xen: credit2: only reset credit on reset condition

The condition for a Credit2 scheduling epoch coming to an
end is that the vcpu at the front of the runqueue has negative
credits. However, it is possible, that runq_candidate() does
not actually return to the scheduler the first vcpu in the
runqueue (e.g., because such vcpu can't run on the cpu that
is going through the scheduler, because of hard-affinity).

If that happens, we should not trigger a credit reset, or we
risk altering the lenght of a scheduler epoch, wrt what the
original idea of the algorithm was.

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

xen/common/sched_credit2.c

@@ -2244,12 +2244,15 @@ void __dump_execstate(void *unused);
 static struct csched2_vcpu *
 runq_candidate(struct csched2_runqueue_data *rqd,
                struct csched2_vcpu *scurr,
-               int cpu, s_time_t now)
+               int cpu, s_time_t now,
+               unsigned int *skipped)
 {
     struct list_head *iter;
     struct csched2_vcpu *snext = NULL;
     struct csched2_private *prv = CSCHED2_PRIV(per_cpu(scheduler, cpu));
 
+    *skipped = 0;
+
     /* Default to current if runnable, idle otherwise */
     if ( vcpu_runnable(scurr->vcpu) )
         snext = scurr;
@@ -2273,7 +2276,10 @@ runq_candidate(struct csched2_runqueue_data *rqd,
 
         /* Only consider vcpus that are allowed to run on this processor. */
         if ( !cpumask_test_cpu(cpu, svc->vcpu->cpu_hard_affinity) )
+        {
+            (*skipped)++;
             continue;
+        }
 
         /*
          * If a vcpu is meant to be picked up by another processor, and such
@@ -2282,6 +2288,7 @@ runq_candidate(struct csched2_runqueue_data *rqd,
         if ( svc->tickled_cpu != -1 && svc->tickled_cpu != cpu &&
              cpumask_test_cpu(svc->tickled_cpu, &rqd->tickled) )
         {
+            (*skipped)++;
             SCHED_STAT_CRANK(deferred_to_tickled_cpu);
             continue;
         }
@@ -2291,6 +2298,7 @@ runq_candidate(struct csched2_runqueue_data *rqd,
         if ( svc->vcpu->processor != cpu
              && snext->credit + CSCHED2_MIGRATE_RESIST > svc->credit )
         {
+            (*skipped)++;
             SCHED_STAT_CRANK(migrate_resisted);
             continue;
         }
@@ -2308,11 +2316,12 @@ runq_candidate(struct csched2_runqueue_data *rqd,
     {
         struct {
             unsigned vcpu:16, dom:16;
-            unsigned tickled_cpu;
+            unsigned tickled_cpu, skipped;
         } d;
         d.dom = snext->vcpu->domain->domain_id;
         d.vcpu = snext->vcpu->vcpu_id;
         d.tickled_cpu = snext->tickled_cpu;
+        d.skipped = *skipped;
         __trace_var(TRC_CSCHED2_RUNQ_CANDIDATE, 1,
                     sizeof(d),
                     (unsigned char *)&d);
@@ -2336,6 +2345,7 @@ csched2_schedule(
     struct csched2_runqueue_data *rqd;
     struct csched2_vcpu * const scurr = CSCHED2_VCPU(current);
     struct csched2_vcpu *snext = NULL;
+    unsigned int skipped_vcpus = 0;
     struct task_slice ret;
 
     SCHED_STAT_CRANK(schedule);
@@ -2385,7 +2395,7 @@ csched2_schedule(
         snext = CSCHED2_VCPU(idle_vcpu[cpu]);
     }
     else
-        snext = runq_candidate(rqd, scurr, cpu, now);
+        snext = runq_candidate(rqd, scurr, cpu, now, &skipped_vcpus);
 
     /* If switching from a non-idle runnable vcpu, put it
      * back on the runqueue. */
@@ -2409,8 +2419,21 @@ csched2_schedule(
             __set_bit(__CSFLAG_scheduled, &snext->flags);
         }
 
-        /* Check for the reset condition */
-        if ( snext->credit <= CSCHED2_CREDIT_RESET )
+        /*
+         * The reset condition is "has a scheduler epoch come to an end?".
+         * The way this is enforced is checking whether the vcpu at the top
+         * of the runqueue has negative credits. This means the epochs have
+         * variable lenght, as in one epoch expores when:
+         *  1) the vcpu at the top of the runqueue has executed for
+         *     around 10 ms (with default parameters);
+         *  2) no other vcpu with higher credits wants to run.
+         *
+         * Here, where we want to check for reset, we need to make sure the
+         * proper vcpu is being used. In fact, runqueue_candidate() may have
+         * not returned the first vcpu in the runqueue, for various reasons
+         * (e.g., affinity). Only trigger a reset when it does.
+         */
+        if ( skipped_vcpus == 0 && snext->credit <= CSCHED2_CREDIT_RESET )
         {
             reset_credit(ops, cpu, now, snext);
             balance_load(ops, cpu, now);