[igt-dev] [PATCH i-g-t] i915/gem_ctx_switch: Use minimum qlen over all engines and measure switches

[Chris Wilson]

Not all engines are created equal, and our weighting ends up favouring
the many faster xCS rings at the expense of RCS. Our qlen estimation
also failed to factor in the context switch overhead, which is a
significant factor for nop batches. So we oversubscribe the number of
batches submitted to RCS and end up waiting for those to complete at the
end of our subtest timeslice.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Caz Yokoyama <caz.yokoyama at intel.com>
---
 tests/i915/gem_ctx_switch.c | 39 +++++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/tests/i915/gem_ctx_switch.c b/tests/i915/gem_ctx_switch.c
index 1208cb8d7..87e13b915 100644
--- a/tests/i915/gem_ctx_switch.c
+++ b/tests/i915/gem_ctx_switch.c
@@ -26,6 +26,7 @@
  */
 
 #include "igt.h"
+#include <limits.h>
 #include <unistd.h>
 #include <stdlib.h>
 #include <stdint.h>
@@ -58,29 +59,50 @@ static int measure_qlen(int fd,
 {
 	const struct drm_i915_gem_exec_object2 * const obj =
 		(struct drm_i915_gem_exec_object2 *)(uintptr_t)execbuf->buffers_ptr;
-	int qlen = 64;
+	uint32_t ctx[64];
+	int min = INT_MAX, max = 0;
+
+	for (int i = 0; i < ARRAY_SIZE(ctx); i++)
+		ctx[i] = gem_context_create(fd);
 
 	for (unsigned int n = 0; n < nengine; n++) {
 		uint64_t saved = execbuf->flags;
 		struct timespec tv = {};
+		int q;
 
 		execbuf->flags |= engine[n];
 
-		igt_nsec_elapsed(&tv);
-		for (int loop = 0; loop < qlen; loop++)
+		for (int i = 0; i < ARRAY_SIZE(ctx); i++) {
+			execbuf->rsvd1 = ctx[i];
 			gem_execbuf(fd, execbuf);
+		}
 		gem_sync(fd, obj->handle);
 
-		execbuf->flags = saved;
+		igt_nsec_elapsed(&tv);
+		for (int i = 0; i < ARRAY_SIZE(ctx); i++) {
+			execbuf->rsvd1 = ctx[i];
+			gem_execbuf(fd, execbuf);
+		}
+		gem_sync(fd, obj->handle);
 
 		/*
 		 * Be conservative and aim not to overshoot timeout, so scale
 		 * down by 8 for hopefully a max of 12.5% error.
 		 */
-		qlen = qlen * timeout * 1e9 / igt_nsec_elapsed(&tv) / 8 + 1;
+		q = ARRAY_SIZE(ctx) * timeout * 1e9 / igt_nsec_elapsed(&tv) / 8 + 1;
+		if (q < min)
+			min = q;
+		if (q > max)
+			max = q;
+
+		execbuf->flags = saved;
 	}
 
-	return qlen;
+	for (int i = 0; i < ARRAY_SIZE(ctx); i++)
+		gem_context_destroy(fd, ctx[i]);
+
+	igt_debug("Estimated qlen: {min:%d, max:%d}\n", min, max);
+	return min;
 }
 
 static void single(int fd, uint32_t handle,
@@ -259,9 +281,10 @@ static void all(int fd, uint32_t handle, unsigned flags, int timeout)
 				clock_gettime(CLOCK_MONOTONIC, &now);
 				gem_close(fd, obj[0].handle);
 
-				igt_info("[%d:%d] %s: %'u cycles: %.3fus%s\n",
+				igt_info("[%d:%d] %s: %'u cycles: %.3fus%s (elapsed: %.3fs)\n",
 					 nctx, child, name[child], count, elapsed(&start, &now)*1e6 / count,
-					 flags & INTERRUPTIBLE ? " (interruptible)" : "");
+					 flags & INTERRUPTIBLE ? " (interruptible)" : "",
+					 elapsed(&start, &now));
 			}
 			igt_waitchildren();
 		}
-- 
2.20.1