Re: [PATCH 0/7] Generic hotplug support for a PMU with a scope

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2024-09-06 11:12 a.m., Peter Zijlstra wrote:
> On Fri, Aug 02, 2024 at 08:16:36AM -0700, kan.liang@linux.intel.com wrote:
>> From: Kan Liang <kan.liang@linux.intel.com>
>>
>> The perf subsystem assumes that the counters of a PMU are per-CPU. So
>> the user space tool reads a counter from each CPU in the system wide
>> mode. However, many PMUs don't have a per-CPU counter. The counter is
>> effective for a scope, e.g., a die or a socket. To address this, a
>> cpumask is exposed by the kernel driver to restrict to one CPU to stand
>> for a specific scope. In case the given CPU is removed,
>> the hotplug support has to be implemented for each such driver.
>>
>> The codes to support the cpumask and hotplug are very similar.
>> - Expose a cpumask into sysfs
>> - Pickup another CPU in the same scope if the given CPU is removed.
>> - Invoke the perf_pmu_migrate_context() to migrate to a new CPU.
>> - In event init, always set the CPU in the cpumask to event->cpu
>> - Usually, an event can be read from any CPU of the scope. (For now,
>>   it is only supported by the pkg scope PMU, via
>>   PERF_EV_CAP_READ_ACTIVE_PKG, e.g., cstate_pkg, rapl, etc)
>>
>> Similar duplicated codes are implemented for each such PMU driver. It
>> would be good to introduce a generic infrastructure to avoid such
>> duplication.
>>
>> The patch series introduce 5 popular scopes, core, die, cluster, pkg,
>> and the system-wide. The PMU drivers for cstate, iommu, idxd and rapl
>> are updated to apply the new infrastructure. The new infrastructure
>> can also be applied for other PMU drivers from different ARCHs as well.
>> But I don't have such platforms. It's not done in this patch series.
>> They can be added later separately.
>>
>> The uncore driver isn't updated either. Because a per-PMU cpumask is
>> required since commit c74443d92f68 ("perf/x86/uncore: Support per PMU
>> cpumask"). Since different types of PMU share the same hotplug codes,
>> more factor out works and verification are expected. The cleanup of the
>> uncore driver can be done later separately.
>>
>> Kan Liang (7):
>>   perf: Generic hotplug support for a PMU with a scope
>>   perf: Add PERF_EV_CAP_READ_SCOPE
>>   perf/x86/intel/cstate: Clean up cpumask and hotplug
>>   iommu/vt-d: Clean up cpumask and hotplug
>>   dmaengine: idxd: Clean up cpumask and hotplug
>>   perf/x86/rapl: Move the pmu allocation out of CPU hotplug
>>   perf/x86/rapl: Clean up cpumask and hotplug
>>
>>  arch/x86/events/intel/cstate.c | 140 +-------------------------
>>  arch/x86/events/rapl.c         | 119 ++++++----------------
> 
> Looks like we have another RAPL driver in:
> 
>   drivers/powercap/intel_rapl_common.c
> 
> that wants to be converted.

Right, but I need to talk with the power guys first. I have a vague
impression that some of the counters are not exactly PKG scope.

Thanks,
Kan