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

Peter Zijlstra
2015-01-27T14:21:45Z

On Mon, Jan 19, 2015 at 04:49:40AM +0000, Xunlei Pang wrote:
> 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.

Steve, I'm not getting this. Why are we using WAKE_AFFINE here?

> 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
>