linux-kernel - Re: [RFC,PATCH 2/2] cputimers/proc: do_task_stat()->thread_group

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Date:	Thu, 25 Mar 2010 13:19:17 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Americo Wang <xiyou.wangcong@...il.com>,
	Balbir Singh <balbir@...ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Roland McGrath <roland@...hat.com>,
	Spencer Candland <spencer@...ehost.com>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC,PATCH 2/2] cputimers/proc:
 do_task_stat()->thread_group_times() is racy and O(n) under ->siglock

On Thu, 2010-03-25 at 13:12 +0100, Oleg Nesterov wrote:
> > Right, so from what I remember the issue is that, yes top et al rely on
> > that monotonicity,
> 
> Really?  So, do you think the change above will break user-space?
> 
> How sad :/ 

IIRC top can give very funny results if you break it hard enough, it
likes to give 9999% cputime if the thing goes backwards over the sample
interval.

But I'm not sure your race is large enough to ever show up like that, so
it might all just work out, but in general it does tend to require
monotonic times.

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