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>
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