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

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Message-ID: <20090622173611.3b583c95@skybase>
Date:	Mon, 22 Jun 2009 17:36:11 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
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

On Mon, 22 Jun 2009 17:29:37 +0200
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Martin Schwidefsky <schwidefsky@...ibm.com> wrote:
> 
> > On Mon, 22 Jun 2009 17:05:53 +0200
> > Ingo Molnar <mingo@...e.hu> wrote:
> > 
> > > 
> > > * Martin Schwidefsky <schwidefsky@...ibm.com> wrote:
> > > 
> > > > On Mon, 22 Jun 2009 16:41:10 +0200
> > > > Ingo Molnar <mingo@...e.hu> wrote:
> > > > 
> > > > > 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
> > > > 
> > > > Hmm, if we replace the HZ based oprofile tick with an hrtimer we 
> > > > should add an interface to configure the sample interval as well, 
> > > > no? Otherwise we just replace one timer event (HZ) with another 
> > > > (hrtimer).
> > > 
> > > Even if the hrtimer is set with a 1/HZ period it's a better 
> > > solution, as it's dynticks safe without invasive changes.
> > 
> > Ok, but the patch will be quite big. All the profile_tick() calls 
> > from the architecture files will have to be removed. [...]
> 
> Hey, that's a bonus :)

It would remove some oddball code :-)
 
> > [...] And if we really want to keep things separate there will be 
> > two sets of per-cpu hrtimer, one for the old style profiler and 
> > one for oprofile. Any preference for the user space interface to 
> > set the sample rate? A sysctl?
> 
> I dont think we want to keep things separate. Regarding old-style 
> profiler, does anyone still use it? There's now a superior in-tree 
> replacement for it, so we could phase it out.

Well, for my part I won't miss it. But to be able to remove the 
profile_tick() calls from the architectures I either have to rip out
the old profiler now, or adapt it to use hrtimer as well.
 
> A sysctl sounds like the most obvious way to set the sample period - 
> and it can default to 1/Hz.

Agreed.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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