[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>

diff --git a/kernel/sched.c b/kernel/sched.c
index 103f705..1dc29de 100644 (file)
--- 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.
                                 */