[Intel-gfx] [PATCH 4/5] drm/i915/execlists: Force preemption

[Chris Wilson]

If the preempted context takes too long to relinquish control, e.g. it
is stuck inside a shader with arbitration disabled, evict that context
with an engine reset. This ensures that preemptions are reasonably
responsive, providing a tighter QoS for the more important context at
the cost of flagging unresponsive contexts more frequently (i.e. instead
of using an ~10s hangcheck, we now evict at ~100ms).  The challenge of
lies in picking a timeout that can be reasonably serviced by HW for
typical workloads, balancing the existing clients against the needs for
responsiveness.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>
---
 drivers/gpu/drm/i915/Kconfig.profile | 12 ++++++
 drivers/gpu/drm/i915/gt/intel_lrc.c  | 62 ++++++++++++++++++++++++++--
 2 files changed, 71 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/Kconfig.profile b/drivers/gpu/drm/i915/Kconfig.profile
index 48df8889a88a..3184e8491333 100644
--- a/drivers/gpu/drm/i915/Kconfig.profile
+++ b/drivers/gpu/drm/i915/Kconfig.profile
@@ -25,3 +25,15 @@ config DRM_I915_SPIN_REQUEST
 	  May be 0 to disable the initial spin. In practice, we estimate
 	  the cost of enabling the interrupt (if currently disabled) to be
 	  a few microseconds.
+
+config DRM_I915_PREEMPT_TIMEOUT
+	int "Preempt timeout (ms)"
+	default 100 # milliseconds
+	help
+	  How long to wait (in milliseconds) for a preemption event to occur
+	  when submitting a new context via execlists. If the current context
+	  does not hit an arbitration point and yield to HW before the timer
+	  expires, the HW will be reset to allow the more important context
+	  to execute.
+
+	  May be 0 to disable the timeout.
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 884dfc1cb033..b85ee12c2451 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -945,6 +945,21 @@ static void record_preemption(struct intel_engine_execlists *execlists)
 	(void)I915_SELFTEST_ONLY(execlists->preempt_hang.count++);
 }
 
+static unsigned long preempt_expires(void)
+{
+	const unsigned long timeout =
+		msecs_to_jiffies_timeout(CONFIG_DRM_I915_PREEMPT_TIMEOUT);
+
+	/*
+	 * Paranoia to make sure the compiler computes the timeout before
+	 * loading 'jiffies' as jiffies is volatile and may be updated in
+	 * the background by a timer tick. All to reduce the complexity
+	 * of the addition and reduce the risk of losing a jiffie.
+	 */
+	barrier();
+	return jiffies + timeout;
+}
+
 static void execlists_dequeue(struct intel_engine_cs *engine)
 {
 	struct intel_engine_execlists * const execlists = &engine->execlists;
@@ -1283,6 +1298,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 		*port = execlists_schedule_in(last, port - execlists->pending);
 		memset(port + 1, 0, (last_port - port) * sizeof(*port));
 		execlists_submit_ports(engine);
+		if (CONFIG_DRM_I915_PREEMPT_TIMEOUT)
+			mod_timer(&execlists->timer, preempt_expires());
 	} else {
 		ring_set_paused(engine, 0);
 	}
@@ -1467,13 +1484,45 @@ static void process_csb(struct intel_engine_cs *engine)
 	invalidate_csb_entries(&buf[0], &buf[num_entries - 1]);
 }
 
-static void __execlists_submission_tasklet(struct intel_engine_cs *const engine)
+static bool __execlists_submission_tasklet(struct intel_engine_cs *const engine)
 {
 	lockdep_assert_held(&engine->active.lock);
 
 	process_csb(engine);
-	if (!engine->execlists.pending[0])
+	if (!engine->execlists.pending[0]) {
 		execlists_dequeue(engine);
+		return true;
+	}
+
+	return false;
+}
+
+static void preempt_reset(struct intel_engine_cs *engine)
+{
+	const unsigned int bit = I915_RESET_ENGINE + engine->id;
+	unsigned long *lock = &engine->gt->reset.flags;
+
+	if (test_and_set_bit(bit, lock))
+		return;
+
+	/* Mark this tasklet as disabled to avoid waiting for it to complete */
+	tasklet_disable_nosync(&engine->execlists.tasklet);
+
+	intel_engine_reset(engine, "preemption time out");
+
+	tasklet_enable(&engine->execlists.tasklet);
+	clear_and_wake_up_bit(bit, lock);
+}
+
+static bool preempt_timeout(struct intel_engine_cs *const engine)
+{
+	if (!CONFIG_DRM_I915_PREEMPT_TIMEOUT)
+		return false;
+
+	if (!intel_engine_has_preemption(engine))
+		return false;
+
+	return !timer_pending(&engine->execlists.timer);
 }
 
 /*
@@ -1484,10 +1533,17 @@ static void execlists_submission_tasklet(unsigned long data)
 {
 	struct intel_engine_cs * const engine = (struct intel_engine_cs *)data;
 	unsigned long flags;
+	bool reset = false;
 
 	spin_lock_irqsave(&engine->active.lock, flags);
-	__execlists_submission_tasklet(engine);
+
+	if (!__execlists_submission_tasklet(engine) && preempt_timeout(engine))
+		reset = true;
+
 	spin_unlock_irqrestore(&engine->active.lock, flags);
+
+	if (reset)
+		preempt_reset(engine);
 }
 
 static void execlists_submission_timer(struct timer_list *timer)
-- 
2.22.0