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Date:   Thu, 15 Apr 2021 10:42:54 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Charan Teja Reddy <charante@codeaurora.org>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched,psi: fix the 'int' underflow for psi
Message-ID: <YHhQ7tDZWF6tHXSQ@cmpxchg.org>
References: <1618496981-6148-1-git-send-email-charante@codeaurora.org>
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On Thu, Apr 15, 2021 at 07:59:41PM +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.

Makes sense, it's more graceful in the event of a bug.

But what motivates this change? Is it something you hit recently with
an upstream kernel and we should investigate?

> Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
> ---
>  kernel/sched/psi.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 967732c..f925468 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -718,7 +718,8 @@ static void psi_group_change(struct psi_group *group, int cpu,
>  					groupc->tasks[3], clear, set);
>  			psi_bug = 1;
>  		}
> -		groupc->tasks[t]--;
> +		if (groupc->tasks[t])
> +			groupc->tasks[t]--;

There is already a branch on the tasks to signal the bug. How about:

		if (groupc->tasksk[t]) {
			groupc->tasks[t]--;
		} else if (!psi_bug) {
			printk_deferred(...
			psi_bug = 1;
		}