[Intel-gfx] [PATCH] drm/i915/pmu: Measure sampler intervals

[Chris Wilson]

hrtimer is not reliable enough to assume fixed intervals, and so even
coarse accuracy (in the face of kasan and similar heavy debugging) we
need to measure the actual interval between sample.

While using a single timestamp to compute the interval does not allow
very fine accuracy (consider the impact of a slow forcewake between
different samples after the timestamp is read) is much better than
assuming the interval.

Testcase: igt/perf_pmu #ivb
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/i915_pmu.c | 20 +++++++++++++-------
 drivers/gpu/drm/i915/i915_pmu.h |  4 ++++
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
index dc87797db500..f5087515eb43 100644
--- a/drivers/gpu/drm/i915/i915_pmu.c
+++ b/drivers/gpu/drm/i915/i915_pmu.c
@@ -127,6 +127,7 @@ static void __i915_pmu_maybe_start_timer(struct drm_i915_private *i915)
 {
 	if (!i915->pmu.timer_enabled && pmu_needs_timer(i915, true)) {
 		i915->pmu.timer_enabled = true;
+		i915->pmu.timestamp = ktime_get();
 		hrtimer_start_range_ns(&i915->pmu.timer,
 				       ns_to_ktime(PERIOD), 0,
 				       HRTIMER_MODE_REL_PINNED);
@@ -160,7 +161,7 @@ update_sample(struct i915_pmu_sample *sample, u32 unit, u32 val)
 	sample->cur += mul_u32_u32(val, unit);
 }
 
-static void engines_sample(struct drm_i915_private *dev_priv)
+static void engines_sample(struct drm_i915_private *dev_priv, u64 period)
 {
 	struct intel_engine_cs *engine;
 	enum intel_engine_id id;
@@ -183,7 +184,7 @@ static void engines_sample(struct drm_i915_private *dev_priv)
 		val = !i915_seqno_passed(current_seqno, last_seqno);
 
 		update_sample(&engine->pmu.sample[I915_SAMPLE_BUSY],
-			      PERIOD, val);
+			      period, val);
 
 		if (val && (engine->pmu.enable &
 		    (BIT(I915_SAMPLE_WAIT) | BIT(I915_SAMPLE_SEMA)))) {
@@ -195,10 +196,10 @@ static void engines_sample(struct drm_i915_private *dev_priv)
 		}
 
 		update_sample(&engine->pmu.sample[I915_SAMPLE_WAIT],
-			      PERIOD, !!(val & RING_WAIT));
+			      period, !!(val & RING_WAIT));
 
 		update_sample(&engine->pmu.sample[I915_SAMPLE_SEMA],
-			      PERIOD, !!(val & RING_WAIT_SEMAPHORE));
+			      period, !!(val & RING_WAIT_SEMAPHORE));
 	}
 
 	if (fw)
@@ -207,7 +208,7 @@ static void engines_sample(struct drm_i915_private *dev_priv)
 	intel_runtime_pm_put(dev_priv);
 }
 
-static void frequency_sample(struct drm_i915_private *dev_priv)
+static void frequency_sample(struct drm_i915_private *dev_priv, u64 period)
 {
 	if (dev_priv->pmu.enable &
 	    config_enabled_mask(I915_PMU_ACTUAL_FREQUENCY)) {
@@ -237,12 +238,17 @@ static enum hrtimer_restart i915_sample(struct hrtimer *hrtimer)
 {
 	struct drm_i915_private *i915 =
 		container_of(hrtimer, struct drm_i915_private, pmu.timer);
+	ktime_t now, period;
 
 	if (!READ_ONCE(i915->pmu.timer_enabled))
 		return HRTIMER_NORESTART;
 
-	engines_sample(i915);
-	frequency_sample(i915);
+	now = ktime_get();
+	period = ktime_sub(now, i915->pmu.timestamp);
+	i915->pmu.timestamp = now;
+
+	engines_sample(i915, period);
+	frequency_sample(i915, period);
 
 	hrtimer_forward_now(hrtimer, ns_to_ktime(PERIOD));
 	return HRTIMER_RESTART;
diff --git a/drivers/gpu/drm/i915/i915_pmu.h b/drivers/gpu/drm/i915/i915_pmu.h
index 2ba735299f7c..0f1e4642077e 100644
--- a/drivers/gpu/drm/i915/i915_pmu.h
+++ b/drivers/gpu/drm/i915/i915_pmu.h
@@ -52,6 +52,10 @@ struct i915_pmu {
 	 * @timer: Timer for internal i915 PMU sampling.
 	 */
 	struct hrtimer timer;
+	/**
+	 * @timestamp: Timestamp of last internal i915 PMU sampling.
+	 */
+	ktime_t timestamp;
 	/**
 	 * @enable: Bitmask of all currently enabled events.
 	 *
-- 
2.17.0