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Date: Fri, 12 Jun 2009 16:25:46 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Thomas Gleixner <tglx@linutronix.de>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>
Subject: Re: [RFC PATCH] posix-cpu-timers: optimize calling
	thread_group_cputime()
Message-ID: <20090612142546.GC3784@redhat.com>
References: <20090612123903.37cfd868@dhcp-lab-109.englab.brq.redhat.com> <1244804986.6691.1224.camel@laptop> <20090612132008.0c6f3e93@dhcp-lab-109.englab.brq.redhat.com>
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To clarify, I am not arguing against this patch, just a queston.

On 06/12, Stanislaw Gruszka wrote:
>
> On Fri, 12 Jun 2009 13:09:46 +0200
> Peter Zijlstra <a.p.zijlstra@chello.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.

I agree with Peter, if we complicate the code it would be nice to know
this really makes it faster. Besides, thread_group_cputime() should not
be called that often.

Perhaps it makes sense to turn ->running into bitmask though, this should
"obviously" speed up account_group_xxx() helpers.

But in that case, perhaps stop_process_timers() should accept bitmask too?
otherwise this doesn't look "complete".

Oleg.