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@chris-wilson.co.uk Cc: Tvrtko Ursulin tvrtko.ursulin@intel.com Cc: stable@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);elseWhen 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