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

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Message-ID: <20140815183630.GA22832@redhat.com>
Date:	Fri, 15 Aug 2014 20:36:30 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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

On 08/15, Rik van Riel wrote:
>
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> 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.

Perhaps. I am obviously biased and can't judge ;) Plus, again, I do
understand that your approach has some advantages too.

> 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);

Yes, perhaps we need something like this in any case. To remind, at least
do_task_stat() calls task_cputime_adjusted() lockless, although we could
fix this separately.

But I do not think the change above is enough. With this change cputime_adjust()
can race with itself. Yes, this guarantees monotonicity even if it is called
lockless, but this can lead to "obviously inconsistent" numbers.

And I don't think we can ignore this. If we could, then we can remove the
scale_stime recalculation and change cputime_adjust() to simply do:

	static void cputime_adjust(struct task_cputime *curr,
				   struct cputime *prev,
				   cputime_t *ut, cputime_t *st)
	{
		/* enforce monotonicity */
		*ut = prev->stime = max(prev->stime, curr->stime);
		*st = prev->utime = max(prev->utime, curr->utime);
	}

Yes, we have this problem either way. And personally I think that this
"enforce monotonicity" logic is pointless, userspace could take care,
but it is too late to complain.

Oleg.

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