linux-kernel - Re: [PATCH 2/6] posix-cpu-timers: Don't start process wide cputime counter if timer is disabled

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Date:   Thu, 10 Jun 2021 12:24:11 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/6] posix-cpu-timers: Don't start process wide cputime
 counter if timer is disabled

On Wed, Jun 09, 2021 at 02:18:34PM +0200, Frederic Weisbecker wrote:
> On Fri, Jun 04, 2021 at 01:31:55PM +0200, Frederic Weisbecker wrote:
> > If timer_settime() is called with a 0 expiration on a timer that is
> > already disabled, the process wide cputime counter will be started
> > and won't ever get a chance to be stopped by stop_process_timer() since
> > no timer is actually armed to be processed.
> > 
> > This process wide counter might bring some performance hit due to the
> > concurrent atomic additions at the thread group scope.
> > 
> > The following snippet is enough to trigger the issue.
> > 
> > 	void trigger_process_counter(void)
> > 	{
> > 		timer_t id;
> > 		struct itimerspec val = { };
> > 
> > 		timer_create(CLOCK_PROCESS_CPUTIME_ID, NULL, &id);
> > 		timer_settime(id, TIMER_ABSTIME, &val, NULL);
> > 		timer_delete(id);
> > 	}
> > 
> > So make sure we don't needlessly start it.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > Cc: Oleg Nesterov <oleg@...hat.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Eric W. Biederman <ebiederm@...ssion.com>
> 
> No Fixes tag for this one. It has been there since year 1 AG.
> 
> I suspect it's the same for most other commits in the series, checking...

Right, so only the first commit needs one.

Thanks.