[PATCH] sched: tweak affine wakeups

Do less affine wakeups.  We're trying to reduce dbt2-pgsql idle time
regressions here...  make sure we don't don't move tasks the wrong way in an
imbalance condition.  Also, remove the cache coldness requirement from the
calculation - this seems to induce sharp cutoff points where behaviour will
suddenly change on some workloads if the load creeps slightly over or under
some point.  It is good for periodic balancing because in that case have
otherwise have no other context to determine what task to move.

But also make a minor tweak to "wake balancing" - the imbalance tolerance is
now set at half the domain's imbalance, so we get the opportunity to do wake
balancing before the more random periodic rebalancing gets preformed.

Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -1016,38 +1016,45 @@ static int try_to_wake_up(task_t * p, unsigned int state, int sync)
 		int idx = this_sd->wake_idx;
 		unsigned int imbalance;
 
+		imbalance = 100 + (this_sd->imbalance_pct - 100) / 2;
+
 		load = source_load(cpu, idx);
 		this_load = target_load(this_cpu, idx);
 
-		/*
-		 * If sync wakeup then subtract the (maximum possible) effect of
-		 * the currently running task from the load of the current CPU:
-		 */
-		if (sync)
-			this_load -= SCHED_LOAD_SCALE;
-
-		 /* Don't pull the task off an idle CPU to a busy one */
-		if (load < SCHED_LOAD_SCALE/2 && this_load > SCHED_LOAD_SCALE/2)
-			goto out_set_cpu;
-
 		new_cpu = this_cpu; /* Wake to this CPU if we can */
 
-		if ((this_sd->flags & SD_WAKE_AFFINE) &&
-			!task_hot(p, rq->timestamp_last_tick, this_sd)) {
+		if (this_sd->flags & SD_WAKE_AFFINE) {
+			unsigned long tl = this_load;
 			/*
-			 * This domain has SD_WAKE_AFFINE and p is cache cold
-			 * in this domain.
+			 * If sync wakeup then subtract the (maximum possible)
+			 * effect of the currently running task from the load
+			 * of the current CPU:
 			 */
-			schedstat_inc(this_sd, ttwu_move_affine);
-			goto out_set_cpu;
-		} else if ((this_sd->flags & SD_WAKE_BALANCE) &&
-				imbalance*this_load <= 100*load) {
-			/*
-			 * This domain has SD_WAKE_BALANCE and there is
-			 * an imbalance.
-			 */
-			schedstat_inc(this_sd, ttwu_move_balance);
-			goto out_set_cpu;
+			if (sync)
+				tl -= SCHED_LOAD_SCALE;
+
+			if ((tl <= load &&
+				tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||
+				100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
+				/*
+				 * This domain has SD_WAKE_AFFINE and
+				 * p is cache cold in this domain, and
+				 * there is no bad imbalance.
+				 */
+				schedstat_inc(this_sd, ttwu_move_affine);
+				goto out_set_cpu;
+			}
+		}
+
+		/*
+		 * Start passive balancing when half the imbalance_pct
+		 * limit is reached.
+		 */
+		if (this_sd->flags & SD_WAKE_BALANCE) {
+			if (imbalance*this_load <= 100*load) {
+				schedstat_inc(this_sd, ttwu_move_balance);
+				goto out_set_cpu;
+			}
 		}
 	}