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From: Viresh Kumar <viresh.kumar@linaro.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Linaro Kernel Mailman List <linaro-kernel@lists.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] hrtimer: update '->active_bases' before calling hrtimer_force_reprogram()
Date: Thu, 2 Apr 2015 19:23:31 +0530	[thread overview]
Message-ID: <CAKohpok=DnS+h-YCF8+=xeK9_4i9S=x+sKan9SWJYVEYiA99vA@mail.gmail.com> (raw)
In-Reply-To: <20150402134713.GB23123@twins.programming.kicks-ass.net>

On 2 April 2015 at 19:17, Peter Zijlstra <peterz@infradead.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 :)

  reply	other threads:[~2015-04-02 13:53 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-04-02 10:51 [PATCH 0/2] hrtimer: Only iterate over active bases Viresh Kumar
2015-04-02 10:51 ` [PATCH 1/2] hrtimer: update '->active_bases' before calling hrtimer_force_reprogram() Viresh Kumar
2015-04-02 13:47   ` Peter Zijlstra
2015-04-02 13:53     ` Viresh Kumar [this message]
2015-04-02 14:16       ` Peter Zijlstra
2015-04-02 14:23         ` Viresh Kumar
2015-04-02 10:51 ` [PATCH 2/2] hrtimer: create for_each_active_base() to iterate over active clock-bases Viresh Kumar
2015-04-02 13:45   ` Peter Zijlstra
2015-04-03  5:42     ` viresh kumar

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