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Subject[tip: sched/core] sched/rt: Remove unnecessary push for unfit tasks
The following commit has been merged into the sched/core branch of tip:

Commit-ID: d94a9df49069ba8ff7c4aaeca1229e6471a01a15
Gitweb: https://git.kernel.org/tip/d94a9df49069ba8ff7c4aaeca1229e6471a01a15
Author: Qais Yousef <qais.yousef@arm.com>
AuthorDate: Mon, 02 Mar 2020 13:27:20
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Fri, 06 Mar 2020 12:57:29 +01:00

sched/rt: Remove unnecessary push for unfit tasks

In task_woken_rt() and switched_to_rto() we try trigger push-pull if the
task is unfit.

But the logic is found lacking because if the task was the only one
running on the CPU, then rt_rq is not in overloaded state and won't
trigger a push.

The necessity of this logic was under a debate as well, a summary of
the discussion can be found in the following thread:

https://lore.kernel.org/lkml/20200226160247.iqvdakiqbakk2llz@e107158-lin.cambridge.arm.com/

Remove the logic for now until a better approach is agreed upon.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
Link: https://lkml.kernel.org/r/20200302132721.8353-6-qais.yousef@arm.com
---
kernel/sched/rt.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index bcb1436..df11d88 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2225,7 +2225,7 @@ static void task_woken_rt(struct rq *rq, struct task_struct *p)
(rq->curr->nr_cpus_allowed < 2 ||
rq->curr->prio <= p->prio);

- if (need_to_push || !rt_task_fits_capacity(p, cpu_of(rq)))
+ if (need_to_push)
push_rt_tasks(rq);
}

@@ -2297,10 +2297,7 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
*/
if (task_on_rq_queued(p) && rq->curr != p) {
#ifdef CONFIG_SMP
- bool need_to_push = rq->rt.overloaded ||
- !rt_task_fits_capacity(p, cpu_of(rq));
-
- if (p->nr_cpus_allowed > 1 && need_to_push)
+ if (p->nr_cpus_allowed > 1 && rq->rt.overloaded)
rt_queue_push_tasks(rq);
#endif /* CONFIG_SMP */
if (p->prio < rq->curr->prio && cpu_online(cpu_of(rq)))
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