Re: [CI] drm/i915/execlists: Prevent GPU death on ELSP[1] promotion to idle context

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On 27/03/2020 11:26, Chris Wilson wrote:
In what seems remarkably similar to the w/a required to not reload an
idle context with HEAD==TAIL, it appears we must prevent the HW from
switching to an idle context in ELSP[1], while simultaneously trying to
preempt the HW to run another context and a continuation of the idle
context (which is no longer idle).

   process_csb: vecs0: cs-irq head=0, tail=1
   process_csb: vecs0: csb[1]: status=0x00000882:0x00000020
   trace_ports: vecs0: preempted { 8c0:30!, 0:0 }
   trace_ports: vecs0: promote { 8b2:32!, 8c0:30 }
   trace_ports: vecs0: submit { 8b8:32, 8c0:32 }
   process_csb: vecs0: cs-irq head=1, tail=2
   process_csb: vecs0: csb[2]: status=0x00000814:0x00000040
   trace_ports: vecs0: completed { 8b2:32!, 8c0:30 }
   process_csb: vecs0: cs-irq head=2, tail=5
   process_csb: vecs0: csb[3]: status=0x00000812:0x00000020
   trace_ports: vecs0: preempted { 8c0:30!, 0:0 }
   trace_ports: vecs0: promote { 8b8:32!, 8c0:32 }
   process_csb: vecs0: csb[4]: status=0x00000814:0x00000060
   trace_ports: vecs0: completed { 8b8:32!, 8c0:32 }
   process_csb: vecs0: csb[5]: status=0x00000818:0x00000020
   trace_ports: vecs0: completed { 8c0:32, 0:0 }
   process_csb: vecs0: ring:{start:0x00021000, head:03f8, tail:03f8, ctl:00000000, mode:00000200}
   process_csb: vecs0: rq:{start:00021000, head:03c0, tail:0400, seqno:8c0:32, hwsp:30},
   process_csb: vecs0: ctx:{start:00021000, head:03f8, tail:03f8},
   process_csb: GEM_BUG_ON("context completed before request")

Fortunately, we just so happen to have a semaphore in place to prevent
the ring HEAD from proceeding past the end of a request that we can use
to fix the HEAD in position as we reprogram ELSP.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1501
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
---
  drivers/gpu/drm/i915/gt/intel_gt_irq.c |  2 +
  drivers/gpu/drm/i915/gt/intel_lrc.c    | 72 +++++++++++++-------------
  2 files changed, 38 insertions(+), 36 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_gt_irq.c b/drivers/gpu/drm/i915/gt/intel_gt_irq.c
index f0e7fd95165a..be3817d99908 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_irq.c
+++ b/drivers/gpu/drm/i915/gt/intel_gt_irq.c
@@ -24,6 +24,8 @@ cs_irq_handler(struct intel_engine_cs *engine, u32 iir)
  {
  	bool tasklet = false;
+ ENGINE_TRACE(engine, "iir: %04x\n", iir);
+
  	if (unlikely(iir & GT_CS_MASTER_ERROR_INTERRUPT)) {
  		u32 eir;
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index b12355048501..53650b452bc9 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -1854,7 +1854,7 @@ static inline void clear_ports(struct i915_request **ports, int count)
  	memset_p((void **)ports, NULL, count);
  }
-static void execlists_dequeue(struct intel_engine_cs *engine)
+static bool execlists_dequeue(struct intel_engine_cs *engine)
  {
  	struct intel_engine_execlists * const execlists = &engine->execlists;
  	struct i915_request **port = execlists->pending;
@@ -1928,13 +1928,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
  				     execlists->queue_priority_hint);
  			record_preemption(execlists);
- /*
-			 * Don't let the RING_HEAD advance past the breadcrumb
-			 * as we unwind (and until we resubmit) so that we do
-			 * not accidentally tell it to go backwards.
-			 */
-			ring_set_paused(engine, 1);
-
  			/*
  			 * Note that we have not stopped the GPU at this point,
  			 * so we are unwinding the incomplete requests as they
@@ -1954,7 +1947,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
  				     last->sched.attr.priority,
  				     execlists->queue_priority_hint);
- ring_set_paused(engine, 1);
  			defer_active(engine);
/*
@@ -1988,7 +1980,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
  				 * of timeslices, our queue might be.
  				 */
  				start_timeslice(engine);
-				return;
+				return false;
  			}
  		}
  	}
@@ -2021,9 +2013,10 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
  			}
if (last && !can_merge_rq(last, rq)) {
+				/* leave this for another sibling */
  				spin_unlock(&ve->base.active.lock);
  				start_timeslice(engine);
-				return; /* leave this for another sibling */
+				return false;
  			}
ENGINE_TRACE(engine,
@@ -2193,32 +2186,31 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
  	 * interrupt for secondary ports).
  	 */
  	execlists->queue_priority_hint = queue_prio(execlists);
+	if (!submit)
+		return false;
- if (submit) {
-		*port = execlists_schedule_in(last, port - execlists->pending);
-		execlists->switch_priority_hint =
-			switch_prio(engine, *execlists->pending);
+	*port = execlists_schedule_in(last, port - execlists->pending);
+	execlists->switch_priority_hint =
+		switch_prio(engine, *execlists->pending);
- /*
-		 * Skip if we ended up with exactly the same set of requests,
-		 * e.g. trying to timeslice a pair of ordered contexts
-		 */
-		if (!memcmp(active, execlists->pending,
-			    (port - execlists->pending + 1) * sizeof(*port))) {
-			do
-				execlists_schedule_out(fetch_and_zero(port));
-			while (port-- != execlists->pending);
-
-			goto skip_submit;
-		}
-		clear_ports(port + 1, last_port - port);
+	/*
+	 * Skip if we ended up with exactly the same set of requests,
+	 * e.g. trying to timeslice a pair of ordered contexts
+	 */
+	if (!memcmp(active, execlists->pending,
+		    (port - execlists->pending + 1) * sizeof(*port))) {
+		do
+			execlists_schedule_out(fetch_and_zero(port));
+		while (port-- != execlists->pending);
- execlists_submit_ports(engine);
-		set_preempt_timeout(engine, *active);
-	} else {
-skip_submit:
-		ring_set_paused(engine, 0);
+		return false;
  	}
+	clear_ports(port + 1, last_port - port);
+
+	execlists_submit_ports(engine);
+	set_preempt_timeout(engine, *active);
+	tasklet_hi_schedule(&execlists->tasklet); /* lost interrupt */
+	return true;
  }
static void
@@ -2478,7 +2470,16 @@ static void __execlists_submission_tasklet(struct intel_engine_cs *const engine)
  	lockdep_assert_held(&engine->active.lock);
  	if (!READ_ONCE(engine->execlists.pending[0])) {
  		rcu_read_lock(); /* protect peeking at execlists->active */
-		execlists_dequeue(engine);
+
+		/*
+		 * Don't let the RING_HEAD advance past the breadcrumb
+		 * as we unwind (and until we resubmit) so that we do
+		 * not accidentally tell it to go backwards.
+		 */
+		ring_set_paused(engine, 1);
+		if (!execlists_dequeue(engine))
+			ring_set_paused(engine, 0);
+
  		rcu_read_unlock();
  	}
  }
@@ -2816,8 +2817,7 @@ static void execlists_reset(struct intel_engine_cs *engine, const char *msg)
  	ring_set_paused(engine, 1); /* Freeze the current request in place */
  	if (execlists_capture(engine))
  		intel_engine_reset(engine, msg);
-	else
-		ring_set_paused(engine, 0);
+	ring_set_paused(engine, 0);
tasklet_enable(&engine->execlists.tasklet);
  	clear_and_wake_up_bit(bit, lock);


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Regards,

Tvrtko
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