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

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Tue, Oct 07, 2025 at 01:34:46PM +0100, James Clark wrote:
> 
> 
> On 07/10/2025 6:47 am, Thomas Richter wrote:
> > On 10/2/25 15:39, Anubhav Shelat wrote:
> > > On Oct 1, 2025 at 9:44 PM, Ian Rogers wrote:
> > > > If cycles is 0 then this will always pass, should this be checking a
> > > range?
> > > 
> > > Yes you're right this will be better.
> > > 
> > > On Oct 2, 2025 at 7:56 AM, Thomas Richter wrote:
> > > > Can we use a larger range to allow the test to pass?
> > > 
> > > What range do you get on s390? When I do group measurements using "perf
> > > record -e "{cycles,cycles}:Su" perf test -w brstack" like in the test I
> > > always get somewhere between 20 and 50 cycles difference. I haven't tested
> > > on s390x, but I see no cycle count difference when testing the same command
> > > on x86. I have observed much larger, more varied differences when using
> > > software events.
> > > 
> > > Anubhav
> > > 
> > 
> > Here is the output of the
> > 
> >   # perf record  -e "{cycles,cycles}:Su" -- ./perf test -w brstack
> >   # perf script | grep brstack
> > 
> > commands:
> > 
> > perf 1110782 426394.696874:    6885000 cycles:           116fc9e brstack_bench+0xae (/r>
> > perf 1110782 426394.696875:    1377000 cycles:           116fb98 brstack_foo+0x0 (/root>
> > perf 1110782 426394.696877:    1377000 cycles:           116fb48 brstack_bar+0x0 (/root>
> > perf 1110782 426394.696878:    1377000 cycles:           116fc94 brstack_bench+0xa4 (/r>
> > perf 1110782 426394.696880:    1377000 cycles:           116fc84 brstack_bench+0x94 (/r>
> > perf 1110782 426394.696881:    1377000 cycles:           116fb7c brstack_bar+0x34 (/roo>
> > perf 1110782 426394.696883:    1377000 cycles:           116fb7c brstack_bar+0x34 (/roo>
> > perf 1110782 426394.696884:    1377000 cycles:           116fb7c brstack_bar+0x34 (/roo>
> > perf 1110782 426394.696885:    1377000 cycles:           116fb7c brstack_bar+0x34 (/roo>
> > perf 1110782 426394.696887:    1377000 cycles:           116fb7c brstack_bar+0x34 (/roo>
> > perf 1110782 426394.696888:    1377000 cycles:           116fc98 brstack_bench+0xa8 (/r>
> > perf 1110782 426394.696890:    1377000 cycles:           116fb7c brstack_bar+0x34 (/roo>
> > perf 1110782 426394.696891:    1377000 cycles:           116fc9e brstack_bench+0xae (/r>
> > perf 1110782 426394.703542:    1377000 cycles:           116fb7c brstack_bar+0x34 (/roo>
> > perf 1110782 426394.703542:   30971975 cycles:           116fb7c brstack_bar+0x34 (/roo>
> > perf 1110782 426394.703543:    1377000 cycles:           116fc76 brstack_bench+0x86 (/r>
> > perf 1110782 426394.703545:    1377000 cycles:           116fc06 brstack_bench+0x16 (/r>
> > perf 1110782 426394.703546:    1377000 cycles:           116fc9e brstack_bench+0xae (/r>
> > perf 1110782 426394.703547:    1377000 cycles:           116fc20 brstack_bench+0x30 (/r>
> > perf 1110782 426394.703549:    1377000 cycles:           116fc9e brstack_bench+0xae (/r>
> > perf 1110782 426394.703550:    1377000 cycles:           116fcbc brstack_bench+0xcc
> > 
> > They are usual identical values beside one or two which are way off. Ignoring those would
> > be good.
> > 
> 
> FWIW I ran 100+ iterations my Arm Juno and N1SDP boards and the test passed
> every time.
> 
> Are we sure there isn't some kind of race condition or bug that the test has
> found? Rather than a bug in the test?

I suspect this too.

> 
> At least "This looks like the result of some hardware property of aarch64
> processors" in the commit message can't be accurate as this isn't the case
> everywhere.

I guess this depends on hardware capability to start/stop the PMU globally
rather than doing it for each counter.

Maybe we can have two set of checks depends on the hardware.  For
example, we keep the existing test on x86 (and selected ARM machines?)
and add a range test for others.  It'd be great if the kernel could
expose that information to userspace.

Thanks,
Namhyung