linux-kernel - Re: [RFC PATCH 6/9] sched: Treat try_to_block

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Message-ID: <20250413150540.3ZW7XJVs@linutronix.de>
Date: Sun, 13 Apr 2025 17:05:40 +0200
From: Nam Cao <namcao@...utronix.de>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tomas Glozar <tglozar@...hat.com>, Juri Lelli <jlelli@...hat.com>
Subject: Re: [RFC PATCH 6/9] sched: Treat try_to_block_task with pending
 signal as wakeup

On Fri, Apr 04, 2025 at 10:45:19AM +0200, Gabriele Monaco wrote:
> If a task sets itself to interruptible and schedules, the __schedule
> function checks whether there's a pending signal and, if that's the
> case, updates the state of the task to runnable instead of dequeuing.
> By looking at the tracepoints, we see the task enters the scheduler
> while sleepable but exits as runnable. From a modelling perspective,
> this is equivalent to a wakeup and the tracepoints should reflect that.
> 
> Add the waking/wakeup tracepoints in the try_to_block_task function and
> set the prev_state used by the sched_switch tracepoint to TASK_RUNNING
> if the task had a pending signal and was not blocked.
> 
> Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
> ---
>  kernel/sched/core.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f2f79236d5811..48cb32abce01a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6584,7 +6584,12 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
>  	int flags = DEQUEUE_NOCLOCK;
>  
>  	if (signal_pending_state(task_state, p)) {
> -		WRITE_ONCE(p->__state, TASK_RUNNING);
> +		/*
> +		 * From a modelling perspective, this is equivalent to a wakeup
> +		 * before dequeuing the task: trace accordingly.
> +		 */
> +		trace_sched_waking(p);
> +		ttwu_do_wakeup(p);

I don't think we should put trace_sched_waking() here. trace_sched_waking()
"is guaranteed to be called from the waking context", and this is not the
waking context.

There is already a trace_sched_waking() in signal_wake_up_state(). This is
duplicating that, in the wrong context.

ttwu_do_wakeup() alone should be sufficient?

Best regards,
Nam