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

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Message-Id: <1244804986.6691.1224.camel@laptop>
Date:	Fri, 12 Jun 2009 13:09:46 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
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, 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? 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.

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