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Subject[PATCH v4] sched/rt : Reduce push_rt_task() retries
Date
Improve push_rt_task() by propagating the double_lock_balance() 
usage from find_lock_lowest_rq(), thereby reducing the number
of cases where we have to assume rq->lock was dropped.
When CONFIG_PREEMPT=n, Unnecessary retries frequently happens
in a simple test case:
Four different rt priority tasks are restricted to run on two
cpus.

Thanks for Steven Rostedt's advice.

Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
---
kernel/sched/rt.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 2e2955a..be0fc43 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1754,7 +1754,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
!task_on_rq_queued(task))) {

double_unlock_balance(rq, lowest_rq);
- lowest_rq = NULL;
+ lowest_rq = RETRY_TASK;
break;
}
}
@@ -1830,7 +1830,9 @@ static int push_rt_task(struct rq *rq)

/* find_lock_lowest_rq locks the rq if found */
lowest_rq = find_lock_lowest_rq(next_task, rq);
- if (!lowest_rq) {
+ if (!lowest_rq)
+ goto out;
+ if (lowest_rq == RETRY_TASK) {
struct task_struct *task;
/*
* find_lock_lowest_rq releases rq->lock
--
1.8.3.1
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