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

[Andi Kleen]

On Thu, Nov 27, 2008 at 12:35:54PM +0100, stephane eranian wrote:
> On Thu, Nov 27, 2008 at 12:31 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> >> 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.
> >
> > The NMI watchdog is now off by default so failing with it enabled
> > is fine.
> 
> Yes, but most likely it is on in distro kernels.

Really? Why?

Old distros of course do it but only because they run old 
kernels.

> You have to handle the case where the NMI fires while you are holding
> a perfmon lock. What you have in the patch (and the the fully-featured version)
> is that we get the NMI and we stop the PMU WITHOUT grabbing any perfmon
> lock, and the we repost the interrupt with the regular vector. We also make sure
> we grab the RIP at NMI. That is the one we want to see reported in the sampling
> buffer.
> 
> I am still wondering how Oprofile handles the case where multiple processes or
> threads access the same file descriptor.

It uses per CPU buffers (so no races on the writer) and readers can
of course use a lock to coordinate between themselves.

I wrote a similar scheme for mce_log() (although the current version
in tree has some issues too)

-Andi
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