linux-kernel - Re: [PATCH 8/8] sched: Relax the set_cpus_allowed

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Message-ID: <YAMECwEi93cWZ1W2@hirez.programming.kicks-ass.net>
Date:   Sat, 16 Jan 2021 16:19:39 +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 8/8] sched: Relax the set_cpus_allowed_ptr() semantics

On Sat, Jan 16, 2021 at 10:39:03PM +0800, Lai Jiangshan wrote:
> On Sat, Jan 16, 2021 at 7:43 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Now that we have KTHREAD_IS_PER_CPU to denote the critical per-cpu
> > tasks to retain during CPU offline, we can relax the warning in
> > set_cpus_allowed_ptr(). Any spurious kthread that wants to get on at
> > the last minute will get pushed off before it can run.
> >
> > While during CPU online there is no harm, and actual benefit, to
> > allowing kthreads back on early, it simplifies hotplug code.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> Thanks!
> 
> Relaxing set_cpus_allowed_ptr() was also one of the choices I listed,
> which can really simplify hotplug code in the workqueue and may be
> other hotplug code.

Indeed you did. Having that KTHREAD_IS_PER_CPU for the offline side of
things made it possible to relax (as per the Changelog above).

> Reviewed-by: Lai jiangshan <jiangshanlai@...il.com>

Thanks!