Quoting Tvrtko Ursulin (2020-05-26 17:00:06)
>
> 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@xxxxxxxxxxxxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx> # 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
>
> When can engines be different and the mask test below brought us here?
We change the mask during evaluation of the bond, which is from the
signaler's signaler's execute_cb before the signaler is submitted. So
there will be a period where the from->execution_mask is constrained to
a single engine, but it is still waiting to be queued. Once it is
executed on HW, it will remain on that engine.
-Chris
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