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

From: Mark Rutland
Date: Wed Aug 31 2016 - 06:46:36 EST


Apologies for the delay in replying.

On Fri, Aug 12, 2016 at 12:01:23PM +0200, Jiri Olsa wrote:
> 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?

Kernel-side, we do not currently expose a cpumask, and I do not have a
current patch series to do so. I wanted to figure out if this was the
right direction or whether I was going off into the weeds.

Clearly that's jsut confusing, so I guess I should respin this long with
the kernel-side patches?

Implementation wise, it's fairly trivial to add (e.g. [1]).