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

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Message-ID: <20090622152937.GA17512@elte.hu>
Date:	Mon, 22 Jun 2009 17:29:37 +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 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 :)

> [...] 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.

A sysctl sounds like the most obvious way to set the sample period - 
and it can default to 1/Hz.

	Ingo
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