[PATCH 03 of 11 v3] xen: sched_credit: when picking, make sure we get an idle one, if any

[Dario Faggioli <dario.faggioli@citrix.com>]

The pcpu picking algorithm treats two threads of a SMT core the same.
More specifically, if one is idle and the other one is busy, they both
will be assigned a weight of 1. Therefore, when picking begins, if the
first target pcpu is the busy thread (and if there are no other idle
pcpu than its sibling), that will never change.

This change fixes this by ensuring that, before entering the core of
the picking algorithm, the target pcpu is an idle one (if there is an
idle pcpu at all, of course).

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -526,6 +526,18 @@ static int
     if ( vc->processor == cpu && IS_RUNQ_IDLE(cpu) )
         cpumask_set_cpu(cpu, &idlers);
     cpumask_and(&cpus, &cpus, &idlers);
+
+    /*
+     * It is important that cpu points to an idle processor, if a suitable
+     * one exists (and we can use cpus to check and, possibly, choose a new
+     * CPU, as we just &&-ed it with idlers). In fact, if we are on SMT, and
+     * cpu points to a busy thread with an idle sibling, both the threads
+     * will be considered the same, from the "idleness" calculation point
+     * of view", preventing vcpu from being moved to the thread that is
+     * actually idle.
+     */
+    if ( !cpumask_empty(&cpus) && !cpumask_test_cpu(cpu, &cpus) )
+        cpu = cpumask_cycle(cpu, &cpus);
     cpumask_clear_cpu(cpu, &cpus);
 
     while ( !cpumask_empty(&cpus) )