Re: [PATCH 1/2] hrtimer: update '->active_bases' before calling hrtimer_force_reprogram()

From: Peter Zijlstra
Date: Thu Apr 02 2015 - 10:16:47 EST

On Thu, Apr 02, 2015 at 07:23:31PM +0530, Viresh Kumar wrote:
> On 2 April 2015 at 19:17, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Thu, Apr 02, 2015 at 04:21:21PM +0530, Viresh Kumar wrote:
> >> 'active_bases' indicates which clock-base have active timers. While it
> >> is updated (almost) correctly, it is hardly used. Next commit will start
> >> using it to make code more efficient, but before that we need to fix a
> >> problem.
> >>
> >> While removing hrtimers, in __remove_hrtimer():
> >> - We first remove the hrtimer from the queue.
> >> - Then reprogram clockevent device if required
> >> (hrtimer_force_reprogram()).
> >> - And then finally clear 'active_bases', if no more timers are pending
> >> on the current clock base (from which we are removing the hrtimer).
> >>
> >> hrtimer_force_reprogram() needs to loop over all active clock bases to
> >> find the next expiry event, and while doing so it will use
> >> 'active_bases' (after next commit). And it will find the current base
> >> active, as we haven't cleared it until now, even if current clock base
> >> has no more hrtimers queued.
> >>
> >> To fix this issue, clear active_bases before calling
> >> hrtimer_force_reprogram().
> >
> > This is a small inefficiency right? Not an actual bug or anything.
> So, what's explained in this patch is a BUG, which isn't harming us today.

So then I'm not seeing how its a bug. Sure __hrtimer_get_next_event()
will iterate all the bases again, and it will not skip the just empty
one. But I don't see how that is anything but an inefficiency. By virtue
of the base being empty it cannot find an event there, so its a
pointless check.

What am I missing?
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