Re: [RFC PATCH 2/3 v2] perf: Implement Nehalem uncore pmu

[Lin Ming <ming.m.lin@intel.com>]

On Tue, 2010-11-23 at 18:00 +0800, Stephane Eranian wrote:
> On Sun, Nov 21, 2010 at 6:44 PM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > On Sun, 2010-11-21 at 22:04 +0800, Lin Ming wrote:
> >> On Sun, 2010-11-21 at 20:46 +0800, Andi Kleen wrote:
> >> > >
> >> > > 2. Uncore pmu NMI handling
> >> > >
> >> > > All the 4 cores are programmed to receive uncore counter overflow
> >> > > interrupt. The NMI handler(running on 1 of the 4 cores) handle all
> >> > > counters enabled by all 4 cores.
> >> >
> >> > Really for uncore monitoring there is no need to use an NMI handler.
> >> > You can't profile a core anyways, so you can just delay the reporting
> >> > a little bit. It may simplify the code to not use one here
> >> > and just use an ordinary handler.
> >>
> >> OK, I can use on ordinary interrupt handler here.
> >
> > Does the hardware actually allow using a different interrupt source?
> >
> It does not. It's using whatever you've programmed into the APIC
> LVT vector, AFAIK. Uncore interrupt mode is enabled via
> IA32_DEBUGCTL. Regarless of sampling or not, you need the interrupt
> to virtualize the counters to 64 bits.

If only counting(perf stat) makes sense for uncore events, do we still
need an interrupt handler?

48 bits counter is not that easy to overflow in practice.