[PATCH] sched: don't kick ALB in the presence of pinned task

Jack Steiner brought this issue at my OLS talk.

Take a scenario where two tasks are pinned to two HT threads in a physical
package.  Idle packages in the system will keep kicking migration_thread on
the busy package with out any success.

We will run into similar scenarios in the presence of CMP/NUMA.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 14 insertions, 1 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 103f705b245c..1dc29dec38a9 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2125,6 +2125,16 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
                 if (unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2)) {
 
                         spin_lock(&busiest->lock);
+
+                        /* don't kick the migration_thread, if the curr
+                         * task on busiest cpu can't be moved to this_cpu
+                         */
+                        if (!cpu_isset(this_cpu, busiest->curr->cpus_allowed)) {
+                                spin_unlock(&busiest->lock);
+                                all_pinned = 1;
+                                goto out_one_pinned;
+                        }
+
                         if (!busiest->active_balance) {
                                 busiest->active_balance = 1;
                                 busiest->push_cpu = this_cpu;
@@ -2165,6 +2175,8 @@ out_balanced:
         schedstat_inc(sd, lb_balanced[idle]);
 
         sd->nr_balance_failed = 0;
+
+out_one_pinned:
         /* tune up the balancing interval */
         if ((all_pinned && sd->balance_interval < MAX_PINNED_INTERVAL) ||
                         (sd->balance_interval < sd->max_interval))
@@ -2357,7 +2369,8 @@ static void rebalance_tick(int this_cpu, runqueue_t *this_rq,
 
                 if (j - sd->last_balance >= interval) {
                         if (load_balance(this_cpu, this_rq, sd, idle)) {
-                                /* We've pulled tasks over so either we're no
+                                /*
+                                 * We've pulled tasks over so either we're no
                                  * longer idle, or one of our SMT siblings is
                                  * not idle.
                                  */