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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Cc:     linux-kernel@vger.kernel.org,
        Valentin Schneider <valentin.schneider@arm.com>,
        Qian Cai <cai@redhat.com>,
        Vincent Donnefort <vincent.donnefort@arm.com>,
        Dexuan Cui <decui@microsoft.com>,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        Paul McKenney <paulmck@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH -tip V3 0/8] workqueue: break affinity initiatively
In-Reply-To: <X/hGHNGB9fltElWB@hirez.programming.kicks-ass.net>
References: <20201226025117.2770-1-jiangshanlai@gmail.com> <X/hGHNGB9fltElWB@hirez.programming.kicks-ass.net>
Date:   Mon, 11 Jan 2021 11:07:34 +0100
Message-ID: <87o8hv7pnd.fsf@nanos.tec.linutronix.de>
MIME-Version: 1.0
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On Fri, Jan 08 2021 at 12:46, Peter Zijlstra wrote:
> On Sat, Dec 26, 2020 at 10:51:08AM +0800, Lai Jiangshan wrote:
>> From: Lai Jiangshan <laijs@linux.alibaba.com>
>> 
>> 06249738a41a ("workqueue: Manually break affinity on hotplug")
>> said that scheduler will not force break affinity for us.
>
> So I've been looking at this the past day or so, and the more I look,
> the more I think commit:
>
>   1cf12e08bc4d ("sched/hotplug: Consolidate task migration on CPU unplug")
>
> is a real problem and we need to revert it (at least for now).
>
> Let me attempt a brain dump:
>
>  - the assumption that per-cpu kernel threads are 'well behaved' on
>    hot-plug has, I think, been proven incorrect, it's far worse than
>    just bounded workqueue. Therefore, it makes sense to provide the old
>    semantics.

I disagree. Per-cpu kernel threads which are magically stopped during
hotplug and then migrated to a random other CPU are just wrong.

We really need to fix that and not proliferate the sloppy and ill
defined behaviour.

Thanks,

        tglx