linux-kernel - Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER

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Message-ID: <X/7c/PZbzFCO8tfe@hirez.programming.kicks-ass.net>
Date:   Wed, 13 Jan 2021 12:43:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Dexuan Cui <decui@...rosoft.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

On Wed, Jan 13, 2021 at 12:36:55AM +0800, Lai Jiangshan wrote:
> On Tue, Jan 12, 2021 at 10:51 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Mark the per-cpu workqueue workers as KTHREAD_IS_PER_CPU.
> >
> > Workqueues have unfortunate semantics in that per-cpu workers are not
> > default flushed and parked during hotplug, however a subset does
> > manual flush on hotplug and hard relies on them for correctness.
> >
> > Therefore play silly games..
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Tested-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> 
> Reviewed-by: Lai Jiangshan <jiangshanlai@...il.com>
> 
> I like this patchset in that the scheduler takes care of the
> affinities of the tasks when we don't want it to be per-cpu.

Thanks! A possibly even simpler approach would be to have
rebind_workers() kill all workers and have create_worker() spawn us new
ones.

That avoids ever having to use set_cpus_allowed_ptr() on per-cpu
kthreads.... with the exception of rescuer.. still pondering that.