On Thu, Aug 11, 2016 at 05:36:06PM +0100, Mark Rutland wrote: > 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 00e727bb389359c8 ("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 00e727bb389359c8), 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, supported_cpumask, 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).
I might have asked before, but what's the kernel side state of this? thanks, jirka