linux-kernel - Re: [PATCH V2] sched,psi: fix the 'int' underflow for psi

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Message-ID: <YHnH6R2c7VW05jc0@hirez.programming.kicks-ass.net>
Date:   Fri, 16 Apr 2021 19:22:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Charan Teja Reddy <charante@...eaurora.org>, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] sched,psi: fix the 'int' underflow for psi

On Fri, Apr 16, 2021 at 11:11:04AM -0400, Johannes Weiner wrote:
> On Fri, Apr 16, 2021 at 08:32:16PM +0530, Charan Teja Reddy wrote:
> > psi_group_cpu->tasks, represented by the unsigned int, stores the number
> > of tasks that could be stalled on a psi resource(io/mem/cpu).
> > Decrementing these counters at zero leads to wrapping which further
> > leads to the psi_group_cpu->state_mask is being set with the respective
> > pressure state. This could result into the unnecessary time sampling for
> > the pressure state thus cause the spurious psi events. This can further
> > lead to wrong actions being taken at the user land based on these psi
> > events.
> > Though psi_bug is set under these conditions but that just for debug
> > purpose. Fix it by decrementing the ->tasks count only when it is
> > non-zero.
> > 
> > Signed-off-by: Charan Teja Reddy <charante@...eaurora.org>
> 
> The subject should be
> 
>   psi: handle potential task count underflow bugs more gracefully

Done.

> since it's not fixing a known bug at this point. Otherwise,
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>

Thanks!