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Subject[PATCH v3 6/6] sched/rt: Fix pushing unfit tasks to a better CPU
Date
If a task was running on unfit CPU we could ignore migrating if the
priority level of the new fitting CPU is the *same* as the unfit one.

Add an extra check to select_task_rq_rt() to allow the push in case:

* p->prio == new_cpu.highest_priority
* task_fits(p, new_cpu)

Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---
kernel/sched/rt.c | 41 ++++++++++++++++++++++++++++-------------
1 file changed, 28 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index ce230bec6847..8aaa442e4867 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1474,20 +1474,35 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
if (test || !rt_task_fits_capacity(p, cpu)) {
int target = find_lowest_rq(p);

- /*
- * Bail out if we were forcing a migration to find a better
- * fitting CPU but our search failed.
- */
- if (!test && target != -1 && !rt_task_fits_capacity(p, target))
- goto out_unlock;
+ if (target != -1) {
+ bool fit_target = rt_task_fits_capacity(p, target);

- /*
- * Don't bother moving it if the destination CPU is
- * not running a lower priority task.
- */
- if (target != -1 &&
- p->prio < cpu_rq(target)->rt.highest_prio.curr)
- cpu = target;
+ /*
+ * Bail out if we were forcing a migration to find a
+ * better fitting CPU but our search failed.
+ */
+ if (!test && !fit_target)
+ goto out_unlock;
+
+ /*
+ * Don't bother moving it if the destination CPU is
+ * not running a lower priority task.
+ */
+ if (p->prio < cpu_rq(target)->rt.highest_prio.curr) {
+
+ cpu = target;
+
+ } else if (p->prio == cpu_rq(target)->rt.highest_prio.curr) {
+
+ /*
+ * If the priority is the same and the new CPU
+ * is a better fit, then move, otherwise don't
+ * bother here either.
+ */
+ if (fit_target)
+ cpu = target;
+ }
+ }
}

out_unlock:
--
2.17.1
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