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Subject[tip: sched/core] sched/rt: Re-instate old behavior in select_task_rq_rt()
The following commit has been merged into the sched/core branch of tip:

Commit-ID: b28bc1e002c23ff8a4999c4a2fb1d4d412bc6f5e
Author: Qais Yousef <>
AuthorDate: Mon, 02 Mar 2020 13:27:17
Committer: Ingo Molnar <>
CommitterDate: Fri, 06 Mar 2020 12:57:27 +01:00

sched/rt: Re-instate old behavior in select_task_rq_rt()

When RT Capacity Aware support was added, the logic in select_task_rq_rt
was modified to force a search for a fitting CPU if the task currently
doesn't run on one.

But if the search failed, and the search was only triggered to fulfill
the fitness request; we could end up selecting a new CPU unnecessarily.

Fix this and re-instate the original behavior by ensuring we bail out
in that case.

This behavior change only affected asymmetric systems that are using
util_clamp to implement capacity aware. None asymmetric systems weren't

Reported-by: Pavan Kondeti <>
Signed-off-by: Qais Yousef <>
Signed-off-by: Peter Zijlstra (Intel) <>
Signed-off-by: Ingo Molnar <>
Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
kernel/sched/rt.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 55a4a50..f0071fa 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1475,6 +1475,13 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
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;
+ /*
* Don't bother moving it if the destination CPU is
* not running a lower priority task.
@@ -1482,6 +1489,8 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
p->prio < cpu_rq(target)->rt.highest_prio.curr)
cpu = target;

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