[Intel-gfx] [PATCH i-g-t] i915/gem_exec_balancer: Race breadcrumb signaling against timeslicing

[Chris Wilson]

This is an attempt to chase down some preempt-to-busy races with
breadcrumb signaling on the virtual engines. By using more semaphore
spinners than available engines, we encourage very short timeslices, and
we make each batch of random duration to try and coincide the end of a
batch with the context being scheduled out.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 tests/i915/gem_exec_balancer.c | 109 +++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)

diff --git a/tests/i915/gem_exec_balancer.c b/tests/i915/gem_exec_balancer.c
index c5c0055fc..e4d9e0464 100644
--- a/tests/i915/gem_exec_balancer.c
+++ b/tests/i915/gem_exec_balancer.c
@@ -2240,6 +2240,112 @@ static void hog(int i915)
 	gem_quiescent_gpu(i915);
 }
 
+static uint32_t sema_create(int i915, uint64_t addr, uint32_t **x)
+{
+	uint32_t handle = gem_create(i915, 4096);
+
+	*x = gem_mmap__device_coherent(i915, handle, 0, 4096, PROT_WRITE);
+	for (int n = 1; n <= 32; n++) {
+		uint32_t *cs = *x + n * 16;
+
+		*cs++ = MI_SEMAPHORE_WAIT |
+			MI_SEMAPHORE_POLL |
+			MI_SEMAPHORE_SAD_GTE_SDD |
+			(4 - 2);
+		*cs++ = n;
+		*cs++ = addr;
+		*cs++ = addr >> 32;
+
+		*cs++ = MI_BATCH_BUFFER_END;
+	}
+
+	return handle;
+}
+
+static uint32_t *sema(int i915, uint32_t ctx)
+{
+	uint32_t *ctl;
+	struct drm_i915_gem_exec_object2 batch = {
+		.handle = sema_create(i915, 64 << 20, &ctl),
+		.offset = 64 << 20,
+		.flags = EXEC_OBJECT_PINNED
+	};
+	struct drm_i915_gem_execbuffer2 execbuf = {
+		.buffers_ptr = to_user_pointer(&batch),
+		.buffer_count = 1,
+		.rsvd1 = gem_context_clone_with_engines(i915, ctx),
+	};
+	int64_t poll = 1;
+
+	for (int n = 1; n <= 32; n++) {
+		execbuf.batch_start_offset = 64 * n,
+		gem_execbuf(i915, &execbuf);
+		/* Force a breadcrumb to be installed on each request */
+		gem_wait(i915, batch.handle, &poll);
+	}
+
+	gem_context_destroy(i915, execbuf.rsvd1);
+
+	igt_assert(gem_bo_busy(i915, batch.handle));
+	gem_close(i915, batch.handle);
+
+	return ctl;
+}
+
+static void __waits(int i915, int timeout, uint32_t ctx, unsigned int count)
+{
+	uint32_t *semaphores[count + 1];
+
+	for (int i = 0; i <= count; i++)
+		semaphores[i] = sema(i915, ctx);
+
+	igt_until_timeout(timeout) {
+		int i = rand() % (count + 1);
+
+		if ((*semaphores[i] += rand() % 32) >= 32) {
+			munmap(semaphores[i], 4096);
+			semaphores[i] = sema(i915, ctx);
+		}
+	}
+
+	for (int i = 0; i <= count; i++) {
+		*semaphores[i] = 0xffffffff;
+		munmap(semaphores[i], 4096);
+	}
+}
+
+static void waits(int i915, int timeout)
+{
+	igt_require(gem_scheduler_has_preemption(i915));
+	igt_require(gem_scheduler_has_semaphores(i915));
+
+	for (int class = 0; class < 32; class++) {
+		struct i915_engine_class_instance *ci;
+		unsigned int count;
+		uint32_t ctx;
+
+		ci = list_engines(i915, 1u << class, &count);
+		if (!ci)
+			continue;
+
+		if (count < 2) {
+			free(ci);
+			continue;
+		}
+
+		ctx = load_balancer_create(i915, ci, count);
+
+		__waits(i915, timeout, ctx, count);
+
+		gem_context_destroy(i915, ctx);
+		igt_waitchildren();
+
+		free(ci);
+	}
+
+	gem_quiescent_gpu(i915);
+}
+
 static void nop(int i915)
 {
 	struct drm_i915_gem_exec_object2 batch = {
@@ -2729,6 +2835,9 @@ igt_main
 	igt_subtest("hog")
 		hog(i915);
 
+	igt_subtest("waits")
+		waits(i915, 5);
+
 	igt_subtest("smoke")
 		smoketest(i915, 20);
 
-- 
2.28.0.rc0