Messages in this thread | | | Date | Mon, 14 Jul 2014 10:11:35 +0530 | Subject | Re: [RFC 0/7] hrtimer: drop active hrtimer checks after adding it | From | Viresh Kumar <> |
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Hi Thomas,
On 10 July 2014 07:04, Frederic Weisbecker <fweisbec@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
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