linux-kernel - Re: [RFC PATCH] posix-cpu-timers: optimize calling thread_group

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Date:	Fri, 12 Jun 2009 13:20:08 +0200
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC PATCH] posix-cpu-timers: optimize calling
 thread_group_cputime()

On Fri, 12 Jun 2009 13:09:46 +0200
Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Fri, 2009-06-12 at 12:39 +0200, Stanislaw Gruszka wrote:
> > -               times->utime = cputime_add(times->utime, t->utime);
> > -               times->stime = cputime_add(times->stime, t->stime);
> > -               times->sum_exec_runtime += t->se.sum_exec_runtime;
> > +               if (mask & TG_CPUCLOCK_UTIME)
> > +                       times->utime = cputime_add(times->utime, t->utime);
> > +               if (mask & TG_CPUCLOCK_STIME)
> > +                       times->stime = cputime_add(times->stime, t->stime);
> > +               if (mask & TG_CPUCLOCK_SCHED)
> > +                       times->sum_exec_runtime += t->se.sum_exec_runtime;
> 
> Does adding 3 branches really make it faster? 
Actually I did not any benchmarking yet, so I don't know what is the real
impact of the patch. I hope it make things taster but the result can be
opposite from my expectations.

> Since you're bound to want
> at least one, I would expect the cacheline to be hot (assuming all three
> variables live in the same cacheline -- if not, they should be!), so all
> you're avoiding is the addition.

utime, stime are probable in the same cache line, se.sum_exec_runtime is far away 
from them in struct task_struct so it's in rather separate cache line. 

Stanislaw
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