[PATCH 2/2] drm/amdgpu: fix fence calculation

[David M Nieto <david.nieto@amd.com>]

The proper metric for fence utilization over several
contexts is an harmonic mean, but such calculation is
prohibitive in kernel space, so the code approximates it.

Because the approximation diverges when one context has a
very small ratio compared with the other context, this change
filter out ratios smaller that 0.01%

Signed-off-by: David M Nieto <david.nieto@amd.com>
Change-Id: I5b6e0ce5f489a5f55855d35354a6a3653e9d613b
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
index 9036c93b4a0c..a26496735080 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
@@ -689,6 +689,8 @@ void amdgpu_ctx_fence_time(struct amdgpu_ctx *ctx, struct amdgpu_ctx_entity *cen
 	}
 }
 
+#define FENCE_USAGE_MIN_RATIO(max, total) (max > 16384ULL*total)
+
 ktime_t amdgpu_ctx_mgr_fence_usage(struct amdgpu_ctx_mgr *mgr, uint32_t hwip,
 		uint32_t idx, uint64_t *elapsed)
 {
@@ -697,17 +699,29 @@ ktime_t amdgpu_ctx_mgr_fence_usage(struct amdgpu_ctx_mgr *mgr, uint32_t hwip,
 	uint32_t id;
 	struct amdgpu_ctx_entity *centity;
 	ktime_t total = 0, max = 0;
+	ktime_t ttotal = 0, tmax = 0;
+
 
 	if (idx >= AMDGPU_MAX_ENTITY_NUM)
 		return 0;
 	idp = &mgr->ctx_handles;
 	mutex_lock(&mgr->lock);
 	idr_for_each_entry(idp, ctx, id) {
+		ttotal = tmax = ktime_set(0, 0);
 		if (!ctx->entities[hwip][idx])
 			continue;
 
 		centity = ctx->entities[hwip][idx];
-		amdgpu_ctx_fence_time(ctx, centity, &total, &max);
+		amdgpu_ctx_fence_time(ctx, centity, &ttotal, &tmax);
+
+		/* Harmonic mean approximation diverges for very small
+		 * values. If ratio < 0.01% ignore
+		 */
+		if (FENCE_USAGE_MIN_RATIO(tmax, ttotal))
+			continue;
+
+		total = ktime_add(total, ttotal);
+		max = ktime_after(tmax, max) ? tmax : max;
 	}
 
 	mutex_unlock(&mgr->lock);
@@ -716,3 +730,5 @@ ktime_t amdgpu_ctx_mgr_fence_usage(struct amdgpu_ctx_mgr *mgr, uint32_t hwip,
 
 	return total;
 }
+
+#undef FENCE_USAGE_MIN_RATIO
-- 
2.17.1

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