linux-kernel - Re: [PATCH v7 16/22] sched: Defer wakeup in ttwu() for unschedulable frozen tasks

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Date:   Thu, 27 May 2021 16:55:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        kernel-team@...roid.com
Subject: Re: [PATCH v7 16/22] sched: Defer wakeup in ttwu() for unschedulable
 frozen tasks

On Thu, May 27, 2021 at 03:44:12PM +0100, Will Deacon wrote:
> I'm not seeing how this handles tasks which weren't put in the freezer
> because they have PF_FREEZER_SKIP set. For these tasks, we need to make
> sure that they don't become runnable before we have onlined a core which
> is capable of running them, and this could occur because of any old
> wakeup (i.e. whatever it was that they blocked on).

I was under the impression that userspace tasks could not have
PF_FREEZER_SKIP set.. let me look harder.