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

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Message-ID: <20150402134713.GB23123@twins.programming.kicks-ass.net>
Date:	Thu, 2 Apr 2015 15:47:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linaro-kernel@...ts.linaro.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] hrtimer: update '->active_bases' before calling
 hrtimer_force_reprogram()

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.
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