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>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5b1e96687b4992f9b1d118028e52ac05fb13a49b..acaf567a03d29f5e0cee33bafcf9b97020344d47 100644 (file)
--- 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.