Re: [PATCH 2/2] selftests/bpf: bpf_cookie: make perf_event subtest trigger reliably

[sun jian]

On Mon, Mar 2, 2026 at 3:59 AM Emil Tsalapatis <emil@xxxxxxxxxxxxxxx> wrote:
>
> On Fri Feb 27, 2026 at 11:40 AM EST, Sun Jian wrote:
> > The perf_event subtest relies on SW_CPU_CLOCK sampling to trigger the BPF
> > Signed-off-by: Sun Jian <sun.jian.kdev@xxxxxxxxx>
>
> Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>
>
> Nit below, but change makes sense.
>
Hi Emil,
Thanks a lot for the review and the Reviewed-by!
> > ---
> > -static void burn_cpu(void)
> > +static void burn_cpu(long loops)
> >  {
> > -     volatile int j = 0;
> > +     long j = 0;
>
> Nit: AFAICT most of these changes are unnecessary, you could just bump
> the cycle count to 100M and add the barrier. Moving everything to longs
> and factoring out the loop count is unnecessary.
>
Good point about keeping the diff small. In v2 I dropped the volatile
and added a barrier().

v2: https://lore.kernel.org/lkml/20260228074555.122950-3-sun.jian.kdev@xxxxxxxxx/

I kept the loop-count parameter mainly to keep the "long burn to
trigger perf_event"
intent explicit at the call site, and to avoid  hard-coding a large
constant in the helper
if it gets reused later.

Thanks again,
Sun Jian