xen: credit1: don't rate limit context switches in case of yields

Rate limiting has been primarily introduced to avoid too
heavy context switch rate due to interrupts, and, in
general, asynchronous events.

If a vcpu "voluntarily" yields, we really should let it
give up the cpu for a while.

In fact, it may be that it is yielding because it's about
to start spinning, and there's few point in forcing a vcpu
to spin for (potentially) the entire rate-limiting period.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>

xen/common/sched_credit.c

@@ -1802,9 +1802,16 @@ csched_schedule(
      *   cpu and steal it.
      */
 
-    /* If we have schedule rate limiting enabled, check to see
-     * how long we've run for. */
-    if ( !tasklet_work_scheduled
+    /*
+     * If we have schedule rate limiting enabled, check to see
+     * how long we've run for.
+     *
+     * If scurr is yielding, however, we don't let rate limiting kick in.
+     * In fact, it may be the case that scurr is about to spin, and there's
+     * no point forcing it to do so until rate limiting expires.
+     */
+    if ( !test_bit(CSCHED_FLAG_VCPU_YIELD, &scurr->flags)
+         && !tasklet_work_scheduled
          && prv->ratelimit_us
          && vcpu_runnable(current)
          && !is_idle_vcpu(current)