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From:   Frederic Weisbecker <frederic@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>
Subject: [PATCH 2/6] posix-cpu-timers: Don't start process wide cputime counter if timer is disabled
Date:   Fri,  4 Jun 2021 13:31:55 +0200
Message-Id: <20210604113159.26177-3-frederic@kernel.org>
In-Reply-To: <20210604113159.26177-1-frederic@kernel.org>
References: <20210604113159.26177-1-frederic@kernel.org>
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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>
---
 kernel/time/posix-cpu-timers.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index aa52fc85dbcb..132fd56fb1cd 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -632,10 +632,15 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
 	 * times (in arm_timer).  With an absolute time, we must
 	 * check if it's already passed.  In short, we need a sample.
 	 */
-	if (CPUCLOCK_PERTHREAD(timer->it_clock))
+	if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
 		val = cpu_clock_sample(clkid, p);
-	else
-		val = cpu_clock_sample_group(clkid, p, true);
+	} else {
+		/*
+		 * Sample group but only start the process wide cputime counter
+		 * if the timer is to be enabled.
+		 */
+		val = cpu_clock_sample_group(clkid, p, !!new_expires);
+	}
 
 	if (old) {
 		if (old_expires == 0) {
-- 
2.25.1