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

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Message-ID: <20150409070917.GF14259@gmail.com>
Date:	Thu, 9 Apr 2015 09:09:17 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Ingo Molnar <mingo@...hat.com>, 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


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Apr 09, 2015 at 08:28:41AM +0200, 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
> > 
> 
> Yeah, that's 3 pointer dereferences from cpu_base, iow you traded a 
> single bit test on an already loaded word for 3 potential cacheline 
> misses.

But the clock bases are not aligned to cachelines, and we have 4 of 
them. So in practice when we access one, we'll load the next one 
anyway.

Furthermore the simplification is measurable, and a fair bit of it is 
in various fast paths. I'd rather trade a bit of a cacheline footprint 
for less overall complexity and faster code.

Thanks,

	Ingo
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