linux-kernel - Re: [PATCH 1/2] hrtimer: update '->active_bases' before calling hrtimer_force

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Date:	Thu, 2 Apr 2015 19:23:31 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] hrtimer: update '->active_bases' before calling hrtimer_force_reprogram()

On 2 April 2015 at 19:17, Peter Zijlstra <peterz@...radead.org> 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.

But overall both patches combined are about improving on the small
inefficiency :)

Sorry for the political answer :)
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