Hi Thomas, On 10 July 2014 07:04, Frederic Weisbecker <fweis...@gmail.com> wrote: > On Wed, Jul 09, 2014 at 11:30:41PM +0200, Thomas Gleixner wrote: >> On Wed, 9 Jul 2014, Viresh Kumar wrote: >> >> So your patch series drops active hrtimer checks after adding it, >> according to your subject line. >> >> Quite useeul to drop something after adding it, right?
I meant "hrtimer" by "it". Will fix it in case this patchset is still required. >> > As hrtimer_start*() never fails, hrtimer_active() is guaranteed to return >> > '1'. >> > So, there is no point calling hrtimer_active(). >> >> Wrong as usual. I cross-checked this with Frederic and Preeti before reaching out to you, to make sure its not 'obviously stupid'. And still couldn't get it right. :( >> It's a common pattern that short timeouts are given which lead to >> immediate expiry so the extra round through schedule is even more >> pointless than the extra check. Just wanted to confirm it again, you are talking about CPU being interrupted by clockevent device's interrupt right after hrtimer_start*() returns and before calling hrtimer_active()? > It may be a common pattern but it's not obvious at all as is in the code > except for timers gurus. > > It looks like error handling while it's actually an optimization. > > Also what about this pattern when it's used in interrupt or > interrupt-disabled code? > In this case the handler is not going to fire right away, unless it's enqueued > on another CPU for unpinned timers. > > For example this code in tick_nohz_stop_sched_tick(): > > hrtimer_start(&ts->sched_timer, expires, HRTIMER_MODE_ABS_PINNED); > /* Check, if the timer was already in the past */ > if (hrtimer_active(&ts->sched_timer)) > goto out; > > It's not clear what this is handling. Concurrent immediate callback > expiration from another CPU? > But the timer is pinned local so it can't execute right away between > hrtimer_start() and hrtimer_active() > check... Actually I was concerned about other cases as well. - Timeouts I do agree that an extra check is better than an extra round of schedule(). But this is already achieved without calling hrtimer_active(), isn't it? All these timeout hrtimers have hrtimer_wakeup() as there handler (as these are initialized with: hrtimer_init_sleeper()). And on expiration hrtimer_wakeup() does this: t->task = NULL; So would this extra call to hrtimer_active() make any difference? - Process-context: sched changes I am not sure if scheduler routines: start_bandwidth_timer() and start_dl_timer() would get called *only* with interrupts disabled. But, it doesn't look obvious that the optimization Thomas mentioned earlier is relevant here as well. These might be added here for error checking. I might be wrong here as I don't have any understanding of this code and so sorry in advance. Note: My tree is monitored by kbuild-bot and these changes are under testing for over a week now. And I haven't received any reports of the WARN() firing in __hrtimer_start_range_ns().. Probably these short timeouts aren't getting hit at all by bot's tests. -- viresh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
