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

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Message-ID: <20200924142751.GF4268@mtj.duckdns.org>
Date:   Thu, 24 Sep 2020 10:27:51 -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: Re: sched: rq->nr_iowait transiently going negative after the recent
 p->on_cpu optimization

Hello,

On Thu, Sep 24, 2020 at 01:50:42PM +0200, Peter Zijlstra wrote:
> Hurmph.. I suppose you're right :/ And this is an actual problem?

Yeah, this got exposed to userspace as a full 64bit number which overflowed
u32 conversion in the rust procfs library which aborted a program I was
working on multiple times over several months.

On a more theoretical side, it might also surprise nr_iowait_cpu() users.
However, a real problem that may be.

> I think the below should cure that, but blergh, not nice. If you could
> confirm, I'll try and think of something nicer.

Rik suggested that it'd be sufficient to return 0 on underflow especially
given that 0 is actually the right number to describe the state. So, maybe
that can be a nicer code-wise?

Thanks.

-- 
tejun