[PATCH v2] sched/core: Prioritize migrating eligible tasks in sched_balance_rq()

From: Hao Jia
Date: Mon Dec 23 2024 - 04:15:37 EST

From: Hao Jia <jiahao1@xxxxxxxxxxx>

When the PLACE_LAG scheduling feature is enabled and
dst_cfs_rq->nr_queued is greater than 1, if a task is
ineligible (lag < 0) on the source cpu runqueue, it will
also be ineligible when it is migrated to the destination
cpu runqueue. Because we will keep the original equivalent
lag of the task in place_entity(). So if the task was
ineligible before, it will still be ineligible after
migration.

So in sched_balance_rq(), we prioritize migrating eligible
tasks, and we soft-limit ineligible tasks, allowing them
to migrate only when nr_balance_failed is non-zero to
avoid load-balancing trying very hard to balance the load.

Below are some benchmark test results. From my test results,
this patch shows a slight improvement on hackbench.

Benchmark
=========

All of the benchmarks are done inside a normal cpu cgroup in a
clean environment with cpu turbo disabled, and test machine is:

Single NUMA machine model is 13th Gen Intel(R) Core(TM)
i7-13700, 12 Core/24 HT.

Based on master b86545e02e8c.

Results
=======

hackbench-process-pipes
vanilla patched
Amean 1 0.5837 ( 0.00%) 0.5733 ( 1.77%)
Amean 4 1.4423 ( 0.00%) 1.4503 ( -0.55%)
Amean 7 2.5147 ( 0.00%) 2.4773 ( 1.48%)
Amean 12 3.9347 ( 0.00%) 3.8880 ( 1.19%)
Amean 21 5.3943 ( 0.00%) 5.3873 ( 0.13%)
Amean 30 6.7840 ( 0.00%) 6.6660 ( 1.74%)
Amean 48 9.8313 ( 0.00%) 9.6100 ( 2.25%)
Amean 79 15.4403 ( 0.00%) 14.9580 ( 3.12%)
Amean 96 18.4970 ( 0.00%) 17.9533 ( 2.94%)

hackbench-process-sockets
vanilla patched
Amean 1 0.6297 ( 0.00%) 0.6223 ( 1.16%)
Amean 4 2.1517 ( 0.00%) 2.0887 ( 2.93%)
Amean 7 3.6377 ( 0.00%) 3.5670 ( 1.94%)
Amean 12 6.1277 ( 0.00%) 5.9290 ( 3.24%)
Amean 21 10.0380 ( 0.00%) 9.7623 ( 2.75%)
Amean 30 14.1517 ( 0.00%) 13.7513 ( 2.83%)
Amean 48 24.7253 ( 0.00%) 24.2287 ( 2.01%)
Amean 79 43.9523 ( 0.00%) 43.2330 ( 1.64%)
Amean 96 54.5310 ( 0.00%) 53.7650 ( 1.40%)

tbench4 Throughput
vanilla patched
Hmean 1 255.97 ( 0.00%) 275.01 ( 7.44%)
Hmean 2 511.60 ( 0.00%) 544.27 ( 6.39%)
Hmean 4 996.70 ( 0.00%) 1006.57 ( 0.99%)
Hmean 8 1646.46 ( 0.00%) 1649.15 ( 0.16%)
Hmean 16 2259.42 ( 0.00%) 2274.35 ( 0.66%)
Hmean 32 4725.48 ( 0.00%) 4735.57 ( 0.21%)
Hmean 64 4411.47 ( 0.00%) 4400.05 ( -0.26%)
Hmean 96 4284.31 ( 0.00%) 4267.39 ( -0.39%)

Signed-off-by: Hao Jia <jiahao1@xxxxxxxxxxx>
Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
Previous discussion link: https://lore.kernel.org/all/20241128084858.25220-1-jiahao.kernel@xxxxxxxxx
Link to v1: https://lore.kernel.org/all/20241218080203.80556-1-jiahao.kernel@xxxxxxxxx

v1 to v2:
- Modify dst_cfs_rq->nr_running to dst_cfs_rq->nr_queued to
resolve conflicts with commit 736c55a02c47 ("sched/fair:
Rename cfs_rq.nr_running into nr_queued").

kernel/sched/fair.c | 34 ++++++++++++++++++++++++++++++++++
1 file changed, 34 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5599b0c1ba9b..c884bf631e66 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9396,6 +9396,30 @@ static inline int migrate_degrades_locality(struct task_struct *p,
}
#endif

+/*
+ * Check whether the task is ineligible on the destination cpu
+ *
+ * When the PLACE_LAG scheduling feature is enabled and
+ * dst_cfs_rq->nr_queued is greater than 1, if the task
+ * is ineligible, it will also be ineligible when
+ * it is migrated to the destination cpu.
+ */
+static inline int task_is_ineligible_on_dst_cpu(struct task_struct *p, int dest_cpu)
+{
+ struct cfs_rq *dst_cfs_rq;
+
+#ifdef CONFIG_FAIR_GROUP_SCHED
+ dst_cfs_rq = task_group(p)->cfs_rq[dest_cpu];
+#else
+ dst_cfs_rq = &cpu_rq(dest_cpu)->cfs;
+#endif
+ if (sched_feat(PLACE_LAG) && dst_cfs_rq->nr_queued &&
+ !entity_eligible(task_cfs_rq(p), &p->se))
+ return 1;
+
+ return 0;
+}
+
/*
* can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
*/
@@ -9420,6 +9444,16 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
return 0;

+ /*
+ * We want to prioritize the migration of eligible tasks.
+ * For ineligible tasks we soft-limit them and only allow
+ * them to migrate when nr_balance_failed is non-zero to
+ * avoid load-balancing trying very hard to balance the load.
+ */
+ if (!env->sd->nr_balance_failed &&
+ task_is_ineligible_on_dst_cpu(p, env->dst_cpu))
+ return 0;
+
/* Disregard percpu kthreads; they are where they need to be. */
if (kthread_is_per_cpu(p))
return 0;
--
2.34.1