Quoting Janusz Krzysztofik (2020-05-11 09:51:12)
> Hi Chris,
>
> On Fri, 2020-05-08 at 18:54 +0100, Chris Wilson wrote:
> > Quoting Janusz Krzysztofik (2020-05-08 14:56:31)
> > > static double nop_on_ring(int fd, uint32_t handle,
> > > const struct intel_execution_engine2 *e, int timeout,
> > > - unsigned long *out)
> > > + unsigned long *count)
> > > {
> > > struct drm_i915_gem_execbuffer2 execbuf;
> > > struct drm_i915_gem_exec_object2 obj;
> > > struct timespec start, now;
> > > - unsigned long count;
> > > + unsigned long total;
> > > +
> > > + igt_assert(*count);
> > >
> > > memset(&obj, 0, sizeof(obj));
> > > obj.handle = handle;
> > > @@ -93,18 +95,18 @@ static double nop_on_ring(int fd, uint32_t handle,
> > > }
> > > intel_detect_and_clear_missed_interrupts(fd);
> > >
> > > - count = 0;
> > > + total = 0;
> > > clock_gettime(CLOCK_MONOTONIC, &start);
> > > do {
> > > - for (int loop = 0; loop < 1024; loop++)
> > > + for (int loop = 0; loop < *count; loop++)
> >
> > This unnerves me. I expect to get this wrong when writing new callers.
> >
> > There's no great reason to even have 1024 here, we can survive with
> > doing clock_gettime() every iteration, and just accept it as part of the
> > systematic cost.
>
> Thanks for fixing this and merging the change.
>
> Following our discussion on IRC which I probably didn't understand
> precisely enough but maybe have a better understanding now after having
> it reread, how about still addressing the "headless" requirement for
> better precision with the following hunk?
>
> @@ -373,10 +372,12 @@ stable_nop_on_ring(int fd, uint32_t handle,
> while (reps--) {
> unsigned long count;
> double time;
>
> time = nop_on_ring(fd, handle, e, timeout, &count);
> + igt_skip_on_f(count < 1000 * timeout,
> + "submicrosecond precision of time measurement\n");
Yes. I just can't remember off the top of my head what order of
magnitude the error was -- I think it was around an extra 100us (50x
worse latency).
-Chris
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