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

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Message-ID: <20240906151243.GY4723@noisy.programming.kicks-ass.net>
Date: Fri, 6 Sep 2024 17:12:43 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: kan.liang@...ux.intel.com
Cc: mingo@...hat.com, acme@...nel.org, namhyung@...nel.org,
	irogers@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] Generic hotplug support for a PMU with a scope

On Fri, Aug 02, 2024 at 08:16:36AM -0700, kan.liang@...ux.intel.com wrote:
> From: Kan Liang <kan.liang@...ux.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.