[PATCH 19/73] drm/i915/gt: Reduce engine runtime stats from seqlock to a latch

[Chris Wilson]

Since we can compute the elapsed time to add to the total, during the
PMU sample we only need to have a consistent view of the (start, total,
active) tuple to be able to locally determine the runtime. That can be
arrange by a pair of memory bariiers and carefully sequencing of the
writes and reads.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/intel_engine_cs.c    | 33 ++++++--------------
 drivers/gpu/drm/i915/gt/intel_engine_stats.h | 24 +++-----------
 drivers/gpu/drm/i915/gt/intel_engine_types.h |  5 ---
 3 files changed, 14 insertions(+), 48 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_engine_cs.c b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
index f870c9429a5c..afb0a52693e6 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
@@ -341,7 +341,6 @@ static int intel_engine_setup(struct intel_gt *gt, enum intel_engine_id id)
 	engine->schedule = NULL;
 
 	ewma__engine_latency_init(&engine->latency);
-	seqcount_init(&engine->stats.lock);
 
 	ATOMIC_INIT_NOTIFIER_HEAD(&engine->context_status_notifier);
 
@@ -1736,22 +1735,6 @@ void intel_engine_dump(struct intel_engine_cs *engine,
 	intel_engine_print_breadcrumbs(engine, m);
 }
 
-static ktime_t __intel_engine_get_busy_time(struct intel_engine_cs *engine,
-					    ktime_t *now)
-{
-	ktime_t total = engine->stats.total;
-
-	/*
-	 * If the engine is executing something at the moment
-	 * add it to the total.
-	 */
-	*now = ktime_get();
-	if (READ_ONCE(engine->stats.active))
-		total = ktime_add(total, ktime_sub(*now, engine->stats.start));
-
-	return total;
-}
-
 /**
  * intel_engine_get_busy_time() - Return current accumulated engine busyness
  * @engine: engine to report on
@@ -1761,13 +1744,17 @@ static ktime_t __intel_engine_get_busy_time(struct intel_engine_cs *engine,
  */
 ktime_t intel_engine_get_busy_time(struct intel_engine_cs *engine, ktime_t *now)
 {
-	unsigned int seq;
-	ktime_t total;
+	ktime_t total = engine->stats.total;
 
-	do {
-		seq = read_seqcount_begin(&engine->stats.lock);
-		total = __intel_engine_get_busy_time(engine, now);
-	} while (read_seqcount_retry(&engine->stats.lock, seq));
+	/*
+	 * If the engine is executing something at the moment
+	 * add it to the total.
+	 */
+	*now = ktime_get();
+	if (READ_ONCE(engine->stats.active)) {
+		smp_rmb(); /* pairs with intel_engine_context_in/out */
+		total = ktime_add(total, ktime_sub(*now, engine->stats.start));
+	}
 
 	return total;
 }
diff --git a/drivers/gpu/drm/i915/gt/intel_engine_stats.h b/drivers/gpu/drm/i915/gt/intel_engine_stats.h
index 24fbdd94351a..1d8314257ffa 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_stats.h
+++ b/drivers/gpu/drm/i915/gt/intel_engine_stats.h
@@ -15,46 +15,30 @@
 
 static inline void intel_engine_context_in(struct intel_engine_cs *engine)
 {
-	unsigned long flags;
-
 	if (engine->stats.active) {
 		engine->stats.active++;
 		return;
 	}
 
 	/* The writer is serialised; but the pmu reader may be from hardirq */
-	local_irq_save(flags);
-	write_seqcount_begin(&engine->stats.lock);
-
 	engine->stats.start = ktime_get();
-	engine->stats.active++;
-
-	write_seqcount_end(&engine->stats.lock);
-	local_irq_restore(flags);
-
-	GEM_BUG_ON(!engine->stats.active);
+	smp_wmb(); /* pairs with intel_engine_get_busy_time() */
+	WRITE_ONCE(engine->stats.active, 1);
 }
 
 static inline void intel_engine_context_out(struct intel_engine_cs *engine)
 {
-	unsigned long flags;
-
 	GEM_BUG_ON(!engine->stats.active);
 	if (engine->stats.active > 1) {
 		engine->stats.active--;
 		return;
 	}
 
-	local_irq_save(flags);
-	write_seqcount_begin(&engine->stats.lock);
-
-	engine->stats.active--;
 	engine->stats.total =
 		ktime_add(engine->stats.total,
 			  ktime_sub(ktime_get(), engine->stats.start));
-
-	write_seqcount_end(&engine->stats.lock);
-	local_irq_restore(flags);
+	smp_wmb(); /* pairs with intel_engine_get_busy_time() */
+	WRITE_ONCE(engine->stats.active, 0);
 }
 
 #endif /* __INTEL_ENGINE_STATS_H__ */
diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
index d2346b425547..ed13012b9338 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
@@ -518,11 +518,6 @@ struct intel_engine_cs {
 		 */
 		unsigned int active;
 
-		/**
-		 * @lock: Lock protecting the below fields.
-		 */
-		seqcount_t lock;
-
 		/**
 		 * @total: Total time this engine was busy.
 		 *
-- 
2.20.1