[Xen-changelog] [xen staging] credit2: during scheduling, update the idle mask before using it

[p***@xen.org]

commit a9400d630b341da544ff355405db779356fe8807
Author: Dario Faggioli <***@suse.com>
AuthorDate: Thu Nov 22 11:54:56 2018 +0000
Commit: George Dunlap <***@citrix.com>
CommitDate: Thu Nov 22 11:54:56 2018 +0000

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 <***@suse.com>
Reviewed-by: George Dunlap <***@citrix.com>
---
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 49a99c0a73..623a325ceb 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -3555,6 +3555,13 @@ 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
@@ -3575,13 +3582,6 @@ 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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