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Subject[PATCH 3.4 167/176] sched/rt: Reduce rq lock contention by eliminating locking of non-feasible target
Date
From: Tim Chen <tim.c.chen@linux.intel.com>

3.4.107-rc1 review patch. If anyone has any objections, please let me know.

------------------


commit 80e3d87b2c5582db0ab5e39610ce3707d97ba409 upstream.

This patch adds checks that prevens futile attempts to move rt tasks
to a CPU with active tasks of equal or higher priority.

This reduces run queue lock contention and improves the performance of
a well known OLTP benchmark by 0.7%.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Shawn Bohrer <sbohrer@rgmadvisors.com>
Cc: Suruchi Kadu <suruchi.a.kadu@intel.com>
Cc: Doug Nelson<doug.nelson@intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1421430374.2399.27.camel@schen9-desk2.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Zefan Li <lizefan@huawei.com>
---
kernel/sched/rt.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 526c77d..71429db 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1293,7 +1293,12 @@ select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
(p->rt.nr_cpus_allowed > 1)) {
int target = find_lowest_rq(p);

- if (target != -1)
+ /*
+ * 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;
}
rcu_read_unlock();
@@ -1570,6 +1575,16 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)

lowest_rq = cpu_rq(cpu);

+ if (lowest_rq->rt.highest_prio.curr <= task->prio) {
+ /*
+ * Target rq has tasks of equal or higher priority,
+ * retrying does not release any lock and is unlikely
+ * to yield a different result.
+ */
+ lowest_rq = NULL;
+ break;
+ }
+
/* if the prio of this runqueue changed, try again */
if (double_lock_balance(rq, lowest_rq)) {
/*
--
1.9.1


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