linux-kernel - Re: [PATCH] Revert "sched/core: Tweak wait_task_inactive() to force dequeue sched

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Message-ID: <CANDhNCr+Q=mitFLQ0Xr8ZkZrJPVtgtu8BFaUSAVTZcAFf+VgsA@mail.gmail.com>
Date: Thu, 25 Sep 2025 17:05:40 -0700
From: John Stultz <jstultz@...gle.com>
To: Matt Fleming <matt@...dmodwrite.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org, 
	kernel-team@...udflare.com, Matt Fleming <mfleming@...udflare.com>, 
	Oleg Nesterov <oleg@...hat.com>, Chris Arges <carges@...udflare.com>, stable@...r.kernel.org
Subject: Re: [PATCH] Revert "sched/core: Tweak wait_task_inactive() to force
 dequeue sched_delayed tasks"

On Thu, Sep 25, 2025 at 6:33 AM Matt Fleming <matt@...dmodwrite.com> wrote:
>
> From: Matt Fleming <mfleming@...udflare.com>
>
> This reverts commit b7ca5743a2604156d6083b88cefacef983f3a3a6.
>
> If we dequeue a task (task B) that was sched delayed then that task is
> definitely no longer on the rq and not tracked in the rbtree.
> Unfortunately, task_on_rq_queued(B) will still return true because
> dequeue_task() doesn't update p->on_rq.

Hey!
  Sorry again my patch has been causing you trouble. Thanks for your
persistence in chasing this down!

It's confusing as this patch uses the similar logic as logic
pick_next_entity() uses when a sched_delayed task is picked to run, as
well as elsewhere in __sched_setscheduler() and in sched_ext, so I'd
fret that similar

And my impression was that dequeue_task() on a sched_delayed task
would update p->on_rq via calling __block_task() at the end of
dequeue_entities().

However, there are two spots where we might exit dequeue_entities()
early when cfs_rq_throttled(rq), so maybe that's what's catching us
here?

Peter: Those cfs_rq_throttled() exits in dequeue_entities() seem a
little odd, as the desired dequeue didn't really complete, but
dequeue_task_fair() will still return true indicating success - not
that too many places are checking the dequeue_task return. Is that
right?

thanks
-john