[Intel-gfx] [PATCH igt] igt/perf_pmu: Tighten semaphore-wait measurement

[Chris Wilson]

Record the before/after semaphore-wait values around the sleep to try to
reduce the inaccuracy from scheduler delays. Previously, the samples
were taken before submitting the batch and then after synchronising its
completion. The measurement will then be the total that the semaphore
was being sampled, but with the extra syscalls intervening may have
drifted from the sleep duration. To further reduce the disparity, wait
for the batch to start executing before taking our samples.

References: https://bugs.freedesktop.org/show_bug.cgi?id=104013
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 tests/perf_pmu.c | 54 ++++++++++++++++++++++++++++++++----------------------
 1 file changed, 32 insertions(+), 22 deletions(-)

diff --git a/tests/perf_pmu.c b/tests/perf_pmu.c
index cb59bf5d..e872f4e5 100644
--- a/tests/perf_pmu.c
+++ b/tests/perf_pmu.c
@@ -355,13 +355,13 @@ no_sema(int gem_fd, const struct intel_execution_engine2 *e, bool busy)
 static void
 sema_wait(int gem_fd, const struct intel_execution_engine2 *e)
 {
-	struct drm_i915_gem_relocation_entry reloc = { };
-	struct drm_i915_gem_execbuffer2 eb = { };
-	struct drm_i915_gem_exec_object2 obj[2];
+	struct drm_i915_gem_relocation_entry reloc[2] = {};
+	struct drm_i915_gem_exec_object2 obj[2] = {};
+	struct drm_i915_gem_execbuffer2 eb = {};
 	uint32_t bb_handle, obj_handle;
 	unsigned long slept;
 	uint32_t *obj_ptr;
-	uint32_t batch[6];
+	uint32_t batch[16];
 	uint64_t val[2];
 	int fd;
 
@@ -378,28 +378,35 @@ sema_wait(int gem_fd, const struct intel_execution_engine2 *e)
 
 	obj_ptr = gem_mmap__wc(gem_fd, obj_handle, 0, 4096, PROT_WRITE);
 
-	batch[0] = MI_SEMAPHORE_WAIT |
+	batch[0] = MI_STORE_DWORD_IMM;
+	batch[1] = sizeof(*obj_ptr);
+	batch[2] = 0;
+	batch[3] = 1;
+	batch[4] = MI_SEMAPHORE_WAIT |
 		   MI_SEMAPHORE_POLL |
 		   MI_SEMAPHORE_SAD_GTE_SDD;
-	batch[1] = 1;
-	batch[2] = 0x0;
-	batch[3] = 0x0;
-	batch[4] = MI_NOOP;
-	batch[5] = MI_BATCH_BUFFER_END;
+	batch[5] = 1;
+	batch[6] = 0x0;
+	batch[7] = 0x0;
+	batch[8] = MI_BATCH_BUFFER_END;
 
 	gem_write(gem_fd, bb_handle, 0, batch, sizeof(batch));
 
-	reloc.target_handle = obj_handle;
-	reloc.offset = 2 * sizeof(uint32_t);
-	reloc.read_domains = I915_GEM_DOMAIN_RENDER;
+	reloc[0].target_handle = obj_handle;
+	reloc[0].offset = 1 * sizeof(uint32_t);
+	reloc[0].read_domains = I915_GEM_DOMAIN_RENDER;
+	reloc[0].write_domain = I915_GEM_DOMAIN_RENDER;
+	reloc[0].delta = sizeof(*obj_ptr);
 
-	memset(obj, 0, sizeof(obj));
+	reloc[1].target_handle = obj_handle;
+	reloc[1].offset = 6 * sizeof(uint32_t);
+	reloc[1].read_domains = I915_GEM_DOMAIN_RENDER;
 
 	obj[0].handle = obj_handle;
 
 	obj[1].handle = bb_handle;
-	obj[1].relocation_count = 1;
-	obj[1].relocs_ptr = to_user_pointer(&reloc);
+	obj[1].relocation_count = 2;
+	obj[1].relocs_ptr = to_user_pointer(reloc);
 
 	eb.buffer_count = 2;
 	eb.buffers_ptr = to_user_pointer(obj);
@@ -413,18 +420,21 @@ sema_wait(int gem_fd, const struct intel_execution_engine2 *e)
 
 	fd = open_pmu(I915_PMU_ENGINE_SEMA(e->class, e->instance));
 
-	val[0] = pmu_read_single(fd);
-
 	gem_execbuf(gem_fd, &eb);
+	do { /* wait for the batch to start executing */
+		usleep(5e3);
+	} while (!obj_ptr[1]);
+	usleep(5e3); /* wait for the register sampling */
 
+	val[0] = pmu_read_single(fd);
 	slept = measured_usleep(batch_duration_ns / 1000);
+	val[1] = pmu_read_single(fd);
+	igt_debug("slept %.3fms, sampled %.3fms\n",
+		  slept*1e-6, (val[1] - val[0])*1e-6);
 
-	*obj_ptr = 1;
-
+	obj_ptr[0] = 1;
 	gem_sync(gem_fd, bb_handle);
 
-	val[1] = pmu_read_single(fd);
-
 	munmap(obj_ptr, 4096);
 	gem_close(gem_fd, obj_handle);
 	gem_close(gem_fd, bb_handle);
-- 
2.15.1