linux-kernel - Re: [PATCH sched_ext/for-6.12] sched_ext: Make scx_rq_online() also test cpu_active() in addition to SCX_RQ

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Message-ID: <20240808173938.GD6223@maniforge>
Date: Thu, 8 Aug 2024 12:39:38 -0500
From: David Vernet <void@...ifault.com>
To: Tejun Heo <tj@...nel.org>
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH sched_ext/for-6.12] sched_ext: Make scx_rq_online() also
 test cpu_active() in addition to SCX_RQ_ONLINE

On Wed, Aug 07, 2024 at 12:13:38PM -1000, Tejun Heo wrote:
> scx_rq_online() currently only tests SCX_RQ_ONLINE. This isn't fully correct
> - e.g. consume_dispatch_q() uses task_run_on_remote_rq() which tests
> scx_rq_online() to see whether the current rq can run the task, and, if so,
> calls consume_remote_task() to migrate the task to @rq. While the test
> itself was done while locking @rq, @rq can be temporarily unlocked by
> consume_remote_task() and nothing prevents SCX_RQ_ONLINE from going offline
> before the migration takes place.
> 
> To address the issue, add cpu_active() test to scx_rq_online(). There is a
> synchronize_rcu() between cpu_active() being cleared and the rq going
> offline, so if an on-going scheduling operation sees cpu_active(), the
> associated rq is guaranteed to not go offline until the scheduling operation
> is complete.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Fixes: 60c27fb59f6c ("sched_ext: Implement sched_ext_ops.cpu_online/offline()")

Acked-by: David Vernet <void@...ifault.com>

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