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

[Chris Wilson]

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.

await_execution is a more restrictive form of the
await_sw_fence(submit), in that 'to' can only be submitted once 'from'
reaches HW and not simply once 'from' reaches its engine submission
queue.
-Chris