[igt-dev] [PATCH i-g-t] igt/gem_exec_gttfill: Avoid pwrite into busy handle

[Chris Wilson]

The goal of gem_exec_gttfill is to exercise execbuf under heavy GTT
pressure (by trying to execute more objects than may fit into the GTT).
We spread the same set of handles across different processes, with the
result that each would occasionally stall waiting for execution of an
unrelated batch, limiting the pressure we were applying. If we using a
steaming write via a WC pointer, we can avoid the serialisation penalty
and so submit faster.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 tests/gem_exec_gttfill.c | 66 +++++++++++++++++++++++++---------------
 1 file changed, 42 insertions(+), 24 deletions(-)

diff --git a/tests/gem_exec_gttfill.c b/tests/gem_exec_gttfill.c
index 4097e4077..efd612bb6 100644
--- a/tests/gem_exec_gttfill.c
+++ b/tests/gem_exec_gttfill.c
@@ -28,18 +28,25 @@ IGT_TEST_DESCRIPTION("Fill the GTT with batches.");
 
 #define BATCH_SIZE (4096<<10)
 
-static void xchg_u32(void *array, unsigned i, unsigned j)
+struct batch {
+	uint32_t handle;
+	void *ptr;
+};
+
+static void xchg_batch(void *array, unsigned int i, unsigned int j)
 {
-	uint32_t *u32 = array;
-	uint32_t tmp = u32[i];
-	u32[i] = u32[j];
-	u32[j] = tmp;
+	struct batch *batches = array;
+	struct batch tmp;
+
+	tmp = batches[i];
+	batches[i] = batches[j];
+	batches[j] = tmp;
 }
 
 static void submit(int fd, int gen,
 		   struct drm_i915_gem_execbuffer2 *eb,
 		   struct drm_i915_gem_relocation_entry *reloc,
-		   uint32_t *handles, unsigned count)
+		   struct batch *batches, unsigned int count)
 {
 	struct drm_i915_gem_exec_object2 obj;
 	uint32_t batch[16];
@@ -80,7 +87,7 @@ static void submit(int fd, int gen,
 
 	eb->buffers_ptr = to_user_pointer(&obj);
 	for (unsigned i = 0; i < count; i++) {
-		obj.handle = handles[i];
+		obj.handle = batches[i].handle;
 		reloc[0].target_handle = obj.handle;
 		reloc[1].target_handle = obj.handle;
 
@@ -88,8 +95,8 @@ static void submit(int fd, int gen,
 		reloc[0].presumed_offset = obj.offset;
 		reloc[1].presumed_offset = obj.offset;
 
-		gem_write(fd, obj.handle, eb->batch_start_offset,
-			  batch, sizeof(batch));
+		memcpy(batches[i].ptr + eb->batch_start_offset,
+		       batch, sizeof(batch));
 
 		gem_execbuf(fd, eb);
 	}
@@ -103,7 +110,7 @@ static void fillgtt(int fd, unsigned ring, int timeout)
 	struct drm_i915_gem_execbuffer2 execbuf;
 	struct drm_i915_gem_relocation_entry reloc[2];
 	volatile uint64_t *shared;
-	unsigned *handles;
+	struct batch *batches;
 	unsigned engines[16];
 	unsigned nengine;
 	unsigned engine;
@@ -145,29 +152,38 @@ static void fillgtt(int fd, unsigned ring, int timeout)
 	if (gen < 6)
 		execbuf.flags |= I915_EXEC_SECURE;
 
-	handles = calloc(count, sizeof(handles));
-	igt_assert(handles);
-	for (unsigned i = 0; i < count; i++)
-		handles[i] = gem_create(fd, BATCH_SIZE);
+	batches = calloc(count, sizeof(*batches));
+	igt_assert(batches);
+	for (unsigned i = 0; i < count; i++) {
+		batches[i].handle = gem_create(fd, BATCH_SIZE);
+		batches[i].ptr =
+			__gem_mmap__wc(fd, batches[i].handle,
+				       0, BATCH_SIZE, PROT_WRITE);
+		if (!batches[i].ptr) {
+			batches[i].ptr =
+				__gem_mmap__gtt(fd, batches[i].handle,
+						BATCH_SIZE, PROT_WRITE);
+		}
+		igt_require(batches[i].ptr);
+	}
 
 	/* Flush all memory before we start the timer */
-	submit(fd, gen, &execbuf, reloc, handles, count);
+	submit(fd, gen, &execbuf, reloc, batches, count);
 
 	igt_fork(child, nengine) {
 		uint64_t cycles = 0;
 		hars_petruska_f54_1_random_perturb(child);
-		igt_permute_array(handles, count, xchg_u32);
+		igt_permute_array(batches, count, xchg_batch);
 		execbuf.batch_start_offset = child*64;
 		execbuf.flags |= engines[child];
 		igt_until_timeout(timeout) {
-			submit(fd, gen, &execbuf, reloc, handles, count);
+			submit(fd, gen, &execbuf, reloc, batches, count);
 			for (unsigned i = 0; i < count; i++) {
-				uint32_t handle = handles[i];
-				uint64_t buf[2];
+				uint64_t offset, delta;
 
-				gem_read(fd, handle, reloc[1].offset, &buf[0], sizeof(buf[0]));
-				gem_read(fd, handle, reloc[0].delta, &buf[1], sizeof(buf[1]));
-				igt_assert_eq_u64(buf[0], buf[1]);
+				offset = *(uint64_t *)(batches[i].ptr + reloc[1].offset);
+				delta = *(uint64_t *)(batches[i].ptr + reloc[0].delta);
+				igt_assert_eq_u64(offset, delta);
 			}
 			cycles++;
 		}
@@ -176,8 +192,10 @@ static void fillgtt(int fd, unsigned ring, int timeout)
 	}
 	igt_waitchildren();
 
-	for (unsigned i = 0; i < count; i++)
-		gem_close(fd, handles[i]);
+	for (unsigned i = 0; i < count; i++) {
+		munmap(batches[i].ptr, BATCH_SIZE);
+		gem_close(fd, batches[i].handle);
+	}
 
 	shared[nengine] = 0;
 	for (unsigned i = 0; i < nengine; i++)
-- 
2.18.0