Messages in this thread | | | Date | Wed, 31 Aug 2016 11:46:16 +0100 | From | Mark Rutland <> | Subject | Re: [RFCv3 2/2] perf: util: support sysfs supported_cpumask file |
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Hi,
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]).
Thanks, Mark.
[1] http://lkml.kernel.org/r/1466529109-21715-9-git-send-email-jeremy.linton@arm.com
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