Messages in this thread | | | Subject | Re: [PATCH 2/3] sched, timer: Use atomics for thread_group_cputimer to improve scalability | From | Jason Low <> | Date | Wed, 15 Apr 2015 13:04:47 -0700 |
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On Wed, 2015-04-15 at 15:32 +0200, Peter Zijlstra wrote: > 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.
So I intended the smp_store_release() here to be mainly for documentation purposes, to say that we would like to set running after the update.
With patch 3/3, even if running happens to get set earlier, the worst case scenario is that update_gt_cputime may have to do go through some retry logic. This isn't much of a performance issue in practice (especially compared to adding smp_load_acquire() to hot paths), since we only enter this path when we need to enable the timers.
In that case, I'm wondering if should just convert this back to WRITE_ONCE(cputimer->running, 1) and avoid adding barriers to the hot paths?
Thanks, Jason
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