[RFCv3 0/2] perf tools: play nicely with CPU PMU cpumasks

[Mark Rutland]

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 existing perf-stat binaries. 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.

Prior to commit 00e727bb389359c8 ("perf stat: Balance opening and reading
events"), from version two of this series, perf-stat would behave erroneously
in the presence of a cpumask file, blocking forever after the workload
completed. While this is now fixed, existing binaries (e.g. those shipped by
distributions) would be broken by the addition of a cpumask file 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? Is there perhaps a better way to do this without using a
cpumask?

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

Thanks,
Mark.

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

Since v2 [2]:
* Drop patches which have been picked up from v2
* Rebase to v4.8-rc1
* Better describe the issue in the supported_cpumask patch

[1] http://lkml.kernel.org/r/1467907474-3290-1-git-send-email-mark.rutland@xxxxxxx
[2] http://lkml.kernel.org/r/1468577293-19667-1-git-send-email-mark.rutland@xxxxxxx

Mark Rutland (2):
  perf: util: only open events on CPUs an evsel permits
  perf: util: support sysfs supported_cpumask file

 tools/perf/util/evlist.c |  8 +++++++-
 tools/perf/util/pmu.c    | 15 ++++++++++++---
 2 files changed, 19 insertions(+), 4 deletions(-)

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1.9.1