linux-kernel - Re: [PATCH 1/2] sched: Fix balance

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Message-ID: <jhjpn6s5tq5.mognet@arm.com>
Date:   Fri, 11 Sep 2020 14:27:30 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     peterz@...radead.org
Cc:     mingo@...nel.org, vincent.guittot@...aro.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bristot@...hat.com, swood@...hat.com
Subject: Re: [PATCH 1/2] sched: Fix balance_callback()


On 11/09/20 13:25, peterz@...radead.org wrote:
> On Fri, Sep 11, 2020 at 01:17:02PM +0100, Valentin Schneider wrote:
>> So that can be say __schedule() tail racing with some setprio; what's the
>> worst that can (currently) happen here? Something like say two consecutive
>> enqueuing of push_rt_tasks() to the callback list?
>
> Yeah, but that isn't in fact the case I worry most about.
>
> What can happen (and what I've spotted once before) is that someone
> attempts to enqueue a balance_callback from a rq->lock region that
> doesn't handle the calls.
>
> Currently that 'works', that is, it will get ran _eventually_. But
> ideally we'd want that to not work and issue a WARN. We want the
> callbacks to be timely.
>
> So basically all of these machinations we in order to add the WARN :-)

Makes sense, thanks!