drm/i915/gt: Wait longer for tasks in migrate selftest

The thread_global_copy subtest of the live migrate selftest creates a
large number of threads and waits 10ms for them all to start.  This is
not enough time to wait for the threaded tasks to start, as some may
need to wait for additional ring space to be granted.  Threads that do
so are at risk of getting stopped (signaled) in the middle of waiting
for additional space, which can result in ERESTARTSYS getting reported
erroneously by i915_request_wait.

Instead of waiting a flat 10ms for the threads to start, wait 10ms per
thread.  This grants enough of a buffer for each thread to wait for
additional ring space when needed.
Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230828192852.2894671-2-jonathan.cavitt@intel.com

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

@@ -710,7 +710,7 @@ static int threaded_migrate(struct intel_migrate *migrate,
 		thread[i].tsk = tsk;
 	}
 
-	msleep(10); /* start all threads before we kthread_stop() */
+	msleep(10 * n_cpus); /* start all threads before we kthread_stop() */
 
 	for (i = 0; i < n_cpus; ++i) {
 		struct task_struct *tsk = thread[i].tsk;