[PATCH v2 2/2] sched/fair: Don't increase sd->balance_interval on newidle balance

[Valentin Schneider <valentin.schneider@arm.com>]

When load_balance() fails to move some load because of task affinity,
we end up increasing sd->balance_interval to delay the next periodic
balance in the hopes that next time we look, that annoying pinned
task(s) will be gone.

However, idle_balance() pays no attention to sd->balance_interval, yet
it will still lead to an increase in balance_interval in case of
pinned tasks.

If we're going through several newidle balances (e.g. we have a
periodic task), this can lead to a huge increase of the
balance_interval in a very small amount of time.

To prevent that, don't increase the balance interval when going
through a newidle balance.

This is a similar approach to what is done in commit 58b26c4c0257
("sched: Increment cache_nice_tries only on periodic lb"), where we
disregard newidle balance and rely on periodic balance for more stable
results.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/fair.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9cf93ba..4c33283 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8782,13 +8782,22 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 	sd->nr_balance_failed = 0;
 
 out_one_pinned:
+	ld_moved = 0;
+
+	/*
+	 * idle_balance() disregards balance intervals, so we could repeatedly
+	 * reach this code, which would lead to balance_interval skyrocketting
+	 * in a short amount of time. Skip the balance_interval increase logic
+	 * to avoid that.
+	 */
+	if (env.idle == CPU_NEWLY_IDLE)
+		goto out;
+
 	/* tune up the balancing interval */
 	if ((env.flags & LBF_ALL_PINNED &&
 	     sd->balance_interval < MAX_PINNED_INTERVAL) ||
 	    sd->balance_interval < sd->max_interval)
 		sd->balance_interval *= 2;
-
-	ld_moved = 0;
 out:
 	return ld_moved;
 }
-- 
2.7.4