[PATCH AUTOSEL 5.8 08/72] sched/fair: Fix NOHZ next idle balance

Sasha Levin
2020-08-08T23:51:43Z

From: Vincent Guittot <vincent.guittot@linaro.org>

[ Upstream commit 3ea2f097b17e13a8280f1f9386c331b326a3dbef ]

With commit:
'b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK")'
rebalance_domains of the local cfs_rq happens before others idle cpus have
updated nohz.next_balance and its value is overwritten.

Move the update of nohz.next_balance for other idles cpus before balancing
and updating the next_balance of local cfs_rq.

Also, the nohz.next_balance is now updated only if all idle cpus got a
chance to rebalance their domains and the idle balance has not been aborted
because of new activities on the CPU. In case of need_resched, the idle
load balance will be kick the next jiffie in order to address remaining
ilb.

Fixes: b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK")
Reported-by: Peng Liu <iwtbavbm@gmail.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20200609123748.18636-1-vincent.guittot@linaro.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/sched/fair.c | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04fa8dbcfa4d7..6b3b59cc51d6c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10027,7 +10027,12 @@ static void kick_ilb(unsigned int flags)
{
int ilb_cpu;

- nohz.next_balance++;
+ /*
+ * Increase nohz.next_balance only when if full ilb is triggered but
+ * not if we only update stats.
+ */
+ if (flags & NOHZ_BALANCE_KICK)
+ nohz.next_balance = jiffies+1;

ilb_cpu = find_new_ilb();

@@ -10348,6 +10353,14 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
}
}

+ /*
+ * next_balance will be updated only when there is a need.
+ * When the CPU is attached to null domain for ex, it will not be
+ * updated.
+ */
+ if (likely(update_next_balance))
+ nohz.next_balance = next_balance;
+
/* Newly idle CPU doesn't need an update */
if (idle != CPU_NEWLY_IDLE) {
update_blocked_averages(this_cpu);
@@ -10368,14 +10381,6 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
if (has_blocked_load)
WRITE_ONCE(nohz.has_blocked, 1);

- /*
- * next_balance will be updated only when there is a need.
- * When the CPU is attached to null domain for ex, it will not be
- * updated.
- */
- if (likely(update_next_balance))
- nohz.next_balance = next_balance;
-
return ret;
}

--
2.25.1