linux-kernel - Re: [PATCHSET v1 wq/for-6.5] workqueue: Improve unbound workqueue execution locality

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Date:   Wed, 24 May 2023 09:34:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Tejun Heo <tj@...nel.org>, jiangshanlai@...il.com,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        kernel-team@...a.com, joshdon@...gle.com, brho@...gle.com,
        briannorris@...omium.org, nhuck@...gle.com, agk@...hat.com,
        snitzer@...nel.org, void@...ifault.com, gautham.shenoy@....com
Subject: Re: [PATCHSET v1 wq/for-6.5] workqueue: Improve unbound workqueue
 execution locality

On Tue, May 23, 2023 at 06:12:45PM +0200, Vincent Guittot wrote:

> Another possibility to investigate would be that each wakeup of a
> worker is mostly unrelated to the previous one and it cares only
> waker. so we should use -1 for the prev_cpu

Tejun is actually overriding p->wake_cpu in this series to target a
specific LLC -- with the explicit purpose to keep the workers near
enough.

But the problem is that with lots of short tasks we then overload the
LLC and are not running long enough for the idle load-balancer to spread
things, leading to idle time.

And that is specific to this lots of little LLC topologies.