Re: [Intel-gfx] [PATCH i-g-t] i915/gem_exec_balancer: Randomise bonded submission

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2020-05-27 16:51:50)
> 
> On 27/05/2020 14:14, Chris Wilson wrote:
> > Randomly submit a paired spinner and its cancellation as a bonded
> > (submit fence) pair. Apply congestion to the engine with more bonded
> > pairs to see if the execution order fails. If we prevent a cancellation
> > from running, then the spinner will remain spinning forever.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > ---
> >   tests/i915/gem_exec_balancer.c | 108 +++++++++++++++++++++++++++++++++
> >   1 file changed, 108 insertions(+)
> > 
> > diff --git a/tests/i915/gem_exec_balancer.c b/tests/i915/gem_exec_balancer.c
> > index 80ae82416..98715d726 100644
> > --- a/tests/i915/gem_exec_balancer.c
> > +++ b/tests/i915/gem_exec_balancer.c
> > @@ -1154,6 +1154,111 @@ static void bonded_semaphore(int i915)
> >       gem_context_destroy(i915, ctx);
> >   }
> >   
> > +static void __bonded_dual(int i915,
> > +                       const struct i915_engine_class_instance *siblings,
> > +                       unsigned int count)
> > +{
> > +     struct drm_i915_gem_exec_object2 batch = {};
> > +     struct drm_i915_gem_execbuffer2 execbuf = {
> > +             .buffers_ptr = to_user_pointer(&batch),
> > +             .buffer_count = 1,
> > +     };
> > +     unsigned long cycles = 0;
> > +     uint32_t A, B;
> > +
> > +     A = gem_context_create(i915);
> > +     set_load_balancer(i915, A, siblings, count, NULL);
> > +
> > +     B = gem_context_create(i915);
> > +     set_load_balancer(i915, B, siblings, count, NULL);
> > +
> > +     igt_until_timeout(5) {
> > +             unsigned int master = rand() % count + 1;
> > +             int timeline, fence;
> > +             igt_spin_t *a, *b;
> > +
> > +             timeline = sw_sync_timeline_create();
> > +             fence = sw_sync_timeline_create_fence(timeline, 1);
> > +
> > +             a = igt_spin_new(i915, A,
> > +                              .engine = master,
> > +                              .fence = fence,
> > +                              .flags = (IGT_SPIN_FENCE_IN |
> > +                                        IGT_SPIN_POLL_RUN |
> > +                                        IGT_SPIN_NO_PREEMPTION |
> > +                                        IGT_SPIN_FENCE_OUT));
> > +             b = igt_spin_new(i915, B,
> > +                              .engine = master,
> > +                              .fence = fence,
> > +                              .flags = (IGT_SPIN_FENCE_IN |
> > +                                        IGT_SPIN_POLL_RUN |
> > +                                        IGT_SPIN_NO_PREEMPTION |
> > +                                        IGT_SPIN_FENCE_OUT));
> > +
> > +             close(fence);
> 
> Does this or close(timeline) releases the submissions?

The timeline is the release.
 
> I suggest a variant without the fence anyway for a slightly different 
> profile.

It didn't make any difference. But yes I contemplated the variant.
 
> > +
> > +             if (rand() % 1)
> > +                     igt_swap(a, b);
> > +
> > +             batch.handle = create_semaphore_to_spinner(i915, a);
> 
> These will be preemptible right? More so they schedule out on semaphore 
> interrupt straight away? So I don't see how slaves can be prevented from 
> running because they always are on a different physical engine.

Right, as I understood your description the masters could only be on one
engine with the bonded requests on the other.

And I don't know how to wait from the GPU other than with the
preemptible semaphore spin. A non preemptible wait would be another
spinner, but that would not coordinate across the bond. To reproduce the
non-preemptible HW might require the actual instruction flow.

> We would need a test flavour where slave spins non-preemptively until 
> either the master or CPU signals to stop. Coming up with the exact 
> scheme to achieve this might be challenging. I can think about this more 
> tomorrow.
> 
> And also I don't know why this would fail with my patch! I'll want to 
> have a experiment with that tomorrow as well.

That was indeed mysterious :) I don't have an explanation either yet.
-Chris
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