drm/i915/selftests: Start kthreads before stopping

An interesting observation made with our parallel selftests was that on
our small/single cpu systems we would call kthread_stop() before the
kthreads were spawned. If this happens, the kthread is never run at all;
completely bypassing the test.

A simple yield() from the parent will ensure that all children have the
opportunity to start before we reap them.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191101084940.31838-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c

@@ -345,6 +345,8 @@ static int live_parallel_switch(void *arg)
 			get_task_struct(data[n].tsk);
 		}
 
+		yield(); /* start all threads before we kthread_stop() */
+
 		for (n = 0; n < count; n++) {
 			int status;
 

drivers/gpu/drm/i915/gem/selftests/i915_gem_object_blt.c

@@ -473,6 +473,8 @@ static int igt_threaded_blt(struct drm_i915_private *i915,
 		get_task_struct(tsk[i]);
 	}
 
+	yield(); /* start all threads before we kthread_stop() */
+
 	for (i = 0; i < n_cpus; ++i) {
 		int status;
 

drivers/gpu/drm/i915/gt/selftest_hangcheck.c

@@ -826,6 +826,8 @@ static int __igt_reset_engines(struct intel_gt *gt,
 			get_task_struct(tsk);
 		}
 
+		yield(); /* start all threads before we begin */
+
 		intel_engine_pm_get(engine);
 		set_bit(I915_RESET_ENGINE + id, &gt->reset.flags);
 		do {

drivers/gpu/drm/i915/gt/selftest_lrc.c

@@ -2252,6 +2252,8 @@ static int smoke_crescendo(struct preempt_smoke *smoke, unsigned int flags)
 		get_task_struct(tsk[id]);
 	}
 
+	yield(); /* start all threads before we kthread_stop() */
+
 	count = 0;
 	for_each_engine(engine, smoke->gt, id) {
 		int status;

drivers/gpu/drm/i915/selftests/i915_request.c

@@ -464,6 +464,7 @@ static int mock_breadcrumbs_smoketest(void *arg)
 		get_task_struct(threads[n]);
 	}
 
+	yield(); /* start all threads before we begin */
 	msleep(jiffies_to_msecs(i915_selftest.timeout_jiffies));
 
 	for (n = 0; n < ncpus; n++) {
@@ -1158,6 +1159,8 @@ static int live_parallel_engines(void *arg)
 			get_task_struct(tsk[idx++]);
 		}
 
+		yield(); /* start all threads before we kthread_stop() */
+
 		idx = 0;
 		for_each_uabi_engine(engine, i915) {
 			int status;
@@ -1314,6 +1317,7 @@ static int live_breadcrumbs_smoketest(void *arg)
 		idx++;
 	}
 
+	yield(); /* start all threads before we begin */
 	msleep(jiffies_to_msecs(i915_selftest.timeout_jiffies));
 
 out_flush: