linux-kernel - Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

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Date:	Fri, 15 Aug 2014 13:25:12 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Mike Galbraith <umgwanakikbuti@...il.com>,
	linux-kernel@...r.kernel.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,signal: protect resource use statistics with
 seqlock

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On 08/15/2014 12:49 PM, Oleg Nesterov wrote:

> Just in case... Yes, sure, "seqlock_t stats_lock" is more scalable.
> Just I do not know it's worth the trouble.

If we don't know whether it is worth the trouble, it is probably best
to stick to a well-known generic locking algorithm, instead of brewing
our own and trying to maintain it.

I have fixed the other locking issue you pointed out, Oleg.

Now to see if this change to cputime_adjust does the trick :)

+++ b/kernel/sched/cputime.c
@@ -605,9 +605,12 @@ static void cputime_adjust(struct task_cputime *curr,
 	 * If the tick based count grows faster than the scheduler one,
 	 * the result of the scaling may go backward.
 	 * Let's enforce monotonicity.
+	 * Atomic exchange protects against concurrent cputime_adjust.
 	 */
- -	prev->stime = max(prev->stime, stime);
- -	prev->utime = max(prev->utime, utime);
+	while (stime > (rtime = ACCESS_ONCE(prev->stime)))
+		cmpxchg(&prev->stime, rtime, stime);
+	while (utime > (rtime = ACCESS_ONCE(prev->utime)))
+		cmpxchg(&prev->utime, rtime, utime);

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