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[Xen-devel] [PATCH] xen: sched: Credit2: during scheduling, update the idle mask before using it

Load balancing, when happening, at the end of a "scheduler epoch", can
trigger vcpu migration, which in its turn may call runq_tickle(). If the
cpu where this happens was idle, but we're now going to schedule a vcpu
on it, let's update the runq's idle cpus mask accordingly _before_ doing
load balancing.

Not doing that, in fact, may cause runq_tickle() to think that the cpu
is still idle, and tickle it to go pick up a vcpu from the runqueue,
which might be wrong/unideal.

Signed-off-by: Dario Faggioli <dfaggioli@xxxxxxxx>
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Backporting: this does not fix a system crash. However, it fixes the
behavior of the scheduler --which I'd call wrong more than just suboptimal.

Therefore, I'd be inclined to ask for this to be backported. It should
be fairly straightforward, but as usual, I'm up for helping with that.

Thanks and Regards,
 xen/common/sched_credit2.c |   14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 2b16bcea21..72fed2dd18 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -3554,6 +3554,13 @@ csched2_schedule(
             __set_bit(__CSFLAG_scheduled, &snext->flags);
+        /* Clear the idle mask if necessary */
+        if ( cpumask_test_cpu(cpu, &rqd->idle) )
+        {
+            __cpumask_clear_cpu(cpu, &rqd->idle);
+            smt_idle_mask_clear(cpu, &rqd->smt_idle);
+        }
          * 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
@@ -3574,13 +3581,6 @@ csched2_schedule(
             balance_load(ops, cpu, now);
-        /* Clear the idle mask if necessary */
-        if ( cpumask_test_cpu(cpu, &rqd->idle) )
-        {
-            __cpumask_clear_cpu(cpu, &rqd->idle);
-            smt_idle_mask_clear(cpu, &rqd->smt_idle);
-        }
         snext->start_time = now;
         snext->tickled_cpu = -1;

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