[PATCH] sched: HT optimisation

If an idle sibling of an HT queue encounters a busy sibling, then make
higher level load balancing of the non-idle variety.

Performance of multiprocessor HT systems with low numbers of tasks
(generally < number of virtual CPUs) can be significantly worse than the
exact same workloads when running in non-HT mode.  The reason is largely
due to poor scheduling behaviour.

This patch improves the situation, making the performance gap far less
significant on one problematic test case (tbench).

Signed-off-by: Nick Piggin <npiggin@suse.de>
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, 28 insertions, 6 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 8535e5c68f5b..46fdd0bb1ed6 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1906,7 +1906,7 @@ out:
  */
 static struct sched_group *
 find_busiest_group(struct sched_domain *sd, int this_cpu,
-                   unsigned long *imbalance, enum idle_type idle)
+                   unsigned long *imbalance, enum idle_type idle, int *sd_idle)
 {
         struct sched_group *busiest = NULL, *this = NULL, *group = sd->groups;
         unsigned long max_load, avg_load, total_load, this_load, total_pwr;
@@ -1931,6 +1931,9 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
                 avg_load = 0;
 
                 for_each_cpu_mask(i, group->cpumask) {
+                        if (*sd_idle && !idle_cpu(i))
+                                *sd_idle = 0;
+
                         /* Bias balancing toward cpus of our domain */
                         if (local_group)
                                 load = target_load(i, load_idx);
@@ -2074,10 +2077,14 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
         unsigned long imbalance;
         int nr_moved, all_pinned = 0;
         int active_balance = 0;
+        int sd_idle = 0;
+
+        if (idle != NOT_IDLE && sd->flags & SD_SHARE_CPUPOWER)
+                sd_idle = 1;
 
         schedstat_inc(sd, lb_cnt[idle]);
 
-        group = find_busiest_group(sd, this_cpu, &imbalance, idle);
+        group = find_busiest_group(sd, this_cpu, &imbalance, idle, &sd_idle);
         if (!group) {
                 schedstat_inc(sd, lb_nobusyg[idle]);
                 goto out_balanced;
@@ -2150,6 +2157,8 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
                         sd->balance_interval *= 2;
         }
 
+        if (!nr_moved && !sd_idle && sd->flags & SD_SHARE_CPUPOWER)
+                return -1;
         return nr_moved;
 
 out_balanced:
@@ -2161,6 +2170,8 @@ out_balanced:
                         (sd->balance_interval < sd->max_interval))
                 sd->balance_interval *= 2;
 
+        if (!sd_idle && sd->flags & SD_SHARE_CPUPOWER)
+                return -1;
         return 0;
 }
 
@@ -2178,9 +2189,13 @@ static int load_balance_newidle(int this_cpu, runqueue_t *this_rq,
         runqueue_t *busiest = NULL;
         unsigned long imbalance;
         int nr_moved = 0;
+        int sd_idle = 0;
+
+        if (sd->flags & SD_SHARE_CPUPOWER)
+                sd_idle = 1;
 
         schedstat_inc(sd, lb_cnt[NEWLY_IDLE]);
-        group = find_busiest_group(sd, this_cpu, &imbalance, NEWLY_IDLE);
+        group = find_busiest_group(sd, this_cpu, &imbalance, NEWLY_IDLE, &sd_idle);
         if (!group) {
                 schedstat_inc(sd, lb_nobusyg[NEWLY_IDLE]);
                 goto out_balanced;
@@ -2205,15 +2220,19 @@ static int load_balance_newidle(int this_cpu, runqueue_t *this_rq,
                 spin_unlock(&busiest->lock);
         }
 
-        if (!nr_moved)
+        if (!nr_moved) {
                 schedstat_inc(sd, lb_failed[NEWLY_IDLE]);
-        else
+                if (!sd_idle && sd->flags & SD_SHARE_CPUPOWER)
+                        return -1;
+        } else
                 sd->nr_balance_failed = 0;
 
         return nr_moved;
 
 out_balanced:
         schedstat_inc(sd, lb_balanced[NEWLY_IDLE]);
+        if (!sd_idle && sd->flags & SD_SHARE_CPUPOWER)
+                return -1;
         sd->nr_balance_failed = 0;
         return 0;
 }
@@ -2338,7 +2357,10 @@ 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 no longer idle */
+                                /* We've pulled tasks over so either we're no
+                                 * longer idle, or one of our SMT siblings is
+                                 * not idle.
+                                 */
                                 idle = NOT_IDLE;
                         }
                         sd->last_balance += interval;