drm/i915/pmu: Do not assume fixed hrtimer period

As Chris has discovered on his Ivybridge, and later automated test runs
have confirmed, on most of our platforms hrtimer faced with heavy GPU load
can occasionally become sufficiently imprecise to affect PMU sampling
calculations.

This means we cannot assume sampling frequency is what we asked for, but
we need to measure the interval ourselves.

This patch is similar to Chris' original proposal for per-engine counters,
but instead of introducing a new set to work around the problem with
frequency sampling, it swaps around the way internal frequency accounting
is done. Instead of accumulating current frequency and dividing by
sampling frequency on readout, it accumulates frequency scaled by each
period.

v2:
* Typo in commit message, comment on period calculation and USEC_PER_SEC.
(Chris Wilson)

Testcase: igt/perf_pmu/*busy* # snb, ivb, hsw
Signed-off-by: Tvrtko Ursulin <[e-mail]>
Suggested-by: Chris Wilson <[e-mail]>
Cc: Chris Wilson <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/i915_pmu.c

···
 {
 	if (!i915->pmu.timer_enabled && pmu_needs_timer(i915, true)) {
 		i915->pmu.timer_enabled = true;
+		i915->pmu.timer_last = ktime_get();
 		hrtimer_start_range_ns(&i915->pmu.timer,
 				       ns_to_ktime(PERIOD), 0,
 				       HRTIMER_MODE_REL_PINNED);
···
 }
 
 static void
-update_sample(struct i915_pmu_sample *sample, u32 unit, u32 val)
+add_sample(struct i915_pmu_sample *sample, u32 val)
 {
-	sample->cur += mul_u32_u32(val, unit);
+	sample->cur += val;
 }
 
-static void engines_sample(struct drm_i915_private *dev_priv)
+static void
+engines_sample(struct drm_i915_private *dev_priv, unsigned int period_ns)
 {
 	struct intel_engine_cs *engine;
 	enum intel_engine_id id;
···
 
 		val = !i915_seqno_passed(current_seqno, last_seqno);
 
-		update_sample(&engine->pmu.sample[I915_SAMPLE_BUSY],
-			      PERIOD, val);
+		if (val)
+			add_sample(&engine->pmu.sample[I915_SAMPLE_BUSY],
+				   period_ns);
 
 		if (val && (engine->pmu.enable &
 		    (BIT(I915_SAMPLE_WAIT) | BIT(I915_SAMPLE_SEMA)))) {
···
 			val = 0;
 		}
 
-		update_sample(&engine->pmu.sample[I915_SAMPLE_WAIT],
-			      PERIOD, !!(val & RING_WAIT));
+		if (val & RING_WAIT)
+			add_sample(&engine->pmu.sample[I915_SAMPLE_WAIT],
+				   period_ns);
 
-		update_sample(&engine->pmu.sample[I915_SAMPLE_SEMA],
-			      PERIOD, !!(val & RING_WAIT_SEMAPHORE));
+		if (val & RING_WAIT_SEMAPHORE)
+			add_sample(&engine->pmu.sample[I915_SAMPLE_SEMA],
+				   period_ns);
 	}
 
 	if (fw)
···
 	intel_runtime_pm_put(dev_priv);
 }
 
-static void frequency_sample(struct drm_i915_private *dev_priv)
+static void
+add_sample_mult(struct i915_pmu_sample *sample, u32 val, u32 mul)
+{
+	sample->cur += mul_u32_u32(val, mul);
+}
+
+static void
+frequency_sample(struct drm_i915_private *dev_priv, unsigned int period_ns)
 {
 	if (dev_priv->pmu.enable &
 	    config_enabled_mask(I915_PMU_ACTUAL_FREQUENCY)) {
···
 			intel_runtime_pm_put(dev_priv);
 		}
 
-		update_sample(&dev_priv->pmu.sample[__I915_SAMPLE_FREQ_ACT],
-			      1, intel_gpu_freq(dev_priv, val));
+		add_sample_mult(&dev_priv->pmu.sample[__I915_SAMPLE_FREQ_ACT],
+				intel_gpu_freq(dev_priv, val),
+				period_ns / 1000);
 	}
 
 	if (dev_priv->pmu.enable &
 	    config_enabled_mask(I915_PMU_REQUESTED_FREQUENCY)) {
-		update_sample(&dev_priv->pmu.sample[__I915_SAMPLE_FREQ_REQ], 1,
-			      intel_gpu_freq(dev_priv,
-					     dev_priv->gt_pm.rps.cur_freq));
+		add_sample_mult(&dev_priv->pmu.sample[__I915_SAMPLE_FREQ_REQ],
+				intel_gpu_freq(dev_priv,
+					       dev_priv->gt_pm.rps.cur_freq),
+				period_ns / 1000);
 	}
 }
 
···
 {
 	struct drm_i915_private *i915 =
 		container_of(hrtimer, struct drm_i915_private, pmu.timer);
+	unsigned int period_ns;
+	ktime_t now;
 
 	if (!READ_ONCE(i915->pmu.timer_enabled))
 		return HRTIMER_NORESTART;
 
-	engines_sample(i915);
-	frequency_sample(i915);
+	now = ktime_get();
+	period_ns = ktime_to_ns(ktime_sub(now, i915->pmu.timer_last));
+	i915->pmu.timer_last = now;
 
-	hrtimer_forward_now(hrtimer, ns_to_ktime(PERIOD));
+	/*
+	 * Strictly speaking the passed in period may not be 100% accurate for
+	 * all internal calculation, since some amount of time can be spent on
+	 * grabbing the forcewake. However the potential error from timer call-
+	 * back delay greatly dominates this so we keep it simple.
+	 */
+	engines_sample(i915, period_ns);
+	frequency_sample(i915, period_ns);
+
+	hrtimer_forward(hrtimer, now, ns_to_ktime(PERIOD));
+
 	return HRTIMER_RESTART;
 }
 
···
 		case I915_PMU_ACTUAL_FREQUENCY:
 			val =
 			   div_u64(i915->pmu.sample[__I915_SAMPLE_FREQ_ACT].cur,
-				   FREQUENCY);
+				   USEC_PER_SEC /* to MHz */);
 			break;
 		case I915_PMU_REQUESTED_FREQUENCY:
 			val =
 			   div_u64(i915->pmu.sample[__I915_SAMPLE_FREQ_REQ].cur,
-				   FREQUENCY);
+				   USEC_PER_SEC /* to MHz */);
 			break;
 		case I915_PMU_INTERRUPTS:
 			val = count_interrupts(i915);

drivers/gpu/drm/i915/i915_pmu.h

···
 	 * event types.
 	 */
 	u64 enable;
+
+	/**
+	 * @timer_last:
+	 *
+	 * Timestmap of the previous timer invocation.
+	 */
+	ktime_t timer_last;
+
 	/**
 	 * @enable_count: Reference counts for the enabled events.
 	 *