Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

[Dietmar Eggemann]

On 06/03/2023 14:24, Zhang Qiao wrote:
> Commit 829c1651e9c4 ("sched/fair: sanitize vruntime of
> entity being placed") fix an overflowing bug, but ignore
> a case that se->exec_start is reset after a migration.
> 
> For fixing this case, we reset the vruntime of a long
> sleeping task in migrate_task_rq_fair().
> 
> Fixes: 829c1651e9c4 ("sched/fair: sanitize vruntime of entity being placed")
> Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Signed-off-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx>

[...]

> @@ -7635,7 +7653,23 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>  	if (READ_ONCE(p->__state) == TASK_WAKING) {
>  		struct cfs_rq *cfs_rq = cfs_rq_of(se);
>  
> -		se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> +		/*
> +		 * We determine whether a task sleeps for long by checking
> +		 * se->exec_start, and if it is, we sanitize its vruntime at
> +		 * place_entity(). However, after a migration, this detection
> +		 * method fails due to se->exec_start being reset.
> +		 *
> +		 * For fixing this case, we add the same check here. For a task
> +		 * which has slept for a long time, its vruntime should be reset
> +		 * to cfs_rq->min_vruntime with a sleep credit. Because waking
> +		 * task's vruntime will be added to cfs_rq->min_vruntime when

Isn't this the other way around? `vruntime += min_vruntime`

> +		 * enqueue, we only need to reset the se->vruntime of waking task
> +		 * to a credit here.

You not reset it to credit, you subtract the credit from vruntime ?

I assume this is done to have sleeper credit accounted on both
(se->vruntime and vruntime) for `se->vruntime =
max_vruntime(se->vruntime, vruntime)` in place_entity() since
entity_is_long_sleep(se)=false for a remove wakeup since `se->exec_start=0`.


> +		 */
> +		if (entity_is_long_sleep(se))
> +			se->vruntime = -sched_sleeper_credit(se);
> +		else
> +			se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);

Not sure I understand this part.
Don't we have to do `vruntime -= min_vruntime` here for long sleeping
task as well?

Since we always do the `vruntime += min_vruntime` on the new CPU for a
remote wakeup.

[...]