drm/i915/gt: Always reset the timeslice after a context switch

Currently, we reset the timer after a pre-eemption event. This has the
side-effect that the timeslice runs into the second context after the
first is completed after a normal promotion event, causing the second
context to be swapped out early and switched for a third context. To be
more fair, we want to reset the clock after promotion as well.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200113214546.1990139-1-chris@chris-wilson.co.uk

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

@@ -1694,7 +1694,7 @@ active_timeslice(const struct intel_engine_cs *engine)
 {
 	const struct i915_request *rq = *engine->execlists.active;
 
-	if (i915_request_completed(rq))
+	if (!rq || i915_request_completed(rq))
 		return 0;
 
 	if (engine->execlists.switch_priority_hint < effective_prio(rq))
@@ -2285,7 +2285,6 @@ static void process_csb(struct intel_engine_cs *engine)
 
 			/* Point active to the new ELSP; prevent overwriting */
 			WRITE_ONCE(execlists->active, execlists->pending);
-			set_timeslice(engine);
 
 			if (!inject_preempt_hang(execlists))
 				ring_set_paused(engine, 0);
@@ -2326,6 +2325,7 @@ static void process_csb(struct intel_engine_cs *engine)
 	} while (head != tail);
 
 	execlists->csb_head = head;
+	set_timeslice(engine);
 
 	/*
 	 * Gen11 has proven to fail wrt global observation point between