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

[David M Nieto]

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 at amd.com>
Change-Id: I5b6e0ce5f489a5f55855d35354a6a3653e9d613b
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c | 13 ++++++++++++-
 drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.h |  1 +
 2 files changed, 13 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..89ee464b9424 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
@@ -698,16 +698,27 @@ ktime_t amdgpu_ctx_mgr_fence_usage(struct amdgpu_ctx_mgr *mgr, uint32_t hwip,
 	struct amdgpu_ctx_entity *centity;
 	ktime_t total = 0, max = 0;
 
+
 	if (idx >= AMDGPU_MAX_ENTITY_NUM)
 		return 0;
 	idp = &mgr->ctx_handles;
 	mutex_lock(&mgr->lock);
 	idr_for_each_entry(idp, ctx, id) {
+		ktime_t 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 (AMDGPU_CTX_FENCE_USAGE_MIN_RATIO(tmax, ttotal))
+			continue;
+
+		total = ktime_add(total, ttotal);
+		max = ktime_after(tmax, max) ? tmax : max;
 	}
 
 	mutex_unlock(&mgr->lock);
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.h
index 10dcf59a5c6b..3541dfb059ec 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.h
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.h
@@ -30,6 +30,7 @@ struct drm_file;
 struct amdgpu_fpriv;
 
 #define AMDGPU_MAX_ENTITY_NUM 4
+#define AMDGPU_CTX_FENCE_USAGE_MIN_RATIO(max, total) (max > 16384ULL*total)
 
 struct amdgpu_ctx_entity {
 	uint64_t		sequence;
-- 
2.17.1