There are several scenarios that can result in posix_cpu_timer_set()
not queueing the timer but still leaving the threadgroup cputime counter
running or keeping the tick dependency around for a random amount of time.

1) 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.

   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);
	}

2) If timer_settime() is called with a 0 expiration on a timer that is
   already armed, the timer is dequeued but not really disarmed. So the
   process wide cputime counter and the tick dependency may still remain
   a while around.

   The following code snippet keeps this overhead around for one week after
   the timer deletion:

	void trigger_process_counter(void)
	{
		timer_t id;
		struct itimerspec val = { };

		val.it_value.tv_sec = 604800;
		timer_create(CLOCK_PROCESS_CPUTIME_ID, NULL, &id);
		timer_settime(id, 0, &val, NULL);
		timer_delete(id);
	}

3) If the timer was initially deactivated, this call to timer_settime()
   with an early expiration may have started the process wide cputime
   counter even though the timer hasn't been queued and armed because it
   has fired early and inline within posix_cpu_timer_set() itself. As a
   result the process wide cputime counter may never stop until a new
   timer is ever armed in the future.

   The following code snippet can reproduce this:

	void trigger_process_counter(void)
	{
		timer_t id;
		struct itimerspec val = { };

		signal(SIGALRM, SIG_IGN);
		timer_create(CLOCK_PROCESS_CPUTIME_ID, NULL, &id);
		val.it_value.tv_nsec = 1;
		timer_settime(id, TIMER_ABSTIME, &val, NULL);
	}

4) If the timer was initially armed with a former expiration value
   before this call to timer_settime() and the current call sets an
   early deadline that has already expired, the timer fires inline
   within posix_cpu_timer_set(). In this case it must have been dequeued
   before firing inline with its new expiration value, yet it hasn't
   been disarmed in this case. So the process wide cputime counter and
   the tick dependency may still be around for a while even after the
   timer fired.

   The following code snippet can reproduce this:

	void trigger_process_counter(void)
	{
		timer_t id;
		struct itimerspec val = { };

		signal(SIGALRM, SIG_IGN);
		timer_create(CLOCK_PROCESS_CPUTIME_ID, NULL, &id);
		val.it_value.tv_sec = 100;
		timer_settime(id, TIMER_ABSTIME, &val, NULL);
		val.it_value.tv_sec = 0;
		val.it_value.tv_nsec = 1;
		timer_settime(id, TIMER_ABSTIME, &val, NULL);
	}

Fix all these issues with triggering the related base next expiration
recalculation on the next tick. This also implies to re-evaluate the need
to keep around the process wide cputime counter and the tick dependency, in
a similar fashion to disarm_timer().
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210726125513.271824-7-frederic@kernel.org