linux-kernel - Re: [PATCH 00/10] workqueue: break affinity initiatively

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Message-ID: <X9eirnnrD/+VqqEo@hirez.programming.kicks-ass.net>
Date:   Mon, 14 Dec 2020 18:36:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Hillf Danton <hdanton@...a.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 00/10] workqueue: break affinity initiatively

On Mon, Dec 14, 2020 at 11:54:47PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> 06249738a41a ("workqueue: Manually break affinity on hotplug")
> said that scheduler will not force break affinity for us.
> 
> But workqueue highly depends on the old behavior. Many parts of the codes
> relies on it, 06249738a41a ("workqueue: Manually break affinity on hotplug")
> is not enough to change it, and the commit has flaws in itself too.
> 
> We need to thoroughly update the way workqueue handles affinity
> in cpu hot[un]plug, what is this patchset intends to do and
> replace the Valentin Schneider's patch [1].

So the actual problem is with per-cpu kthreads, the new assumption is
that hot-un-plug will make all per-cpu kthreads for the dying CPU go
away.

Workqueues violated that. I fixed the obvious site, and Valentin's patch
avoids workqueues from quickly creating new ones while we're not
looking.

What other problems did you find?