linux-kernel - Re: [PATCHSET sched/urgent, sched_ext/for-6.12-fixes] sched/core, sched_ext: Add ENQUEUE_RQ_SELECTED to fix ->select_task_rq() skip detection in sched

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Message-ID: <ZwRCiCnaKWqgQyRz@slm.duckdns.org>
Date: Mon, 7 Oct 2024 10:20:24 -1000
From: Tejun Heo <tj@...nel.org>
To: void@...ifault.com, peterz@...radead.org, mingo@...hat.com
Cc: kernel-team@...a.com, linux-kernel@...r.kernel.org, sched-ext@...a.com
Subject: Re: [PATCHSET sched/urgent, sched_ext/for-6.12-fixes] sched/core,
 sched_ext: Add ENQUEUE_RQ_SELECTED to fix ->select_task_rq() skip detection
 in sched_ext schedulers

On Fri, Sep 27, 2024 at 01:46:10PM -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.
> 
> This patchset adds ENQUEUE_RQ_SELECTED to indicate whether
> ->select_task_rq() was called to ->enqueue_task() and propagate that to
> sched_ext schedulers so that they can reliably detect whether
> ->select_task_rq() was skipped.
> 
> Peter, please let me know how 0001-0002 should be routed. If they get
> applied to sched/urgent (when it opens), I'll pull it into
> sched_ext/for-6.12-fixes and apply 0003 on top. If you'd prefer, I can take
> all three through sched_ext/for-6.12-fixes too.
> 
> This patchset contains the following patches:
> 
>  0001-sched-core-Make-select_task_rq-take-the-pointer-to-w.patch
>  0002-sched-core-Add-ENQUEUE_RQ_SELECTED-to-indicate-wheth.patch
>  0003-sched_ext-scx_qmap-Add-and-use-SCX_ENQ_CPU_SELECTED.patch
> 
>  0001-0002 are sched/core patches to add ENQUEUE_RQ_SELECTED.
> 
>  0003 makes sched_ext propagate the flag as SCX_ENQ_CPU_SELECTED and fix
>  scx_qmap using the new flag.

Applied 1-3 to sched_ext/for-6.12-fixes.

Thanks.

-- 
tejun