linux-kernel - Re: [RFC PATCH 6/7] sched/fair: fix tasks

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Message-ID: <28267ba4-94a2-4a77-9c34-7273a102c183@amd.com>
Date: Fri, 14 Mar 2025 09:44:45 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Aaron Lu <ziqianlu@...edance.com>, Valentin Schneider
	<vschneid@...hat.com>, Ben Segall <bsegall@...gle.com>, Peter Zijlstra
	<peterz@...radead.org>, Josh Don <joshdon@...gle.com>, Ingo Molnar
	<mingo@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>
CC: <linux-kernel@...r.kernel.org>, Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
	<rostedt@...dmis.org>, Mel Gorman <mgorman@...e.de>, Chengming Zhou
	<chengming.zhou@...ux.dev>, Chuyi Zhou <zhouchuyi@...edance.com>
Subject: Re: [RFC PATCH 6/7] sched/fair: fix tasks_rcu with task based
 throttle

Hello Aaron,

On 3/13/2025 12:52 PM, Aaron Lu wrote:
> Taskes throttled on exit to user path are scheduled by cond_resched() in
> task_work_run() but that is a preempt schedule and doesn't mark a task
> rcu quiescent state.
> 
> Fix this by directly calling schedule() in throttle_cfs_rq_work().

Perhaps that can be gotten around by just using set_ti_thread_flag()
resched_curr() will also call set_preempt_need_resched() which allows
cond_resched() to resched the task.

Since exit_to_user_mode_loop() will run once again seeing that
TIF_NEED_RESCHED is set, schedule() should follow soon. Thoughts?

> 
> Signed-off-by: Aaron Lu <ziqianlu@...edance.com>
> ---
>   kernel/sched/fair.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f26d53ac143fe..be96f7d32998c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5847,6 +5847,7 @@ static void throttle_cfs_rq_work(struct
> callback_head *work)
>   	struct cfs_rq *cfs_rq;
>   	struct rq *rq;
>   	struct rq_flags rf;
> +	bool sched = false;
> 
>   	WARN_ON_ONCE(p != current);
>   	p->sched_throttle_work.next = &p->sched_throttle_work;
> @@ -5879,9 +5880,13 @@ static void throttle_cfs_rq_work(struct
> callback_head *work)
>   	dequeue_task_fair(rq, p, DEQUEUE_SLEEP | DEQUEUE_SPECIAL);
>   	list_add(&p->throttle_node, &cfs_rq->throttled_limbo_list);
>   	resched_curr(rq);
> +	sched = true;
> 
>   out_unlock:
>   	task_rq_unlock(rq, p, &rf);
> +
> +	if (sched)
> +		schedule();
>   }
> 
>   void init_cfs_throttle_work(struct task_struct *p)

-- 
Thanks and Regards,
Prateek