linux-kernel - Re: [PATCH v23 7/9] sched: Have try_to_wake_up() handle return-migration for PROXY

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Message-ID: <CANDhNCp-V5sHK6Nbo0_kxwROocwMyxd2NjeO03p6JboCeq+kLw@mail.gmail.com>
Date: Wed, 19 Nov 2025 23:34:12 -0800
From: John Stultz <jstultz@...gle.com>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: LKML <linux-kernel@...r.kernel.org>, Joel Fernandes <joelagnelf@...dia.com>, 
	Qais Yousef <qyousef@...alina.io>, 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>, 
	Valentin Schneider <vschneid@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Metin Kaya <Metin.Kaya@....com>, 
	Xuewen Yan <xuewen.yan94@...il.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>, Suleiman Souhlal <suleiman@...gle.com>, 
	kuyo chang <kuyo.chang@...iatek.com>, hupu <hupu.gm@...il.com>, kernel-team@...roid.com
Subject: Re: [PATCH v23 7/9] sched: Have try_to_wake_up() handle
 return-migration for PROXY_WAKING case

On Wed, Nov 19, 2025 at 7:16 PM K Prateek Nayak <kprateek.nayak@....com> wrote:
> On 11/20/2025 6:35 AM, John Stultz wrote:
> >> Sounds like block_task() would be better than deactivate_task() above
> >> in that case. Anything that is waiting on the task's state change takes
> >> the pi_lock afaik and the wakeup is always done with pi_lock held so
> >> blocking the task shouldn't cause any problems based on my reading.
> >
> > So earlier I did try using block_task() but it always seemed to run
> > into crashes, which I assumed was because other cpus were picking the
> > task up as it wasn't on_rq (any references to a task after
> > block_task() in other situations often runs into this trouble).
> >
> > But your point about the pi_lock being held is a good one, so I will
> > tinker and think a bit more on this.
>
> So if you hadn't used DEQUEUE_SPECIAL previously with block_task(),

Yeah, you're right, DEQUEUE_SPECIAL definitely solves the crashes I
was seeing without it.
I'll switch over to that.

thanks so much!
-john