On 04/08/2026 10:06 pm, Colton Lewis wrote:
James Clark <[email protected]> writes:
On 23/07/2026 9:57 pm, Colton Lewis wrote:
James Clark <[email protected]> writes:
When running the guest on a single CPU I get different counts for the
same event for a single process, although this never happens on a
host.
I think there might even be some Perf tests which expect them to
be the
same, and this doesn't depend on whether any events are running on
the
host or not. Not sure if you ran all the Perf selftests in a guest or
not?
I'll investigate but I'm not sure perf is intended to guarantee
that. perf stat just runs the event counters but may not write or read
them at exactly the same time.
Is that true? The perf core calls perf_pmu_disable() when a process is
scheduled out before reading the count of each event of that process in
an inner loop. The perf_pmu_disable() clears PMCR_EL0.E which freezes
all of the counters so they can be read out in a consistent state.
It's important that they're all stopped at the same time because
counters might be used in metrics as ratios of each other. So I think
it's deliberately designed that way and appears to not be working in a
guest now.
By default I think perf assumes events can be measured independently, If
you want to guarantee events are scheduled together to avoid measurement
skew you need to make sure the events are grouped.
The common way to do that is with {} around the event list:
perf stat -e {branches,branches}
Please see if that resolves the issue.
Groups only change how the events are scheduled, not how the driver
starts or stops multiple events running on the same PMU (grouped or
ungrouped). In my repro I had less events than counters in HW, so they
will always be scheduled at the same time regardless of grouping.
I did notice something extra though, you have to first open some amount
of counters, and then open more than that. Then the second time the ones
with different counts will be however many were opened first time, as if
some state has stuck.
For example if I open two counters then 6, the first two always have
different counts the second time:
$ perf stat -e branches,branches true
Performance counter stats for 'true':
106129 branches
106129 branches
$ perf stat -e
'{branches,branches,branches,branches,branches,branches}' true
Performance counter stats for 'true':
117013 branches
117013 branches
110364 branches
110364 branches
110364 branches
110364 branches
After opening 6 again a third time they'll all have the same counts.
Interesting. Thanks. To clarify, is this happening in the host or VM?
This is in the VM
