Re: [PATCH] sched: Account blkio delay independently of delayed dequeue

[Kayra Cizmeci]

Hi Christian,

> Starting blkio delay accounting in __block_task() ties the interval to
> removal from the runqueue. With EEVDF delayed dequeue, this misses the
> initial part of an I/O sleep, or the entire sleep if the task wakes
> before it is dequeued. Final dequeue can also run in another task's
> context, causing delayacct_blkio_start() to timestamp the wrong task.

> Start accounting in try_to_block_task(), when the current task commits
> to blocking, before delayed dequeue or proxy execution can retain it on
> the runqueue. End accounting when clearing is_blocked, including queued
> wakeups and the proxy execution paths that let the current task continue.

> Reported-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> Closes: https://lore.kernel.org/r/20261014081731.1068794-1-zhanxusheng@xxxxxxxxxx
> Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
> Signed-off-by: Christian Loehle <christian.loehle@xxxxxxx>

Aha. Again one of these Proxy Exec. &  Delayed Dequeue dou causing issue type of issue.

> @@ -636,7 +636,7 @@ int task_llc(const struct task_struct *p)
>   *
>   * p->is_blocked <- { 0, 1 }:
>   *
> - *   is set by try_to_block_task() and cleared by ttwu_do_wakeup() and tracks
> + *   is set by try_to_block_task() and cleared by unblock_task() and tracks
>   *   if the task is blocked. Traditionally this would mirror p->on_rq, however
>   *   due things like DELAY_DEQUEUE and PROXY_EXEC, this can diverge.
>   *

Nice...

> @@ -3737,12 +3737,20 @@ ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
>  		__schedstat_inc(p->stats.nr_wakeups_sync);
>  }
>  
> +static inline void unblock_task(struct task_struct *p)
> +{
> +	if (p->is_blocked && p->in_iowait)
> +		delayacct_blkio_end(p);
> +
> +	p->is_blocked = 0;
> +}
> +

Hm. So if I'm understanding this correctly were trying to track the changes on is_blocked here, correct?
try_to_block_task() finally found his lost twin!

> static inline void ttwu_do_wakeup(struct task_struct *p)
>  {
> -	p->is_blocked = 0;
> +	unblock_task(p);
>  	WRITE_ONCE(p->__state, TASK_RUNNING);
>  	trace_sched_wakeup(p);
>  }
> @@ -3839,10 +3847,8 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
>  		en_flags |= ENQUEUE_RQ_SELECTED;
>  	if (wake_flags & WF_MIGRATED)
>  		en_flags |= ENQUEUE_MIGRATED;
> -	else if (p->in_iowait) {
> -		delayacct_blkio_end(p);
> +	else if (p->in_iowait)
>  		atomic_dec(&task_rq(p)->nr_iowait);
> -	}
>  
>  	activate_task(rq, p, en_flags);
>  	wakeup_preempt(rq, p, wake_flags);
> @@ -4415,10 +4421,8 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  
>  		cpu = select_task_rq(p, p->wake_cpu, &wake_flags);
>  		if (task_cpu(p) != cpu) {
> -			if (p->in_iowait) {
> -				delayacct_blkio_end(p);
> +			if (p->in_iowait)
>  				atomic_dec(&task_rq(p)->nr_iowait);
> -			}
>  
>  			wake_flags |= WF_MIGRATED;
>  			psi_ttwu_dequeue(p);
> @@ -6762,6 +6766,9 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
>  
>  	p->is_blocked = 1;
>  
> +	if (p->in_iowait)
> +		delayacct_blkio_start();
> +
>  	/*
>  	 * We check should_block after signal_pending because we
>  	 * will want to wake the task in that case. But if
> @@ -6934,7 +6941,7 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  		if (!mutex) {
>  			clear_task_blocked_on(p, mutex);
>  			if (task_current(rq, p)) {
> -				p->is_blocked = 0;
> +				unblock_task(p);
>  				return p;
>  			}
>  			goto deactivate;
> @@ -6970,7 +6977,7 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  			 */
>  			__clear_task_blocked_on(p, NULL);
>  			if (task_current(rq, p)) {
> -				p->is_blocked = 0;
> +				unblock_task(p);
>  				return p;
>  			}
>  			goto deactivate;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 4c25fbe84fb5..638c7709654a 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3072,10 +3072,8 @@ static inline void __block_task(struct rq *rq, struct task_struct *p)
>  	if (p->sched_contributes_to_load)
>  		rq->nr_uninterruptible++;
>  
> -	if (p->in_iowait) {
> +	if (p->in_iowait)
>  		atomic_inc(&rq->nr_iowait);
> -		delayacct_blkio_start();
> -	}
>  
>  	ASSERT_EXCLUSIVE_WRITER(p->on_rq);
>  

Hmmmm.... HMMMMMMMM....

I mean, looks clean to me. The is_blocked status is followed correctly.
The only place its made 0 is only unblock_task(). And blocking is only on try_to_block_task().

Hope I'm not missing something...

Reviewed-by: Kayra Cizmeci <kayracizmeci@xxxxxxxxx>
















I have no work todo tomorrow, so I can review as many patches as I want.
But I guess no one cares, so I put this down here. As a joke too,
I need to do more jokes. To entertain our sched people.

Thanks,
Kayra