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

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Message-ID: <20200924145758.GG4268@mtj.duckdns.org>
Date:   Thu, 24 Sep 2020 10:57:58 -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

On Thu, Sep 24, 2020 at 04:50:41PM +0200, Peter Zijlstra wrote:
> > 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?
> 
> I worry about things where one CPU has a positive value and one or more
> (other) CPUs have a temporary negative value.

I think it'd make more sense to max'ing them per-cpu as that's the right
per-cpu state. nr_iowait_cpu() needs that anyway, so maybe the summing
function can just use nr_iowait_cpu()?

Thanks.

-- 
tejun