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

["stephane eranian" <eranian@googlemail.com>]

On Thu, Nov 27, 2008 at 1:32 PM, Andi Kleen <andi@firstfloor.org> wrote:
> 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@firstfloor.org> 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?
>
To make sure they can get a crash dump off of production systems.

> 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.
>
What if a threads reprograms the counters while another is reading them?
How is the buffer reset?