[PATCH 5/5] sched/rt: Optimize find_lowest_rq() to select a cache hot cpu

Xunlei Pang
2015-01-19T04:52:37Z

In find_lowest_rq(), if we can't find a wake_affine cpu from
sched_domain, then we can actually determine a cache hot cpu
instead of simply calling "cpumask_any(lowest_mask)" which
always returns the first cpu in the mask.

So, we can determine the cache hot cpu during the interation of
sched_domain() in passing.

Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
---
kernel/sched/rt.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index d28cfa4..e6a42e6 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1535,6 +1535,7 @@ static int find_lowest_rq(struct task_struct *task)
struct cpumask *lowest_mask = this_cpu_cpumask_var_ptr(local_cpu_mask);
int this_cpu = smp_processor_id();
int cpu = task_cpu(task);
+ int cachehot_cpu = nr_cpu_ids;

/* Make sure the mask is initialized first */
if (unlikely(!lowest_mask))
@@ -1566,8 +1567,12 @@ static int find_lowest_rq(struct task_struct *task)

rcu_read_lock();
for_each_domain(cpu, sd) {
+ if (cachehot_cpu >= nr_cpu_ids)
+ cachehot_cpu = cpumask_first_and(lowest_mask,
+ sched_domain_span(sd));
+
if (sd->flags & SD_WAKE_AFFINE) {
- int best_cpu;
+ int wakeaffine_cpu;

/*
* "this_cpu" is cheaper to preempt than a
@@ -1579,16 +1584,20 @@ static int find_lowest_rq(struct task_struct *task)
return this_cpu;
}

- best_cpu = cpumask_first_and(lowest_mask,
+ wakeaffine_cpu = cpumask_first_and(lowest_mask,
sched_domain_span(sd));
- if (best_cpu < nr_cpu_ids) {
+ if (wakeaffine_cpu < nr_cpu_ids) {
rcu_read_unlock();
- return best_cpu;
+ return wakeaffine_cpu;
}
}
}
rcu_read_unlock();

+ /* most likely cache-hot */
+ if (cachehot_cpu < nr_cpu_ids)
+ return cachehot_cpu;
+
/*
* And finally, if there were no matches within the domains
* just give the caller *something* to work with from the compatible
--
1.9.1