linux-kernel - Re: [PATCH cgroup/for-6.19] cgroup: Fix sleeping from invalid context warning on PREEMPT

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Message-ID: <aQzczJZFiGPOocKb@slm.duckdns.org>
Date: Thu, 6 Nov 2025 07:37:16 -1000
From: Tejun Heo <tj@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Calvin Owens <calvin@...nvd.org>, linux-kernel@...r.kernel.org,
	Dan Schatzberg <dschatzberg@...a.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH cgroup/for-6.19] cgroup: Fix sleeping from invalid
 context warning on PREEMPT_RT

Hello,

On Thu, Nov 06, 2025 at 04:07:17PM +0100, Sebastian Andrzej Siewior wrote:
> How important is it, that it happens right away? Written as-is, this

Not important at all.

> leads to an interrupt then wakes irq_work/$cpu thread which then runs
> this callback. That thread runs as SCHED_FIFO-1. This means the
> termination of a SCHED_OTHER tasks on a single CPU will run as follows:
>  - TASK_DEAD
>    schedule()
>      - queue IRQ_WORK
>      -> INTERRUPT
>      -> WAKE irq_work
>    -> preempt to irq_work/
>       -> handle one callback
>       schedule()
>  back to next TASK_DEAD
> 
> So cgrp_dead_tasks_iwork_fn() will never have to opportunity to batch.
> Unless the exiting task's priority is > 1. Then it will be delayed
> until all RT tasks are done.
> 
> My proposal would be to init the irq_work item with
> 	*per_cpu_ptr(&cgrp_dead_tasks_iwork, cpu) = IRQ_WORK_INIT_LAZY(cgrp_dead_tasks_iwork_fn);
> 
> instead which won't raise an IRQ immediately and delay the callback
> until the next timer tick. So it could batch multiple tasks.
> 
> [ queue_work() should work, too but the overhead to schedule is greater
>   imho so this makes sense ]

Will switch to IRQ_WORK_LAZY_INIT.

Thanks for the review.

-- 
tejun