On 23/09/2019 11:32, Chris Wilson wrote:
If we are asked to submit a completed request, just move it onto the
active-list without modifying it's payload. If we try to emit the
modified payload of a completed request, we risk racing with the
ring->head update during retirement which may advance the head past our
breadcrumb and so we generate a warning for the emission being behind
the RING_HEAD.
v2: Commentary for the sneaky, shared responsibility between functions.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/gt/intel_lrc.c | 60 +++++++++++++++++------------
drivers/gpu/drm/i915/i915_request.c | 44 +++++++++++++++------
drivers/gpu/drm/i915/i915_request.h | 2 +-
3 files changed, 68 insertions(+), 38 deletions(-)
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 0a4812ebd184..8c1ea5c315ac 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -799,6 +799,17 @@ static bool can_merge_rq(const struct i915_request *prev,
GEM_BUG_ON(prev == next);
GEM_BUG_ON(!assert_priority_queue(prev, next));
+ /*
+ * We not submit known completed requests. Therefore if the next
"We _do_ not submit"?
+ * request is already completed, we can pretend to merge it in
+ * with the previous context (and we will skip updating the ELSP
+ * and tracking). Thus hopefully keeping the ELSP full with active
+ * contexts, despite the best efforts of preempt-to-busy to confuse
+ * us.
+ */
+ if (i915_request_completed(next))
+ return true;
It works with the current use of can_merge_rq but leaves a bit of a
concern for the future. I did not come up with any interesting
GEM_BUG_ONs to add. I was thinking along the lines of making sure we
never end up coalescing different contexts to the same port. But no
ideas how to do that.
+
if (!can_merge_ctx(prev->hw_context, next->hw_context))
return false;
@@ -1181,21 +1192,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
continue;
}
- if (i915_request_completed(rq)) {
- ve->request = NULL;
- ve->base.execlists.queue_priority_hint = INT_MIN;
- rb_erase_cached(rb, &execlists->virtual);
- RB_CLEAR_NODE(rb);
-
- rq->engine = engine;
- __i915_request_submit(rq);
-
- spin_unlock(&ve->base.active.lock);
-
- rb = rb_first_cached(&execlists->virtual);
- continue;
- }
-
if (last && !can_merge_rq(last, rq)) {
spin_unlock(&ve->base.active.lock);
return; /* leave this for another */
@@ -1249,11 +1245,23 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
GEM_BUG_ON(ve->siblings[0] != engine);
}
- __i915_request_submit(rq);
- if (!i915_request_completed(rq)) {
+ if (__i915_request_submit(rq)) {
submit = true;
last = rq;
}
+
+ /*
+ * Hmm, we have a bunch of virtual engine requests,
+ * but the first one was already complete (thanks
Complete or completed? Not sure myself..
+ * preempt-to-busy!). Keep looking at the veng queue
+ * until we have no more relevent requests (i.e.
relevant
+ * the normal submit queue has higher priority).
+ */
+ if (!submit) {
+ spin_unlock(&ve->base.active.lock);
+ rb = rb_first_cached(&execlists->virtual);
+ continue;
+ }
}
spin_unlock(&ve->base.active.lock);
@@ -1266,8 +1274,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
int i;
priolist_for_each_request_consume(rq, rn, p, i) {
- if (i915_request_completed(rq))
- goto skip;
+ bool merge = true;
/*
* Can we combine this request with the current port?
@@ -1308,14 +1315,17 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
ctx_single_port_submission(rq->hw_context))
goto done;
- *port = execlists_schedule_in(last, port - execlists->pending);
- port++;
+ merge = false;
}
- last = rq;
- submit = true;
-skip:
- __i915_request_submit(rq);
+ if (__i915_request_submit(rq)) {
+ if (!merge) {
+ *port = execlists_schedule_in(last, port - execlists->pending);
+ port++;
+ }
+ submit = true;
+ last = rq;
+ }
}
rb_erase_cached(&p->node, &execlists->queue);
diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 9bd8538b1907..0ca43ca15ca0 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -377,9 +377,10 @@ __i915_request_await_execution(struct i915_request *rq,
return 0;
}
-void __i915_request_submit(struct i915_request *request)
+bool __i915_request_submit(struct i915_request *request)
{
struct intel_engine_cs *engine = request->engine;
+ bool result = false;
GEM_TRACE("%s fence %llx:%lld, current %d\n",
engine->name,
@@ -389,6 +390,25 @@ void __i915_request_submit(struct i915_request *request)
GEM_BUG_ON(!irqs_disabled());
lockdep_assert_held(&engine->active.lock);
+ /*
+ * With the advent of preempt-to-busy, we frequently encounter
+ * requests that we have unsubmitted from HW, but left running
+ * until the next ack and have completed in the meantime. On
+ * resubmission of that completed request, we can skip
+ * updating the payload (and execlists can even skip submitting
+ * the request).
+ *
+ * We must remove the request from the caller's priority queue,
+ * and the caller must only call us when the request is in their
+ * priority queue, under the active.lock. This ensures that the
+ * request has *not* yet been retired and we can safely move
+ * the request into the engine->active.list where it will be
+ * dropped upon retiring. (Otherwise if resubmit a *retired*
+ * request, this would be a horrible use-after-free.)
+ */
+ if (i915_request_completed(request))
+ goto xfer;
+
if (i915_gem_context_is_banned(request->gem_context))
i915_request_skip(request, -EIO);
@@ -412,13 +432,18 @@ void __i915_request_submit(struct i915_request *request)
i915_sw_fence_signaled(&request->semaphore))
engine->saturated |= request->sched.semaphores;
- /* We may be recursing from the signal callback of another i915 fence */
- spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
+ engine->emit_fini_breadcrumb(request,
+ request->ring->vaddr + request->postfix);
+
+ trace_i915_request_execute(request);
+ engine->serial++;
+ result = true;
- list_move_tail(&request->sched.link, &engine->active.requests);
+xfer: /* We may be recursing from the signal callback of another i915 fence */
+ spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
- GEM_BUG_ON(test_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags));
- set_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags);
+ if (!test_and_set_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags))
+ list_move_tail(&request->sched.link, &engine->active.requests);
if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags) &&
!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &request->fence.flags) &&
@@ -429,12 +454,7 @@ void __i915_request_submit(struct i915_request *request)
spin_unlock(&request->lock);
- engine->emit_fini_breadcrumb(request,
- request->ring->vaddr + request->postfix);
-
- engine->serial++;
-
- trace_i915_request_execute(request);
+ return result;
}
void i915_request_submit(struct i915_request *request)
diff --git a/drivers/gpu/drm/i915/i915_request.h b/drivers/gpu/drm/i915/i915_request.h
index b18f49528ded..ec5bb4c2e5ae 100644
--- a/drivers/gpu/drm/i915/i915_request.h
+++ b/drivers/gpu/drm/i915/i915_request.h
@@ -292,7 +292,7 @@ int i915_request_await_execution(struct i915_request *rq,
void i915_request_add(struct i915_request *rq);
-void __i915_request_submit(struct i915_request *request);
+bool __i915_request_submit(struct i915_request *request);
void i915_request_submit(struct i915_request *request);
void i915_request_skip(struct i915_request *request, int error);
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Regards,
Tvrtko
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