linux-kernel - Re: [PATCH 04/29] x86/perfcounters: rework pmc_amd_save_disable_all() and pmc_amd_restore

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Message-ID: <20090429111934.GA8298@elte.hu>
Date:	Wed, 29 Apr 2009 13:19:34 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Robert Richter <robert.richter@....com>,
	Paul Mackerras <paulus@...ba.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/29] x86/perfcounters: rework
	pmc_amd_save_disable_all() and pmc_amd_restore_all()


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Wed, 2009-04-29 at 13:11 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > 
> > > On Wed, 2009-04-29 at 12:47 +0200, Robert Richter wrote:
> > > > MSR reads and writes are expensive. This patch adds checks to avoid
> > > > its usage where possible.
> > > 
> > > save_disable_all()
> > > enable(1)
> > > restore_all()
> > > 
> > > would not correctly enable 1 with the below modification as we do 
> > > not write the configuration into the msr, on which restore relies, 
> > > as it only toggles the _ENABLE bit.
> > > 
> > > That said, I'm not sure if that's really an issue, but its why the 
> > > does does as it does.
> > > 
> > > A better abstraction could perhaps avoid this issue all-together.
> > 
> > Could we remove the disable-all facility altogether and make the 
> > core code NMI-safe? The current approach wont scale on CPUs that 
> > dont have global-disable features.
> > 
> > disable-all was arguably a hack i introduced and which spread too 
> > far. Can you see a conceptual need for it?
> 
> power suffers the same issue and simply iterates the things like 
> amd does now.
> 
> The thing is, with a global disable you get slightly better 
> coupling, so in that respect it might be nice to have.

ok. With system-wide profiling there's no global disable/enable in 
the fastpath. Do we have any of them in the per task counter 
fastpath?

	Ingo
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