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

[stephane eranian]

Andi, Thomas,

On Thu, Nov 27, 2008 at 11:06 AM, Andi Kleen <andi@xxxxxxxxxxxxxx> 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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/