[Intel-gfx] [PATCH 2/2] drm/i915/gt: Do not schedule normal requests immediately along virtual

[Chris Wilson]

Quoting Tvrtko Ursulin (2020-05-27 08:32:05)
> 
> On 27/05/2020 08:03, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2020-05-27 07:51:44)
> >>
> >> On 26/05/2020 10:07, Chris Wilson wrote:
> >>> When we push a virtual request onto the HW, we update the rq->engine to
> >>> point to the physical engine. A request that is then submitted by the
> >>> user that waits upon the virtual engine, but along the physical engine
> >>> in use, will then see that it is due to be submitted to the same engine
> >>> and take a shortcut (and be queued without waiting for the completion
> >>> fence). However, the virtual request may be preempted (either by higher
> >>> priority users, or by timeslicing) and removed from the physical engine
> >>> to be migrated over to one of its siblings. The dependent normal request
> >>> however is oblivious to the removal of the virtual request and remains
> >>> queued to execute on HW, believing that once it reaches the head of its
> >>> queue all of its predecessors will have completed executing!
> >>>
> >>> v2: Beware restriction of signal->execution_mask prior to submission.
> >>>
> >>> Fixes: 6d06779e8672 ("drm/i915: Load balancing across a virtual engine")
> >>> Testcase: igt/gem_exec_balancer/sliced
> >>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>> Cc: <stable at vger.kernel.org> # v5.3+
> >>> ---
> >>>    drivers/gpu/drm/i915/i915_request.c | 25 +++++++++++++++++++++----
> >>>    1 file changed, 21 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> >>> index 33bbad623e02..0b07ccc7e9bc 100644
> >>> --- a/drivers/gpu/drm/i915/i915_request.c
> >>> +++ b/drivers/gpu/drm/i915/i915_request.c
> >>> @@ -1237,6 +1237,25 @@ i915_request_await_execution(struct i915_request *rq,
> >>>        return 0;
> >>>    }
> >>>    
> >>> +static int
> >>> +await_request_submit(struct i915_request *to, struct i915_request *from)
> >>> +{
> >>> +     /*
> >>> +      * If we are waiting on a virtual engine, then it may be
> >>> +      * constrained to execute on a single engine *prior* to submission.
> >>> +      * When it is submitted, it will be first submitted to the virtual
> >>> +      * engine and then passed to the physical engine. We cannot allow
> >>> +      * the waiter to be submitted immediately to the physical engine
> >>> +      * as it may then bypass the virtual request.
> >>> +      */
> >>> +     if (to->engine == READ_ONCE(from->engine))
> >>> +             return i915_sw_fence_await_sw_fence_gfp(&to->submit,
> >>> +                                                     &from->submit,
> >>> +                                                     I915_FENCE_GFP);
> >>> +     else
> >>> +             return __i915_request_await_execution(to, from, NULL);
> >>
> >> If I am following correctly this branch will be the virtual <-> physical
> >> or virtual <-> virtual dependency on the same physical engine. Why is
> >> await_execution sufficient in this case? Is there something preventing
> >> timeslicing between the two (not wanted right!) once from start
> >> execution (not finishes).
> > 
> > Timeslicing is only between independent requests. When we expire a
> > request, we also expire all of its waiters along the same engine, so
> > that the execution order remains intact.
> 
> Via scheduler dependencies - they are enough?

Yes.
-Chris