linux-kernel - Re: [PATCH 1/1] sched/cputime: Mitigate performance regression in times()/clock

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Date:	Mon, 15 Aug 2016 10:58:04 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
Cc:	Giovanni Gherdovich <ggherdovich@...e.cz>,
	Ingo Molnar <mingo@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <mgalbraith@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] sched/cputime: Mitigate performance regression in
 times()/clock_gettime()

On Mon, Aug 15, 2016 at 11:19:01AM +0200, Stanislaw Gruszka wrote:
> > Is this really equivalent though? It updates one task instead of all
> > tasks in the group and there is no guarantee that tsk == current.
> 
> Oh, my intention was to update runtime on current.
> 

Ok, so minimally that would need addressing. However, then I would worry
that two tasks in a group calling the function at the same time would
see different results because each of them updated a different task.
Such a situation is inherently race-prone anyway but it's a large enough
functional difference to be worth calling out.

Minimally, I don't think such a patch is a replacement for Giovanni's
which is functionally equivalent to the current code but could be layered
on top if it is proven to be ok.

> > Glancing at it, it should monotonically increase but it looks like it
> > would calculate stale data.
> 
> Yes, until the next tick on a CPU, the patch does not count partial
> runtime of thread running on that CPU. However that was the behaviour
> before commit d670ec13178d0 - that how old thread_group_sched_runtime()
> function worked:
> 

Sure, but does this patch not reintroduce the "SMP wobble" and the
problem of "the diff of 'process' should always be >= the diff of
'thread'" ?

-- 
Mel Gorman
SUSE Labs