linux-kernel - sched: rq->nr_iowait transiently going negative after the recent p->on

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Message-ID: <20200918172759.GA4247@mtj.thefacebook.com>
Date:   Fri, 18 Sep 2020 13:27:59 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Rik van Riel <riel@...riel.com>
Subject: sched: rq->nr_iowait transiently going negative after the recent
 p->on_cpu optimization

Hello,

Peter, I noticed /proc/stat::procs_blocked going U64_MAX transiently once in
the blue moon without any other persistent issues. After looking at the code
with Rik for a bit, the culprit seems to be c6e7bd7afaeb ("sched/core:
Optimize ttwu() spinning on p->on_cpu") - it changed where ttwu dec's
nr_iowait and it looks like that can happen before the target task gets to
inc.

Thanks.

-- 
tejun