linux-kernel - Re: [PATCH 1/6] sched_ext: Simplify scx_can_stop_tick() invocation in sched_can_stop

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Message-ID: <20240805175556.GC42857@maniforge>
Date: Mon, 5 Aug 2024 12:55:56 -0500
From: David Vernet <void@...ifault.com>
To: Tejun Heo <tj@...nel.org>
Cc: peterz@...radead.org, linux-kernel@...r.kernel.org,
	kernel-team@...a.com, mingo@...hat.com
Subject: Re: [PATCH 1/6] sched_ext: Simplify scx_can_stop_tick() invocation
 in sched_can_stop_tick()

On Sat, Aug 03, 2024 at 04:40:08PM -1000, Tejun Heo wrote:
> The way sched_can_stop_tick() used scx_can_stop_tick() was rather confusing
> and the behavior wasn't ideal when SCX is enabled in partial mode. Simplify
> it so that:
> 
> - scx_can_stop_tick() can say no if scx_enabled().
> 
> - CFS tests rq->cfs.nr_running > 1 instead of rq->nr_running.
> 
> This is easier to follow and leads to the correct answer whether SCX is
> disabled, enabled in partial mode or all tasks are switched to SCX.
> 
> Peter, note that this is a bit different from your suggestion where
> sched_can_stop_tick() unconditionally returns scx_can_stop_tick() iff
> scx_switched_all(). The problem is that in partial mode, tick can be stopped
> when there is only one SCX task even if the BPF scheduler didn't ask and
> isn't ready for it.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>

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

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