Commit c96f5471 authored by Josh Snyder's avatar Josh Snyder Committed by Ingo Molnar
Browse files

delayacct: Account blkio completion on the correct task 



Before commit:

  e33a9bba ("sched/core: move IO scheduling accounting from io_schedule_timeout() into scheduler")

delayacct_blkio_end() was called after context-switching into the task which
completed I/O.

This resulted in double counting: the task would account a delay both waiting
for I/O and for time spent in the runqueue.

With e33a9bba, delayacct_blkio_end() is called by try_to_wake_up().
In ttwu, we have not yet context-switched. This is more correct, in that
the delay accounting ends when the I/O is complete.

But delayacct_blkio_end() relies on 'get_current()', and we have not yet
context-switched into the task whose I/O completed. This results in the
wrong task having its delay accounting statistics updated.

Instead of doing that, pass the task_struct being woken to delayacct_blkio_end(),
so that it can update the statistics of the correct task.

Signed-off-by: default avatarJosh Snyder <joshs@netflix.com>
Acked-by: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarBalbir Singh <bsingharora@gmail.com>
Cc: <stable@vger.kernel.org>
Cc: Brendan Gregg <bgregg@netflix.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-block@vger.kernel.org
Fixes: e33a9bba ("sched/core: move IO scheduling accounting from io_schedule_timeout() into scheduler")
Link: http://lkml.kernel.org/r/1513613712-571-1-git-send-email-joshs@netflix.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent a8750ddc

include/linux/delayacct.h

+4 −4
Original line number Diff line number Diff line
@@ -71,7 +71,7 @@ extern void delayacct_init(void); 
extern void __delayacct_tsk_init(struct task_struct *);

extern void __delayacct_tsk_exit(struct task_struct *);

extern void __delayacct_blkio_start(void);

extern void __delayacct_blkio_end(void);

extern void __delayacct_blkio_end(struct task_struct *);

extern int __delayacct_add_tsk(struct taskstats *, struct task_struct *);

extern __u64 __delayacct_blkio_ticks(struct task_struct *);

extern void __delayacct_freepages_start(void);
@@ -122,10 +122,10 @@ static inline void delayacct_blkio_start(void) 
		__delayacct_blkio_start();

}



static inline void delayacct_blkio_end(void)

static inline void delayacct_blkio_end(struct task_struct *p)

{

	if (current->delays)

		__delayacct_blkio_end();

		__delayacct_blkio_end(p);

	delayacct_clear_flag(DELAYACCT_PF_BLKIO);

}
@@ -169,7 +169,7 @@ static inline void delayacct_tsk_free(struct task_struct *tsk) 
{}

static inline void delayacct_blkio_start(void)

{}

static inline void delayacct_blkio_end(void)

static inline void delayacct_blkio_end(struct task_struct *p)

{}

static inline int delayacct_add_tsk(struct taskstats *d,

					struct task_struct *tsk)

kernel/delayacct.c

+26 −16
Original line number Diff line number Diff line
@@ -51,16 +51,16 @@ void __delayacct_tsk_init(struct task_struct *tsk) 
 * Finish delay accounting for a statistic using its timestamps (@start),

 * accumalator (@total) and @count

 */

static void delayacct_end(u64 *start, u64 *total, u32 *count)

static void delayacct_end(spinlock_t *lock, u64 *start, u64 *total, u32 *count)

{

	s64 ns = ktime_get_ns() - *start;

	unsigned long flags;



	if (ns > 0) {

		spin_lock_irqsave(&current->delays->lock, flags);

		spin_lock_irqsave(lock, flags);

		*total += ns;

		(*count)++;

		spin_unlock_irqrestore(&current->delays->lock, flags);

		spin_unlock_irqrestore(lock, flags);

	}

}
@@ -69,17 +69,25 @@ void __delayacct_blkio_start(void) 
	current->delays->blkio_start = ktime_get_ns();

}



void __delayacct_blkio_end(void)

/*

 * We cannot rely on the `current` macro, as we haven't yet switched back to

 * the process being woken.

 */

void __delayacct_blkio_end(struct task_struct *p)

{

	if (current->delays->flags & DELAYACCT_PF_SWAPIN)

		/* Swapin block I/O */

		delayacct_end(&current->delays->blkio_start,

			&current->delays->swapin_delay,

			&current->delays->swapin_count);

	else	/* Other block I/O */

		delayacct_end(&current->delays->blkio_start,

			&current->delays->blkio_delay,

			&current->delays->blkio_count);

	struct task_delay_info *delays = p->delays;

	u64 *total;

	u32 *count;



	if (p->delays->flags & DELAYACCT_PF_SWAPIN) {

		total = &delays->swapin_delay;

		count = &delays->swapin_count;

	} else {

		total = &delays->blkio_delay;

		count = &delays->blkio_count;

	}



	delayacct_end(&delays->lock, &delays->blkio_start, total, count);

}



int __delayacct_add_tsk(struct taskstats *d, struct task_struct *tsk)
@@ -153,7 +161,9 @@ void __delayacct_freepages_start(void) 


void __delayacct_freepages_end(void)

{

	delayacct_end(&current->delays->freepages_start,

	delayacct_end(

		&current->delays->lock,

		&current->delays->freepages_start,

		&current->delays->freepages_delay,

		&current->delays->freepages_count);

}

kernel/sched/core.c

+3 −3
Original line number Diff line number Diff line
@@ -2056,7 +2056,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) 
	p->state = TASK_WAKING;



	if (p->in_iowait) {

		delayacct_blkio_end();

		delayacct_blkio_end(p);

		atomic_dec(&task_rq(p)->nr_iowait);

	}
@@ -2069,7 +2069,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) 
#else /* CONFIG_SMP */



	if (p->in_iowait) {

		delayacct_blkio_end();

		delayacct_blkio_end(p);

		atomic_dec(&task_rq(p)->nr_iowait);

	}
@@ -2122,7 +2122,7 @@ static void try_to_wake_up_local(struct task_struct *p, struct rq_flags *rf) 


	if (!task_on_rq_queued(p)) {

		if (p->in_iowait) {

			delayacct_blkio_end();

			delayacct_blkio_end(p);

			atomic_dec(&rq->nr_iowait);

		}

		ttwu_activate(rq, p, ENQUEUE_WAKEUP | ENQUEUE_NOCLOCK);

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