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Date:   Wed, 9 Jun 2021 14:18:32 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 2/6] posix-cpu-timers: Don't start process wide cputime
 counter if timer is disabled
Message-ID: <20210609121832.GE104634@lothringen>
References: <20210604113159.26177-1-frederic@kernel.org>
 <20210604113159.26177-3-frederic@kernel.org>
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In-Reply-To: <20210604113159.26177-3-frederic@kernel.org>
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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@kernel.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Eric W. Biederman <ebiederm@xmission.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...