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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 00e727b ("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 00e727b), 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 <[email protected]> Cc: Alexander Shishkin <[email protected]> Cc: Jiri Olsa <[email protected]> Cc: Mark Rutland <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Will Deacon <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
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