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

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Message-ID: <20160815102948.GC19741@redhat.com>
Date:	Mon, 15 Aug 2016 12:29:49 +0200
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Mel Gorman <mgorman@...hsingularity.net>
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 10:58:04AM +0100, Mel Gorman wrote:
> 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.

It races bacause we don't know which thread will call the clock_gettime()
first. But once that happen, second thread will see updated runtime value
from first thread as we call update_curr() for it with task_rq_lock (change
from commit 6e998916dfe3).

> 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.

I agree. I wanted to post my patch on top of Giovanni's.

> > > 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'" ?

It should not reintroduce that problem, also because of change from
commit 6e998916dfe3. When a thread reads sum_exec_runtime it also
update that value, then process reads updated value. I run test
case from  "SMP wobble" commit and the problem do not happen
on my tests.

Perhaps I should post patch with a descriptive changelog and things
would be clearer ...

Stanislaw