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

[stephane eranian]

On Thu, Nov 27, 2008 at 1:38 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> On Thu, Nov 27, 2008 at 12:49:37PM +0100, Thomas Gleixner wrote:
>> On Thu, 27 Nov 2008, David Miller wrote:
>>
>> > From: "stephane eranian" <eranian@xxxxxxxxxxxxxx>
>> > Date: Thu, 27 Nov 2008 12:35:54 +0100
>> >
>> > > I am still wondering how Oprofile handles the case where multiple
>> > > processes or threads access the same file descriptor.
>> >
>> > There's only one profiling buffer active on a given cpu,
>> > so it's pure per-cpu value insertion.
>> >
>> > In any event I think that NMI profiling is a must, especially
>> > for the kernel.  You get total unusable crap otherwise.  I
>> > just learned this the hard way having gotten an NMI'ish scheme
>> > working on sparc64 just the other day.
>>
>> Not arguing about that, I'm just not agreeing with the implementation.
>>
>> So for the moment we can go w/o the NMI and implement it cleanly after
>> we got the initial lot in.
>
> Note once Stephane readds PEBS support (it is currently stripped out)
> you'll be also able to get somewhat reasonable results at least on modern
> Intel x86 without NMI profiling. But longer term it is still
> very useful because PEBS has some drawbacks too

That's true for Intel.
On AMD64, I think you can get at most one sample out of a irq-off
region using IBS.
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