Re: [PATCH 2/3] sched, timer: Use atomics for thread_group_cputimer to improve scalability

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 15, 2015 at 03:25:36PM +0200, Frederic Weisbecker wrote:
> On Tue, Apr 14, 2015 at 04:09:45PM -0700, Jason Low wrote:
> >  void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
> >  {
> >  	struct thread_group_cputimer *cputimer = &tsk->signal->cputimer;
> >  	struct task_cputime sum;
> >  
> >  	if (!cputimer->running) {
> >  		/*
> >  		 * The POSIX timer interface allows for absolute time expiry
> >  		 * values through the TIMER_ABSTIME flag, therefore we have
> > +		 * to synchronize the timer to the clock every time we start it.
> >  		 */
> >  		thread_group_cputime(tsk, &sum);
> > +		update_gt_cputime(cputimer, &sum);
> > +		/* Start 'running' after update_gt_cputime() */
> > +		smp_store_release(&cputimer->running, 1);
> 
> This barrier should be mirrored somewhere but I can't see where in this patch.
> Maybe in another one in the series. Or maybe there is already a barrier in the
> existing code that I'm missing. I would expect to see it in account_group_*_time().
> In any case, there should be comment about what it mirrors.

I think it should be in cputimer_running(), which should use
smp_load_acquire() to read cputimer->running.

That way you guarantee that everything observing 'running' will indeed
observe the initialized state.