linux-kernel - Re: [PATCH 2/3] sched/core: Add ENQUEUE_RQ_SELECTED to indicate whether ->select_task

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Message-ID: <20240928003855.GB2717@maniforge>
Date: Fri, 27 Sep 2024 19:38:55 -0500
From: David Vernet <void@...ifault.com>
To: Tejun Heo <tj@...nel.org>
Cc: peterz@...radead.org, mingo@...hat.com, kernel-team@...a.com,
	linux-kernel@...r.kernel.org, sched-ext@...a.com
Subject: Re: [PATCH 2/3] sched/core: Add ENQUEUE_RQ_SELECTED to indicate
 whether ->select_task_rq() was called

On Fri, Sep 27, 2024 at 01:46:12PM -1000, Tejun Heo wrote:
> During ttwu, ->select_task_rq() can be skipped if only one CPU is allowed or
> migration is disabled. sched_ext schedulers may perform operations such as
> direct dispatch from ->select_task_rq() path and it is useful for them to
> know whether ->select_task_rq() was skipped in the ->enqueue_task() path.
> 
> Currently, sched_ext schedulers are using ENQUEUE_WAKEUP for this purpose
> and end up assuming incorrectly that ->select_task_rq() was called for tasks
> that are bound to a single CPU or migration disabled.
> 
> Make select_task_rq() indicate whether ->select_task_rq() was called by
> setting WF_RQ_SELECTED in *wake_flags and make ttwu_do_activate() map that
> to ENQUEUE_RQ_SELECTED for ->enqueue_task().
> 
> This will be used by sched_ext to fix ->select_task_rq() skip detection.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>

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

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