linux-kernel - Re: [regression] cpuset: offlined CPUs removed from affinity masks

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Message-ID: <20200219161222.GF698990@mtj.thefacebook.com>
Date:   Wed, 19 Feb 2020 11:12:22 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Li Zefan <lizefan@...wei.com>, cgroups <cgroups@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [regression] cpuset: offlined CPUs removed from affinity masks

On Wed, Feb 19, 2020 at 11:08:39AM -0500, Mathieu Desnoyers wrote:
> I wonder if applying the online cpu masks to the per-thread affinity mask
> is the correct approach ? I suspect what we may be looking for here is to keep

Oh, the whole thing is wrong.

> the affinity mask independent of cpu hotplug, and look-up both the per-thread
> affinity mask and the online cpu mask whenever the scheduler needs to perform
> "is_cpu_allowed()" to check task placement.

Yes, that's what it should have done from the get-go. The way it's
implemented now, maybe we can avoid some specific cases like cpuset
not being used at all but it'll constantly get in the way if you're
expecting thread affinity to retain its value across offlines.

-- 
tejun