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@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxx>
---
drivers/gpu/drm/i915/gt/intel_engine_cs.c | 34 +++++++-------------
drivers/gpu/drm/i915/gt/intel_engine_stats.h | 24 +++-----------
drivers/gpu/drm/i915/gt/intel_engine_types.h | 5 ---
3 files changed, 15 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 fb1b1d096975..bd33f8a3e9b6 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
@@ -342,7 +342,6 @@ 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);
@@ -1744,22 +1743,6 @@ 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
@@ -1769,13 +1752,18 @@ 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;
+ ktime_t start = engine->stats.start;
- 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, 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
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