perf pmu: Support alternative sysfs cpumask

The perf tools can read a cpumask file for a PMU, describing a subset of
CPUs which that PMU covers. So far this has only been used to cater for
uncore PMUs, which in practice happen to only have a single CPU
described in the mask.

Until recently, the perf tools only correctly handled cpumask containing
a single CPU, and only when monitoring in system-wide mode. For example,
prior to commit 00e727bb ("perf stat: Balance opening and
reading events"), a mask with more than a single CPU could cause perf
stat to hang. When a CPU PMU covers a subset of CPUs, but lacks a
cpumask, perf record will fail to open events (on the cores the PMU does
not support), and gives up.

For systems with heterogeneous CPUs such as ARM big.LITTLE systems, this
presents a problem. We have a PMU for each microarchitecture (e.g. a big
PMU and a little PMU), and would like to expose a cpumask for each (so
as to allow perf record and other tools to do the right thing). However,
doing so kernel-side will cause old perf binaries to not function (e.g.
hitting the issue solved by 00e727bb

), and thus commits the
cardinal sin of breaking (existing) userspace.

To address this chicken-and-egg problem, this patch adds support got a
new file, cpus, which is largely identical to the existing cpumask file.
A kernel can expose this file, knowing that new perf binaries will
correctly support it, while old perf binaries will not look for it (and
thus will not be broken).

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/1473330112-28528-8-git-send-email-mark.rutland@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>