linux-kernel - Re: [PATCH RFC] time: drop do_sys

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Date:	Tue, 12 Aug 2014 18:27:41 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sanjay Rao <srao@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH RFC] time: drop do_sys_times spinlock

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On 08/12/2014 03:12 PM, Oleg Nesterov wrote:

> Afaics, the problem is that a single thread can observe the
> decreasing (say) sum_exec_runtime if it calls do_sys_times() twice
> without the lock.
> 
> This is because it can account the exiting sub-thread twice if it
> races with __exit_signal() which increments sig->sum_sched_runtime,
> but this exiting thread can still be visible to
> thread_group_cputime().
> 
> IOW, it is not actually about decreasing, the problem is that the
> lockless thread_group_cputime() can return the wrong result, and
> the next ys_times() can show the right value.

You are right, changing the test case to call times() many
times in a row in each thread can result in the wrong value
being returned.

Not entirely sure what I can do there...

Replacing the spinlock with a seqlock, and taking it for
write in most places is pretty gross, and may lead to other
issues like reader livelock when there is a lot of write
activity.

Having a seqlock just for the stats?  Not sure the calls
to times() are a big enough issue for most workloads to
justify that...

Any other ideas?

- -- 
All rights reversed
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