linux-kernel - Re: [patch 05/24] perfmon: X86 generic code (x86)

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Message-ID: <20081127100602.GF6703@one.firstfloor.org>
Date:	Thu, 27 Nov 2008 11:06:02 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andi Kleen <andi@...stfloor.org>, eranian@...glemail.com,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, x86@...nel.org, eranian@...il.com,
	sfr@...b.auug.org.au
Subject: Re: [patch 05/24] perfmon: X86 generic code (x86)

On Wed, Nov 26, 2008 at 11:54:30PM +0100, Thomas Gleixner wrote:
> On Wed, 26 Nov 2008, Andi Kleen wrote:
> > On Wed, Nov 26, 2008 at 02:35:18PM +0100, Thomas Gleixner wrote:
> > > > +	 */
> > > > +	pfm_arch_resend_irq(ctx);
> > > 
> > > Do we really need this whole NMI business ?
> > 
> > Without it you cannot profile interrupts off regions well.
> 
> Fair enough, but I doubt that this is a real solution.
> 
> There is not even an attempt to avoid the obvious wrmrsl races, while
> there are several comments which explain how expensive wrmrsl is. In
> the NMI handler we enable the NMI right away. This might cause
> multiple NMIs for nothing when the NMIs hit between the manipulations
> of the counters. Not likely but can happen depending on the counter
> settings.
> 
> Sending an self-IPI from NMI simply sucks: For every NMI we get an
> extra local interrupt and we have an extra of 2 * NR_ACTIVE_COUNTERS
> accesses to MSRs.

In newer Intel the counters can be reset/rearmed by accessing
only a few global control msrs. But it's probably still a problem 
on other PMUs.

On the other hand it also has PEBS which allows at least some 
profiling of irq-off regions without using NMIs.
> 
> Designing that code to use lockless buffers instead is not really
> rocket science.

Lockless buffers are nasty, but it works in oprofile at least.

Taking out NMis in the first version at least seems like a reasonable
solution. After all you can still use standard oprofile where they work
just fine.

-Andi

-- 
ak@...ux.intel.com
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