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

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Message-ID: <7c86c4470811270209q18d4e83aq8901837159838cc4@mail.gmail.com>
Date:	Thu, 27 Nov 2008 11:09:37 +0100
From:	"stephane eranian" <eranian@...glemail.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	"Thomas Gleixner" <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, x86@...nel.org, sfr@...b.auug.org.au
Subject: Re: [patch 05/24] perfmon: X86 generic code (x86)

Andi, Thomas,

On Thu, Nov 27, 2008 at 11:06 AM, Andi Kleen <andi@...stfloor.org> wrote:
> 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.
>
The only reason why I have to deal with NMI is not so much to allow
for profiling irq-off regions but because I have to share the PMU with
the NMI watchdog. Otherwise I'd have to fail  or disable the NMI watchdog
on the fly.
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