sched/deadline: Make find_later_rq() choose a closer CPU in topology

When cpudl_find() returns any among free_cpus, the CPU might not be
closer than others, considering sched domain. For example:

   this_cpu: 15
   free_cpus: 0, 1,..., 14 (== later_mask)
   best_cpu: 0

   topology:

   0 --+
       +--+
   1 --+  |
          +-- ... --+
   2 --+  |         |
       +--+         |
   3 --+            |

   ...             ...

   12 --+           |
        +--+        |
   13 --+  |        |
           +-- ... -+
   14 --+  |
        +--+
   15 --+

In this case, it would be best to select 14 since it's a free CPU and
closest to 15 (this_cpu). However, currently the code selects 0 (best_cpu)
even though that's just any among free_cpus. Fix it.

This (re)aligns the deadline behaviour with the rt behaviour.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: <bristot@redhat.com>
Cc: <juri.lelli@gmail.com>
Cc: <kernel-team@lge.com>
Cc: <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1495504859-10960-2-git-send-email-byungchul.park@lge.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index a205ac7cf435c78be8cfdafc30d48ef1b6aad78e..ac07d7c3a978d92d6b3d95c1b1a61737c24b6c7f 100644 (file)
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1798,7 +1798,7 @@ static int find_later_rq(struct task_struct *task)
        struct sched_domain *sd;
        struct cpumask *later_mask = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
        int this_cpu = smp_processor_id();
-       int best_cpu, cpu = task_cpu(task);
+       int cpu = task_cpu(task);
 
        /* Make sure the mask is initialized first */
        if (unlikely(!later_mask))
@@ -1811,17 +1811,14 @@ static int find_later_rq(struct task_struct *task)
         * We have to consider system topology and task affinity
         * first, then we can look for a suitable cpu.
         */
-       best_cpu = cpudl_find(&task_rq(task)->rd->cpudl,
-                       task, later_mask);
-       if (best_cpu == -1)
+       if (cpudl_find(&task_rq(task)->rd->cpudl, task, later_mask) == -1)
                return -1;
 
        /*
-        * If we are here, some target has been found,
-        * the most suitable of which is cached in best_cpu.
-        * This is, among the runqueues where the current tasks
-        * have later deadlines than the task's one, the rq
-        * with the latest possible one.
+        * If we are here, some targets have been found, including
+        * the most suitable which is, among the runqueues where the
+        * current tasks have later deadlines than the task's one, the
+        * rq with the latest possible one.
         *
         * Now we check how well this matches with task's
         * affinity and system topology.
@@ -1841,6 +1838,7 @@ static int find_later_rq(struct task_struct *task)
        rcu_read_lock();
        for_each_domain(cpu, sd) {
                if (sd->flags & SD_WAKE_AFFINE) {
+                       int best_cpu;
 
                        /*
                         * If possible, preempting this_cpu is
@@ -1852,12 +1850,15 @@ static int find_later_rq(struct task_struct *task)
                                return this_cpu;
                        }
 
+                       best_cpu = cpumask_first_and(later_mask,
+                                                       sched_domain_span(sd));
                        /*
-                        * Last chance: if best_cpu is valid and is
-                        * in the mask, that becomes our choice.
+                        * Last chance: if a cpu being in both later_mask
+                        * and current sd span is valid, that becomes our
+                        * choice. Of course, the latest possible cpu is
+                        * already under consideration through later_mask.
                         */
-                       if (best_cpu < nr_cpu_ids &&
-                           cpumask_test_cpu(best_cpu, sched_domain_span(sd))) {
+                       if (best_cpu < nr_cpu_ids) {
                                rcu_read_unlock();
                                return best_cpu;
                        }