sched: Prevent to re-select dst-cpu in load_balance()

Commit 88b8dac0 makes load_balance() consider other cpus in its
group. But, in that, there is no code for preventing to
re-select dst-cpu. So, same dst-cpu can be selected over and
over.

This patch add functionality to load_balance() in order to
exclude cpu which is selected once. We prevent to re-select
dst_cpu via env's cpus, so now, env's cpus is a candidate not
only for src_cpus, but also dst_cpus.

With this patch, we can remove lb_iterations and
max_lb_iterations, because we decide whether we can go ahead or
not via env's cpus.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Tested-by: Jason Low <jason.low2@hp.com>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1366705662-3587-7-git-send-email-iamjoonsoo.kim@lge.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 15 insertions, 18 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5b1e96687b49..acaf567a03d2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3905,7 +3905,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
                 return 0;
 
         if (!cpumask_test_cpu(env->dst_cpu, tsk_cpus_allowed(p))) {
-                int new_dst_cpu;
+                int cpu;
 
                 schedstat_inc(p, se.statistics.nr_failed_migrations_affine);
 
@@ -3920,12 +3920,15 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
                 if (!env->dst_grpmask || (env->flags & LBF_SOME_PINNED))
                         return 0;
 
-                new_dst_cpu = cpumask_first_and(env->dst_grpmask,
-                                                tsk_cpus_allowed(p));
-                if (new_dst_cpu < nr_cpu_ids) {
-                        env->flags |= LBF_SOME_PINNED;
-                        env->new_dst_cpu = new_dst_cpu;
+                /* Prevent to re-select dst_cpu via env's cpus */
+                for_each_cpu_and(cpu, env->dst_grpmask, env->cpus) {
+                        if (cpumask_test_cpu(cpu, tsk_cpus_allowed(p))) {
+                                env->flags |= LBF_SOME_PINNED;
+                                env->new_dst_cpu = cpu;
+                                break;
+                        }
                 }
+
                 return 0;
         }
 
@@ -5008,7 +5011,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
                         int *balance)
 {
         int ld_moved, cur_ld_moved, active_balance = 0;
-        int lb_iterations, max_lb_iterations;
         struct sched_group *group;
         struct rq *busiest;
         unsigned long flags;
@@ -5028,15 +5030,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
          * For NEWLY_IDLE load_balancing, we don't need to consider
          * other cpus in our group
          */
-        if (idle == CPU_NEWLY_IDLE) {
+        if (idle == CPU_NEWLY_IDLE)
                 env.dst_grpmask = NULL;
-                /*
-                 * we don't care max_lb_iterations in this case,
-                 * in following patch, this will be removed
-                 */
-                max_lb_iterations = 0;
-        } else
-                max_lb_iterations = cpumask_weight(env.dst_grpmask);
 
         cpumask_copy(cpus, cpu_active_mask);
 
@@ -5064,7 +5059,6 @@ redo:
         schedstat_add(sd, lb_imbalance[idle], env.imbalance);
 
         ld_moved = 0;
-        lb_iterations = 1;
         if (busiest->nr_running > 1) {
                 /*
                  * Attempt to move tasks. If find_busiest_group has found
@@ -5121,14 +5115,17 @@ more_balance:
                  * moreover subsequent load balance cycles should correct the
                  * excess load moved.
                  */
-                if ((env.flags & LBF_SOME_PINNED) && env.imbalance > 0 &&
-                                lb_iterations++ < max_lb_iterations) {
+                if ((env.flags & LBF_SOME_PINNED) && env.imbalance > 0) {
 
                         env.dst_rq       = cpu_rq(env.new_dst_cpu);
                         env.dst_cpu      = env.new_dst_cpu;
                         env.flags       &= ~LBF_SOME_PINNED;
                         env.loop         = 0;
                         env.loop_break   = sched_nr_migrate_break;
+
+                        /* Prevent to re-select dst_cpu via env's cpus */
+                        cpumask_clear_cpu(env.dst_cpu, env.cpus);
+
                         /*
                          * Go back to "more_balance" rather than "redo" since we
                          * need to continue with same src_cpu.