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Subject[tip:sched/core] sched/fair: Don't increase sd->balance_interval on newidle balance
Commit-ID:  3f130a37c442d5c4d66531b240ebe9abfef426b5
Gitweb: https://git.kernel.org/tip/3f130a37c442d5c4d66531b240ebe9abfef426b5
Author: Valentin Schneider <valentin.schneider@arm.com>
AuthorDate: Wed, 26 Sep 2018 16:12:07 +0100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Sun, 4 Nov 2018 00:59:23 +0100

sched/fair: Don't increase sd->balance_interval on newidle balance

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Dietmar.Eggemann@arm.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: patrick.bellasi@arm.com
Cc: vincent.guittot@linaro.org
Link: http://lkml.kernel.org/r/1537974727-30788-2-git-send-email-valentin.schneider@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
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 4e298931a715..a17ca4254427 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8876,13 +8876,22 @@ out_all_pinned:
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;
}
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