Re: [PATCH v2 2/2] selftests/bpf: bpf_cookie: make perf_event subtest trigger reliably

[Jiri Olsa]

On Tue, Mar 03, 2026 at 10:15:26AM +0800, sun jian wrote:
> On Mon, Mar 2, 2026 at 6:02 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> >
> > On Sat, Feb 28, 2026 at 03:45:55PM +0800, Sun Jian wrote:
> > > The perf_event subtest relies on SW_CPU_CLOCK sampling to trigger the BPF
> > > -static void burn_cpu(void)
> > > +static void burn_cpu(long loops)
> >
> > nit, there's another burn_cpu in prog_tests/perf_link.c,
> > we could add it to trace_helpers.c or test_progs.c
> >
> 
> happy to refactor into a shared helper if maintainers prefer, but I keep it
> local to minimize the diff.
> 
> > >  {
> > > -     volatile int j = 0;
> > > +     long j = 0;
> > >       cpu_set_t cpu_set;
> > > -     int i, err;
> > > +     long i;
> > > +     int err;
> > >
> > >       /* generate some branches on cpu 0 */
> > >       CPU_ZERO(&cpu_set);
> > > @@ -443,9 +445,10 @@ static void burn_cpu(void)
> > >       err = pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set);
> > >       ASSERT_OK(err, "set_thread_affinity");
> > >
> > > -     /* spin the loop for a while (random high number) */
> > > -     for (i = 0; i < 1000000; ++i)
> > > +     for (i = 0; i < loops; ++i) {
> > >               ++j;
> > > +             barrier();
> >
> > what's the rationale for barrier call in here,
> > together with the volatile change above?
> >
> 
> The burn_cpu() loop is only meant to consume CPU time to reliably trigger the
> SW_CPU_CLOCK perf_event overflow. With an side-effect-free loop, the
> compiler may optimize the loop away or significantly shrink it under -O2.
> 
> The old version relied on volatile to prevent the loop from being optimized, but
> checkpatch warns against it. Using barrier() achieves the same goal  — keep the
> loop intact as a CPU-burn  — while making the intent more explicit.

ok, would be great to have this in the changelog, other than that:

Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>

thanks,
jirka