Re: [RFCv3 2/2] perf: util: support sysfs supported_cpumask file

[Jiri Olsa]

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