Re: [PATCH v8 00/21] ARM64 PMU Partitioning

[James Clark <james.clark@linaro.org>]

On 04/08/2026 10:06 pm, Colton Lewis wrote:
> James Clark <james.clark@linaro.org> writes:
> 
>> On 23/07/2026 9:57 pm, Colton Lewis wrote:
>>> James Clark <james.clark@linaro.org> 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