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

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Message-ID: <20150409091836.GA11494@gmail.com>
Date:	Thu, 9 Apr 2015 11:18:36 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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


* Thomas Gleixner <tglx@...utronix.de> wrote:

> 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.

In the (typical) case it will touch one extra cacheline - and removes 
a fair bit of complexity which 80 bytes (that touches two cachelines):

   7502     427       0    7929    1ef9 hrtimer.o.before
   7422     427       0    7849    1ea9 hrtimer.o.after

So even if we were to optimize for cache footprint (which isn't the 
only factor we optimize for)it looks like a win-win scenario to me, 
even if you ignore the speedup and the simpler code structure...

Ok?

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

So please lets do this series first - it achieves the same thing, with 
less cache used and faster code.

Thanks,

	Ingo
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