Re: [PATCH 1/3 v2] perf/amd/uncore: Prepare L3 thread mask code for Family 19h support

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 18, 2020 at 09:46:41AM -0500, Kim Phillips wrote:

> > But this does not work with the cpumask programmed for the amd_l3 PMU. This mask
> > shows, as it should, one CPU/CCX. So that means that when I do:
> > 
> > $ perf stat -a amd_l3/event=llc_event/
> > 
> > This only collects on the CPUs listed in the cpumask: 0,4,8,12 ....
> > That means that L3 events generated by the other CPUs on the CCX are
> > not monitored.
> > I can easily see the problem by pinning a memory bound program to
> > CPU64, for instance.
> 
> Right, the higher level code calls the driver with a single cpu==0
> call if the perf tool is invoked with a simple -a style system-wide.
> If the tool is invoked with supplemental switches to -a, like -C 0-255,
> and -A, the driver gets called multiple times with all the unique cpu
> values.  The latter is the expected invocation style when measuring
> a benchmark pinned on a subset of cpus, i.e., when evaluating
> the driver, and is the more deterministic behaviour for the driver
> to have, given it cannot tell the difference otherwise.

That seems to suggest it is all horribly broken.