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Subject[PATCH 2/5] sched/fair: Explain LLC nohz kick condition
Date
Provide a comment explaining the LLC related nohz kick in
nohz_balancer_kick().

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
kernel/sched/fair.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 380a72e42678..62f6aead7e73 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9543,8 +9543,13 @@ static void nohz_balancer_kick(struct rq *rq)
sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
if (sds) {
/*
- * XXX: write a coherent comment on why we do this.
- * See also: http://lkml.kernel.org/r/20111202010832.602203411@sbsiddha-desk.sc.intel.com
+ * If there is an imbalance between LLC domains (IOW we could
+ * increase the overall cache use), we need some less-loaded LLC
+ * domain to pull some load. Likewise, we may need to spread
+ * load within the current LLC domain (e.g. packed SMT cores but
+ * other CPUs are idle). We can't really know from here how busy
+ * the others are - so just get a nohz balance going if it looks
+ * like this LLC domain has tasks we could move.
*/
nr_busy = atomic_read(&sds->nr_busy_cpus);
if (nr_busy > 1) {
--
2.20.1
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