linux-kernel - Re: [patch 0/2] NOHZ vs. profile/oprofile v2

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Date:	Mon, 22 Jun 2009 16:41:10 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Rob van der Heij <rvdheij@...il.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	john stultz <johnstul@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch 0/2] NOHZ vs. profile/oprofile v2


* Martin Schwidefsky <schwidefsky@...ibm.com> wrote:

> On Tue, 9 Jun 2009 22:52:51 +0200 (CEST)
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > > version 2 of the profile patches. The only change is the in_interrupt()
> > > fix in tick_nohz_stop_idle(). I would like to know how to proceed with
> > > the issue.
> > > Andy, do you still prefer to handle the old style profiler analog to
> > > the oprofile patch? If yes I would drop patch #1 and extend patch #2
> > > with another tick_nohz_disable().  
> > 
> > Any update on this one ?
> 
> A solution to this problem should go upstream soon, no? How about this
> patch, it uses the tick_nohz_disable/tick_nohz_enable mechanic for 
> oprofile and the old style kernel profiler. Good enough ?
> 
> ---
> Subject: [PATCH] keep on ticking if a profiler is active
> 
> From: Martin Schwidefsky <schwidefsky@...ibm.com>
> 
> On a NOHZ system with oprofile or the old style kernel profiler enabled
> the timer tick should not be stopped when a cpu goes idle. Currently
> a maximum of 1 tick is accounted if a cpu sleeps for a longer period of
> time. This does bad things to the percentages in the profiler output.
> 
> Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
> ---
> 
>  drivers/oprofile/oprof.c |    3 +++
>  include/linux/tick.h     |    4 ++++
>  kernel/profile.c         |    4 ++++
>  kernel/time/tick-sched.c |   27 ++++++++++++++++++++++++++-
>  4 files changed, 37 insertions(+), 1 deletion(-)

Hm, this is rather ugly. Why not use hrtimers like 'perf' does when 
it fallback-samples based on the timer tick?

That method has three advantages:

 - no weird hookery needed
 - resolution can go far beyond HZ
 - it is evidently dynticks-safe

	Ingo
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