Re: [PATCH v4 08/10] drivers/perf: hisi: Export associated CPUs of each PMU through sysfs

[Will Deacon <will@kernel.org>]

On Tue, Dec 10, 2024 at 06:49:15PM +0800, Yicong Yang wrote:
> On 2024/12/9 23:48, Will Deacon wrote:
> > On Tue, Dec 03, 2024 at 08:50:47PM +0800, Yicong Yang wrote:
> >> From: Yicong Yang <yangyicong@hisilicon.com>
> >>
> >> Although the event of the uncore PMU can only be opened on a single
> >> CPU, some PMU does have the affinity on a range of CPUs. For example
> >> the L3C PMU is associated to the CPUs sharing the L3T it monitors.
> >> Users may infer this affinity by the PMU name which may have SCCL ID
> >> and CCL ID encoded (for L3C etc), but it's not that straightforward.
> >> So export this information by adding an "associated_cpus" sysfs
> >> attribute then user can get this directly.
> >>
> >> Reviewed-by: Jonathan Cameron <Joanthan.Cameron@huawei.com>
> >> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> >> ---
> >>  Documentation/admin-guide/perf/hisi-pmu.rst |  5 ++++-
> >>  drivers/perf/hisilicon/hisi_uncore_pmu.c    | 10 ++++++++++
> >>  2 files changed, 14 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/Documentation/admin-guide/perf/hisi-pmu.rst b/Documentation/admin-guide/perf/hisi-pmu.rst
> >> index 5cc248d18c63..48992a0b8e94 100644
> >> --- a/Documentation/admin-guide/perf/hisi-pmu.rst
> >> +++ b/Documentation/admin-guide/perf/hisi-pmu.rst
> >> @@ -35,7 +35,10 @@ e.g. hisi_sccl1_hha0/rx_operations is RX_OPERATIONS event of HHA index #0 in
> >>  SCCL ID #1.
> >>  
> >>  The driver also provides a "cpumask" sysfs attribute, which shows the CPU core
> >> -ID used to count the uncore PMU event.
> >> +ID used to count the uncore PMU event. An "associated_cpus" sysfs attribute is
> >> +also provided to show the CPUs associated with this PMU. The "cpumask" indicates
> >> +the CPUs to open the events, usually as a hint for userspaces tools like perf.
> >> +It only contains one associated CPU from the "associated_cpus".
> > 
> > What is userspace expected to do with this information? Can you point me
> > to patches that add the corresponding support to the 'perf' tool, please?
> > 
> 
> two attributes here, "cpumask" and "associated_cpus". For "cpumask" it's used in
> perf [1], this patch does no change to this attribute but only update the
> description in the doc.
> 
> I think you're questioning about the "associated_cpus" introduced by this patch,
> which is not intended to be used by the perf tools but provides a hint for the
> user to know the CPUs monitored by which uncore PMU, for example which uncore
> L3C PMU's monitoring the events by the CPU(s) the application's running on, or
> if the counts is imbalanced among the L3C PMUs user can figure out what's running
> on the CPUs provided by the "associated_cpus". As described in the commit this
> can also inferred by the PMU name to map SICL ID and CCL ID to certain CPU cluster,
> but these ID's aren't readable enough and it'll be more straightforward and
> friendly to provide an "associated_cpus" information, considering we've already
> maintain it in the drvier.

Bah. I just noticed that there's precedent for this in the Arm CSPMU driver,
so I don't really have a leg to stand on despite my objections to funneling
firmware information through to userspace without a confirmed user.

Will