linux-kernel - [RFCv2 0/4] perf tools: play nicely with CPU PMU cpumasks

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Date:	Fri, 15 Jul 2016 11:08:09 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	linux-kernel@...r.kernel.org
Cc:	acme@...nel.org, adrian.hunter@...el.com,
	alexander.shishkin@...ux.intel.com, hekuang@...wei.com,
	jolsa@...nel.org, kan.liang@...el.com, mark.rutland@....com,
	mingo@...hat.com, peterz@...radead.org, wangnan0@...wei.com
Subject: [RFCv2 0/4] perf tools: play nicely with CPU PMU cpumasks

Hi,

I'm trying to make the perf tool play better with PMUs in heterogeneous systems
(e.g. big.LITTLE). These patches fix some brokenness that exists today, but
they require the addition of a cpumask file to each CPU PMU sysfs directory,
and this happens to break prior versions of perf-stat. Due to this, I have not
yet added a cpumask attribute to the ARM PMU code.

In these system we have separate logical PMUs for discrete sets of CPUs. For
example, on an ARM Juno system we have a logical PMU for all Cortex-A53 CPUs,
and a logical PMU for all the Cortex-A57 CPUs. The logical PMUs allow
task-bound events, but reject CPU-bound events for CPUs they do not cover.

Currently perf-record doesn't work for these PMUs, unless forced to use
per-thread mmaps. In the absence of a cpumask, it tries to open events on CPUs
not supported by a PMU, and gives up. In the presence of a cpumask, it ends up
failing to mmap, as the evlist->cpus map contains a different set of CPUs from
the evsel->cpus map populated from the cpumask.

Today's perf-stat can profile a task in the absence of a cpumask file, but in
the presence of one ends up blocking after the profiled task exits. Due to an
inconsistency between __perf_evsel__open and read_counter, it ends up treating
some uninitialised memory as a file descriptor, and typically ends up blocked
on stdin. That can avoided as in patch 1, but existing binaries would be broken
by the addition of the cpumask kernel-side.

To cater for this, this series adds support for a new PMU sysfs file,
supported_cpus, listing a number of CPUs that a logical PMU covers. As old
binaries will not look for this, this can be safely added to the kernel without
risk of breakage.

Does using a sysfs cpumask to handle (heterogeneous) CPU PMUs feel like the
right approach?

Does it make sense to have a differently-named cpumask file that only new tools
will look at?

Since v1 [1]:
* Avoid double cpu_map__idx() call in perf_evlist__mmap_per_evsel
* Look for a supported_cpumask file when a cpumask file is not present

Thanks,
Mark.

[1] http://lkml.kernel.org/r/1467907474-3290-1-git-send-email-mark.rutland@arm.com

Mark Rutland (4):
  perf stat: balance opening and reading events
  perf: util: Add more cpu_map helpers
  perf: util: only open events on CPUs an evsel permits
  perf: util: support sysfs supported_cpumask file

 tools/perf/builtin-stat.c |  8 ++++++--
 tools/perf/util/cpumap.c  | 14 ++++++++++++--
 tools/perf/util/cpumap.h  |  2 ++
 tools/perf/util/evlist.c  |  8 +++++++-
 tools/perf/util/pmu.c     | 15 ++++++++++++---
 5 files changed, 39 insertions(+), 8 deletions(-)

-- 
1.9.1