Re: [tip: sched/core] sched/uclamg: Handle delayed dequeue

[Christian Loehle <christian.loehle@arm.com>]

On 8/18/24 07:23, tip-bot2 for Peter Zijlstra wrote:
> The following commit has been merged into the sched/core branch of tip:
> 
> Commit-ID:     dfa0a574cbc47bfd5f8985f74c8ea003a37fa078
> Gitweb:        https://git.kernel.org/tip/dfa0a574cbc47bfd5f8985f74c8ea003a37fa078
> Author:        Peter Zijlstra <peterz@infradead.org>
> AuthorDate:    Wed, 05 Jun 2024 12:09:11 +02:00
> Committer:     Peter Zijlstra <peterz@infradead.org>
> CommitterDate: Sat, 17 Aug 2024 11:06:42 +02:00
> 
> sched/uclamg: Handle delayed dequeue

Nit, but I haven't seen the typo until now.

> 
> Delayed dequeue has tasks sit around on the runqueue that are not
> actually runnable -- specifically, they will be dequeued the moment
> they get picked.
> 
> One side-effect is that such a task can get migrated, which leads to a
> 'nested' dequeue_task() scenario that messes up uclamp if we don't
> take care.
> 
> Notably, dequeue_task(DEQUEUE_SLEEP) can 'fail' and keep the task on
> the runqueue. This however will have removed the task from uclamp --
> per uclamp_rq_dec() in dequeue_task(). So far so good.
> 
> However, if at that point the task gets migrated -- or nice adjusted
> or any of a myriad of operations that does a dequeue-enqueue cycle --
> we'll pass through dequeue_task()/enqueue_task() again. Without
> modification this will lead to a double decrement for uclamp, which is
> wrong.
> 
> Reported-by: Luis Machado <luis.machado@arm.com>
> Reported-by: Hongyan Xia <hongyan.xia2@arm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Reviewed-by: Valentin Schneider <vschneid@redhat.com>
> Tested-by: Valentin Schneider <vschneid@redhat.com>
> Link: https://lkml.kernel.org/r/20240727105029.315205425@infradead.org
> ---
>  kernel/sched/core.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7356464..80e639e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1691,6 +1691,9 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
>  	if (unlikely(!p->sched_class->uclamp_enabled))
>  		return;
>  
> +	if (p->se.sched_delayed)
> +		return;
> +
>  	for_each_clamp_id(clamp_id)
>  		uclamp_rq_inc_id(rq, p, clamp_id);
>  
> @@ -1715,6 +1718,9 @@ static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
>  	if (unlikely(!p->sched_class->uclamp_enabled))
>  		return;
>  
> +	if (p->se.sched_delayed)
> +		return;
> +
>  	for_each_clamp_id(clamp_id)
>  		uclamp_rq_dec_id(rq, p, clamp_id);
>  }
> @@ -1994,8 +2000,12 @@ void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
>  		psi_enqueue(p, (flags & ENQUEUE_WAKEUP) && !(flags & ENQUEUE_MIGRATED));
>  	}
>  
> -	uclamp_rq_inc(rq, p);
>  	p->sched_class->enqueue_task(rq, p, flags);
> +	/*
> +	 * Must be after ->enqueue_task() because ENQUEUE_DELAYED can clear
> +	 * ->sched_delayed.
> +	 */
> +	uclamp_rq_inc(rq, p);
>  
>  	if (sched_core_enabled(rq))
>  		sched_core_enqueue(rq, p);
> @@ -2017,6 +2027,10 @@ inline bool dequeue_task(struct rq *rq, struct task_struct *p, int flags)
>  		psi_dequeue(p, flags & DEQUEUE_SLEEP);
>  	}
>  
> +	/*
> +	 * Must be before ->dequeue_task() because ->dequeue_task() can 'fail'
> +	 * and mark the task ->sched_delayed.
> +	 */
>  	uclamp_rq_dec(rq, p);
>  	return p->sched_class->dequeue_task(rq, p, flags);
>  }
>