[igt-dev] [PATCH i-g-t] i915/gem_exec_balancer: Check for scheduling bonded-pairs on the same engine

[Chris Wilson]

The expectation for bonded submission is that they are run concurrently,
in parallel on multiple engines. However, given a lack of constraints in
the scheduler's selection combined with timeslicing could mean that the
bonded requests could be run in opposite order on the same engine. With
just the right pair of requests, this can cause a GPU hang (or at least
trigger hangchecker), best (worst) case would be execution running
several times slower than ideal.

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 | 151 +++++++++++++++++++++++++++++++++
 1 file changed, 151 insertions(+)

diff --git a/tests/i915/gem_exec_balancer.c b/tests/i915/gem_exec_balancer.c
index 407dc0eca..e4fe75747 100644
--- a/tests/i915/gem_exec_balancer.c
+++ b/tests/i915/gem_exec_balancer.c
@@ -30,6 +30,15 @@
 
 IGT_TEST_DESCRIPTION("Exercise in-kernel load-balancing");
 
+#define MI_SEMAPHORE_WAIT		(0x1c << 23)
+#define   MI_SEMAPHORE_POLL             (1 << 15)
+#define   MI_SEMAPHORE_SAD_GT_SDD       (0 << 12)
+#define   MI_SEMAPHORE_SAD_GTE_SDD      (1 << 12)
+#define   MI_SEMAPHORE_SAD_LT_SDD       (2 << 12)
+#define   MI_SEMAPHORE_SAD_LTE_SDD      (3 << 12)
+#define   MI_SEMAPHORE_SAD_EQ_SDD       (4 << 12)
+#define   MI_SEMAPHORE_SAD_NEQ_SDD      (5 << 12)
+
 #define INSTANCE_COUNT (1 << I915_PMU_SAMPLE_INSTANCE_BITS)
 
 static size_t sizeof_load_balance(int count)
@@ -694,6 +703,145 @@ static void bonded(int i915, unsigned int flags)
 	gem_context_destroy(i915, master);
 }
 
+static unsigned int offset_in_page(void *addr)
+{
+	return (uintptr_t)addr & 4095;
+}
+
+static uint32_t create_semaphore_to_spinner(int i915, igt_spin_t *spin)
+{
+	uint32_t *cs, *map;
+	uint32_t handle;
+
+	handle = gem_create(i915, 4096);
+	cs = map = gem_mmap__cpu(i915, handle, 0, 4096, PROT_WRITE);
+
+	/* Wait until the spinner is running */
+	*cs++ = MI_SEMAPHORE_WAIT |
+		MI_SEMAPHORE_POLL |
+		MI_SEMAPHORE_SAD_NEQ_SDD |
+		(4 - 2);
+	*cs++ = 0;
+	*cs++ = spin->obj[0].offset + 4 * SPIN_POLL_START_IDX;
+	*cs++ = 0;
+
+	/* Then cancel the spinner */
+	*cs++ = MI_STORE_DWORD_IMM;
+	*cs++ = spin->obj[IGT_SPIN_BATCH].offset +
+		offset_in_page(spin->condition);
+	*cs++ = 0;
+	*cs++ = MI_BATCH_BUFFER_END;
+
+	*cs++ = MI_BATCH_BUFFER_END;
+	munmap(map, 4096);
+
+	return handle;
+}
+
+static void bonded_slice(int i915)
+{
+	uint32_t ctx;
+	int *stop;
+
+	igt_require(gem_scheduler_has_semaphores(i915));
+
+	stop = mmap(0, 4096, PROT_WRITE, MAP_SHARED | MAP_ANON, -1, 0);
+	igt_assert(stop != MAP_FAILED);
+
+	ctx = gem_context_create(i915);
+
+	for (int class = 0; class < 32; class++) {
+		struct i915_engine_class_instance *siblings;
+		struct drm_i915_gem_exec_object2 obj[3] = {};
+		struct drm_i915_gem_execbuffer2 eb = {};
+		unsigned int count;
+		igt_spin_t *spin;
+
+		siblings = list_engines(i915, 1u << class, &count);
+		if (!siblings)
+			continue;
+
+		if (count < 2) {
+			free(siblings);
+			continue;
+		}
+
+		/*
+		 * A: semaphore wait on spinner; cancel spinner
+		 * B: unpreemptable spinner
+		 *
+		 * A waits for running ack from B, if scheduled on the same
+		 * engine -> hang.
+		 *
+		 * C+: background load across engines
+		 */
+
+		set_load_balancer(i915, ctx, siblings, count, NULL);
+
+		spin = __igt_spin_new(i915,
+				      .ctx = ctx,
+				      .flags = (IGT_SPIN_NO_PREEMPTION |
+						IGT_SPIN_POLL_RUN));
+		igt_spin_end(spin); /* we just want its address for later */
+		gem_sync(i915, spin->handle);
+		igt_spin_reset(spin);
+
+		obj[0] = spin->obj[0];
+		obj[1] = spin->obj[1];
+		obj[2].handle = create_semaphore_to_spinner(i915, spin);
+
+		eb.buffers_ptr = to_user_pointer(obj);
+		eb.rsvd1 = ctx;
+
+		*stop = 0;
+		igt_fork(child, count + 1) {
+			igt_list_del(&spin->link);
+
+			ctx = gem_context_clone(i915, ctx,
+						I915_CONTEXT_CLONE_ENGINES, 0);
+
+			while (!READ_ONCE(*stop)) {
+				spin = igt_spin_new(i915,
+						    .ctx = ctx,
+						    .engine = (1 + rand() % count),
+						    .flags = IGT_SPIN_POLL_RUN);
+				igt_spin_busywait_until_started(spin);
+				usleep(50000);
+				igt_spin_free(i915, spin);
+			}
+
+			gem_context_destroy(i915, ctx);
+		}
+
+		igt_until_timeout(5) {
+			igt_spin_reset(spin);
+
+			/* A: Submit the semaphore wait */
+			eb.buffer_count = 3;
+			eb.flags = (1 + rand() % count) | I915_EXEC_FENCE_OUT;
+			gem_execbuf_wr(i915, &eb);
+
+			/* B: Submit the spinner (in parallel) */
+			eb.buffer_count = 2;
+			eb.flags = 0 | I915_EXEC_FENCE_SUBMIT;
+			eb.rsvd2 >>= 32;
+			gem_execbuf(i915, &eb);
+			close(eb.rsvd2);
+
+			gem_sync(i915, obj[2].handle);
+		}
+
+		*stop = 1;
+		igt_waitchildren();
+
+		gem_close(i915, obj[2].handle);
+		igt_spin_free(i915, spin);
+	}
+
+	gem_context_destroy(i915, ctx);
+	munmap(stop, 4096);
+}
+
 static void indices(int i915)
 {
 	I915_DEFINE_CONTEXT_PARAM_ENGINES(engines, I915_EXEC_RING_MASK + 1);
@@ -1320,6 +1468,9 @@ igt_main
 	igt_subtest("bonded-cork")
 		bonded(i915, CORK);
 
+	igt_subtest("bonded-slice")
+		bonded_slice(i915);
+
 	igt_fixture {
 		igt_stop_hang_detector();
 	}
-- 
2.23.0