is done on IO completion, but that can result
- * in more than a second's worth of IO being accounted for within any one
- * second, leading to >100% utilisation.  To deal with that, we call this
- * function to do a round-off before returning the results when reading
- * /proc/diskstats.  This accounts immediately for all queue usage up to
- * the current jiffies and restarts the counters again.
- */
-void part_round_stats(struct request_queue *q, int cpu, struct hd_struct *part)
-{
-	struct hd_struct *part2 = NULL;
-	unsigned long now = jiffies;
-	unsigned int inflight[2];
-	int stats = 0;
-
-	if (part->stamp != now)
-		stats |= 1;
-
-	if (part->partno) {
-		part2 = &part_to_disk(part)->part0;
-		if (part2->stamp != now)
-			stats |= 2;
-	}
-
-	if (!stats)
-		return;
-
-	part_in_flight(q, part, inflight);
-
-	if (stats & 2)
-		part_round_stats_single(q, cpu, part2, now, inflight[1]);
-	if (stats & 1)
-		part_round_stats_single(q, cpu, part, now, inflight[0]);
-}
-EXPORT_SYMBOL_GPL(part_round_stats);
-
 void blk_put_request(struct request *req)
 {
 	blk_mq_free_request(req);

block/blk-core.c void blk_account_io_done(struct request · line 1357

Quoted 8 lines

block/blk-core.c +2 -1

@@ -1408,9 +1351,11 @@ void blk_account_io_done(struct request
 		cpu = part_stat_lock();
 		part = req->part;
 
+		update_io_ticks(cpu, part, jiffies);
 		part_stat_inc(cpu, part, ios[sgrp]);
 		part_stat_add(cpu, part, nsecs[sgrp], now - req->start_time_ns);
-		part_round_stats(req->q, cpu, part);
+		part_stat_add(cpu, part, time_in_queue, nsecs_to_jiffies64(now - req->start_time_ns));

(Extra indentation offered a clue...)
Are you missing an 'if (part->partno)' conditional here?

block/blk-core.c void blk_account_io_done(struct request · line 1361

Quoted 4 lines

block/blk-core.c +1 -0

+			part_stat_add(cpu, &part_to_disk(part)->part0, time_in_queue, nsecs_to_jiffies64(now - req->start_time_ns));
 		part_dec_in_flight(req->q, part, rq_data_dir(req));
 
 		hd_struct_put(part);