linux-kernel - Re: [PATCH] hrtimer: Replace cpu_base->active_bases with a direct check of the active list

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Date:	Thu, 9 Apr 2015 10:53:40 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...nel.org>
cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [PATCH] hrtimer: Replace cpu_base->active_bases with a direct
 check of the active list

On Thu, 9 Apr 2015, Ingo Molnar wrote:
> Btw., does cpu_base->active_bases even make sense? hrtimer bases are 
> fundamentally percpu, and to check whether there are any pending 
> timers is a very simple check:
> 
> 	base->active->next != NULL
> 
> So I'd rather suggest taking a direct look at the head, instead of 
> calculating bit positions, masks, etc.
>
> Furthermore, we never actually use cpu_base->active_bases as a 
> 'summary' value (which is the main point of bitmasks in general), so 
> I'd remove that complication altogether.
> 
> This would speed up various hrtimer primitives like 
> hrtimer_remove()/add and simplify the code. It would be a net code 
> shrink as well.

Well. You trade a bit more code against touching cache lines to figure
out whether the clock base has active timers or not. So for a lot of
scenarios where only clock monotonic is used you touch 3 cache lines
for nothing.

I'm about to send out a patch which actually makes better use of the
active_bases field without creating a code size explosion.

Thanks,

	tglx
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