Messages in this thread | | | Date | Fri, 7 Nov 2014 10:09:59 +0000 | From | Matt Fleming <> | Subject | Re: [PATCH 09/11] perf/x86/intel: Support task events with Intel CQM |
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On Fri, 07 Nov, at 10:08:04AM, Peter Zijlstra wrote: > > How is that supposed to work? You call __intel_cqm_event_count() on the > one cpu per socket, but then you use a local_add, not an atomic_add, > even though these adds can happen concurrently as per IPI broadcast.
Ouch, right. That's broken.
> Also, I think smp_call_function_many() ignores the current cpu, if this > cpu happens to be the cpu for this socket, you're up some creek without > no paddle, right?
OK, I didn't realise that. Yeah that sounds very problematic. I think my eyes skipped over the word "other" in the smp_call_function_many() docs,
* smp_call_function_many(): Run a function on a set of other CPUs.
So, the correct way to do this is to iterate over cqm_cpumask and invoke smp_call_function_single(), right?
> Thirdly, there is no serialization around calling perf_event_count() [or > your pmu::count method] so you cannot temporarily put it to 0.
Urgh, thanks. Good spot. I'm gonna have to think of a suitable serialisation mechanism because all the current ones are pretty heavy-handed. And of course, there's the added fun that it needs to be held across the IPIs.
Perhaps a per-cache-group mutex?
-- Matt Fleming, Intel Open Source Technology Center
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