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Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation

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On Tue, Dec 27, 2016 at 3:10 PM, Andi Kleen <andi@firstfloor.org> wrote:
> On Tue, Dec 27, 2016 at 01:33:46PM -0800, David Carrillo-Cisneros wrote:
>> When using one intel_cmt/llc_occupancy/ cgroup perf_event in one CPU, the
>> avg time to do __perf_event_task_sched_out + __perf_event_task_sched_in is
>> ~1170ns
>>
>> most of the time is spend in cgroup ctx switch (~1120ns) .
>>
>> When using continuous monitoring in CQM driver, the avg time to
>> find the rmid to write inside of pqr_context switch  is ~16ns
>>
>> Note that this excludes the MSR write. It's only the overhead of
>> finding the RMID
>> to write in PQR_ASSOC. Both paths call the same routine to find the
>> RMID, so there are
>> about 1100 ns of overhead in perf_cgroup_switch. By inspection I assume most
>> of it comes from iterating over the pmu list.
>
> Do Kan's pmu list patches help?
>
> https://patchwork.kernel.org/patch/9420035/

I think these are independent problems. Kan's patches aim to reduce the overhead
of multiples events in the same task context. The overhead numbers I posted
measure only _one_ event in the cpu's context.

>
>>
>> > Or is there some other overhead other than the MSR write
>> > you're concerned about?
>>
>> No, that problem is solved with the PQR software cache introduced in the series.
>
> So it's already fixed?

Sort of, with PQR sw cache there is only one write to MSR and is only
when either the
RMID or the CLOSID actually changes.

>
> How much is the cost with your cache?

If there is no change on CLOSID or RMID, the hook and comparison takes
about 60 ns.
If there is a change, the write to the MSR + other overhead is about
610 ns (dominated by the MSR write).

>
>>
>>
>> > Perhaps some optimization could be done in the code to make it faster,
>> > then the new interface wouldn't be needed.
>>
>> There are some. One in my list is to create a list of pmus with at
>> least one cgroup event
>> and use it to iterate over in perf_cgroup_switch, instead of using the
>> "pmus" list.
>> The pmus list has grown a lot recently with the addition of all the uncore pmus.
>
> Kan's patches above already do that I believe.

see previous answer.

>
>>
>> Despite this optimization, it's unlikely that the whole sched_out +
>> sched_in gets that
>> close to the 15 ns of the non perf_event approach.
>
> It would be good to see how close we can get. I assume
> there is more potential for optimizations and fast pathing.

I will work on the optimization I described earlier that avoids iterating
over all pmus on the cgroup switch. That should take the bulk of the
overhead, but still more work will probably be needed to get close to the
15ns overhead.

Thanks,
David

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[PATCH V4 00/14] Cqm2: Intel Cache Monitoring fixes and enhancements Vikas Shivappa <vikas.shivappa@linux.intel.com> - 2016-12-17 00:20 +0100
  [PATCH 03/14] x86/rdt: Add rdt common/cqm compile option Vikas Shivappa <vikas.shivappa@linux.intel.com> - 2016-12-17 00:20 +0100
  [PATCH 05/14] x86/cqm,perf/core: Cgroup support prepare Vikas Shivappa <vikas.shivappa@linux.intel.com> - 2016-12-17 00:20 +0100
  [PATCH 13/14] perf/stat: fix bug in handling events in error state Vikas Shivappa <vikas.shivappa@linux.intel.com> - 2016-12-17 00:20 +0100
  [PATCH 07/14] x86/rdt,cqm: Scheduling support update Vikas Shivappa <vikas.shivappa@linux.intel.com> - 2016-12-17 00:20 +0100
  [PATCH 02/14] x86/cqm: Remove cqm recycling/conflict handling Vikas Shivappa <vikas.shivappa@linux.intel.com> - 2016-12-17 00:20 +0100
  [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation Vikas Shivappa <vikas.shivappa@linux.intel.com> - 2016-12-17 00:20 +0100
    Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation Peter Zijlstra <peterz@infradead.org> - 2016-12-23 13:40 +0100
      Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation Shivappa Vikas <vikas.shivappa@intel.com> - 2016-12-23 20:40 +0100
        Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation Peter Zijlstra <peterz@infradead.org> - 2016-12-23 22:00 +0100
          Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation Shivappa Vikas <vikas.shivappa@intel.com> - 2016-12-23 22:50 +0100
          Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation Shivappa Vikas <vikas.shivappa@intel.com> - 2016-12-25 03:00 +0100
            Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation David Carrillo-Cisneros <davidcc@google.com> - 2016-12-27 08:20 +0100
            Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation Andi Kleen <andi@firstfloor.org> - 2016-12-27 21:10 +0100
              Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation Shivappa Vikas <vikas.shivappa@intel.com> - 2016-12-27 21:30 +0100
                Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation David Carrillo-Cisneros <davidcc@google.com> - 2016-12-27 22:40 +0100
              Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation David Carrillo-Cisneros <davidcc@google.com> - 2016-12-27 22:40 +0100
                Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation Andi Kleen <andi@firstfloor.org> - 2016-12-28 00:20 +0100
                Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation David Carrillo-Cisneros <davidcc@google.com> - 2016-12-28 02:30 +0100
                Re: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation Shivappa Vikas <vikas.shivappa@intel.com> - 2016-12-28 21:10 +0100
  [PATCH 06/14] x86/cqm: Add cgroup hierarchical monitoring support Vikas Shivappa <vikas.shivappa@linux.intel.com> - 2016-12-17 00:20 +0100
  [PATCH 11/14] x86/cqm: Add failure on open and read Vikas Shivappa <vikas.shivappa@linux.intel.com> - 2016-12-17 00:20 +0100
    Re: [PATCH 11/14] x86/cqm: Add failure on open and read David Carrillo-Cisneros <davidcc@google.com> - 2016-12-23 13:00 +0100
  [PATCH 08/14] x86/cqm: Add support for monitoring task and cgroup together Vikas Shivappa <vikas.shivappa@linux.intel.com> - 2016-12-17 00:20 +0100

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