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

[Stephane Eranian]

On Fri, Nov 26, 2010 at 12:36 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> On Fri, 2010-11-26 at 12:25 +0100, Stephane Eranian wrote:
>> On Fri, Nov 26, 2010 at 12:24 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
>> > On Fri, 2010-11-26 at 09:18 +0100, Stephane Eranian wrote:
>> >
>> >> In the perf_event model, given that any one of the 4 cores can be used
>> >> to program uncore events, you have no choice but to broadcast to all
>> >> 4 cores. Each has to demultiplex and figure out which of its counters
>> >> have overflowed.
>> >
>> > Not really, you can redirect all these events to the first online cpu of
>> > the node.
>> >
>> > You can re-write event->cpu in pmu::event_init(), and register cpu
>> > hotplug notifiers to migrate the state around.
>> >
>> I am sure you could. But then the user thinks the event is controlled
>> from CPUx when it's actually from CPUz. I am sure it can work but
>> that's confusing, especially interrupt-wise.
>
> Well, its either that or keeping a node wide state like we do for AMD
> and serialize everything from there.
>
> And I'm not sure what's most expensive, steering the interrupt to one
> core only, or broadcasting every interrupt, I'd favour the first
> approach.

I think the one core-only approach will limit the spurious interrupt aspect.
In perfmon, that's how I had it setup. The first CPU where uncore is
accessed owns the uncore PMU for the socket, thus all interrupts are
routed there. What you are proposing is the same. Now you can chose
you hardcode which is the default core to handle this, or (better) you
use the first core that accesses uncore.

>
> The whole thing is a node-wide resource, so the user needs to think in
> nodes anyway, we already do a cpu->node mapping for identifying the
> thing.
>
Agreed.
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