Re: [PATCH] perf tests record: allow for some difference in cycle count in leader sampling test on aarch64

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

On 09/10/2025 3:43 pm, Anubhav Shelat wrote:
> The first machine was running kernel 6.12.0-55.37.1.el10_0.aarch64 on a KVM
> virtual machine.
> The second machine was running kernel 6.12.0-119.el10.aarch64 also on a KVM.
> 

That's quite old. Make sure you test on the latest kernel before sending 
patches. The tests in mainline should be targeting the latest kernel, 
especially in this case because the throttling fix didn't have a fixes 
tag so won't be backported.

That change to fix throttling and group sampling is only from v6.16.

Also what hardware is the VM running on?

> On Thu, Oct 9, 2025 at 3:17 PM James Clark <james.clark@linaro.org> wrote:
>> After reading that patch it seems like we should actually be removing
>> the 80% tolerance from the leader sampling test. Both instances of the
>> cycles counts should be the same now.
> 
> If there's no tolerance then the leader sampling test would fail much more
> often. In most of the runs there's at least one case where the leader event
> has much fewer cycles.
> 

That's assuming that we've agreed that any difference in cycle counts is 
expected and valid. I don't agree that's the case yet and I think it's a 
bug. I only see identical counts, and the commit message in Kan's fix 
describes that the values should be the same for all architectures.

>> (Excluding s390) I'm starting to think you were hitting this bug on an
>> older kernel? Or something else is going wrong that we should  get to
>> the bottom of. The test could have found something and we shouldn't
>> ignore it yet.
> 
> I agree that the first bug I mentioned might be from an older kernel, but
> there's still the case here where the cycle counts don't match. I'll keep
> looking into it.
> 
> Anubhav
>