From: Peter Zijlstra <peterz@infradead.org>
To: Tejun Heo <tj@kernel.org>
Cc: linux-kernel@vger.kernel.org, Rik van Riel <riel@surriel.com>
Subject: Re: sched: rq->nr_iowait transiently going negative after the recent p->on_cpu optimization
Date: Thu, 24 Sep 2020 16:50:41 +0200 [thread overview]
Message-ID: <20200924145041.GP2628@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200924142751.GF4268@mtj.duckdns.org>
On Thu, Sep 24, 2020 at 10:27:51AM -0400, Tejun Heo wrote:
> 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?
I worry about things where one CPU has a positive value and one or more
(other) CPUs have a temporary negative value.
next prev parent reply other threads:[~2020-09-24 14:50 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-18 17:27 sched: rq->nr_iowait transiently going negative after the recent p->on_cpu optimization Tejun Heo
2020-09-24 11:50 ` Peter Zijlstra
2020-09-24 14:27 ` Tejun Heo
2020-09-24 14:50 ` Peter Zijlstra [this message]
2020-09-24 14:57 ` Tejun Heo
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