[igt-dev] [PATCH igt] igt/perf_pmu: Disable all cpus

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-02-21 09:11:15)
> 
> On 20/02/2018 21:40, Chris Wilson wrote:
> > Rather than iteratively disable and then immediately reenable a CPU,
> > turn off each in turn, forcing the PMU events onto the next CPU without
> > allowing them to retreat back to CPU0 after the first. If this fails,
> 
> Hm, interesting and I think it possibly makes sense to test both 
> migration patterns.

Yup.

> > @@ -988,35 +999,43 @@ static void cpu_hotplug(int gem_fd)
> >        */
> >       igt_fork(child, 1) {
> >               int cpu = 0;
> > +             int cpufd;
> > +             int err;
> >   
> >               close(link[0]);
> >   
> > +             /* Offline each cpu in turn */
> >               for (;;) {
> > -                     char name[128];
> > -                     int cpufd;
> > -
> > -                     igt_assert_lt(snprintf(name, sizeof(name),
> > -                                            "/sys/devices/system/cpu/cpu%d/online",
> > -                                            cpu), sizeof(name));
> > -                     cpufd = open(name, O_WRONLY);
> > -                     if (cpufd == -1) {
> > -                             igt_assert(cpu > 0);
> > -                             /*
> > -                              * Signal parent that we cycled through all
> > -                              * CPUs and we are done.
> > -                              */
> > -                             igt_assert_eq(write(link[1], "*", 1), 1);
> > +                     cpufd = open_cpu_online(cpu);
> > +                     igt_assert(cpufd != -1);
> > +
> > +                     err = write(cpufd, "0", 2);
> > +                     close(cpufd);
> > +                     if (err < 0)
> >                               break;
> 
> Keep off-lining until no more CPUs to offline? I had to try it! :) Ok, 
> last one will fail to offline. But I think it needs a comment.

I thought that was a fun trick to try and offline the last cpu :)

> > -                     }
> >   
> > -                     /* Offline followed by online a CPU. */
> > -                     igt_assert_eq(write(cpufd, "0", 2), 2);
> >                       usleep(1e6);
> > -                     igt_assert_eq(write(cpufd, "1", 2), 2);
> > +                     cpu++;
> > +             }
> >   
> > +             /* Then bring them back online */
> > +             while (cpu--) {
> > +                     cpufd = open_cpu_online(cpu);
> > +                     err = write(cpufd, "1", 2);
> >                       close(cpufd);
> 
> Need to online in the same order or the PMU will stay on some higher CPU 
> making the subsequent tests fail. Or I need to improve the helpers to 
> hunt for the correct CPU, as perf tool does.

Ah. I was expecting everytime we onlined a new cpu, the notifier would
move the pmu. Why do the subsequent tests fail? In my naivety I expected
one CPU is as good as any other for pmu. Do we need to put a trivial
test inside the online/offline loops?
-Chris