[PATCH v2 3/9] sched/fair: Add cgroup LCA finder for hierarchical yield

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

Implement yield_deboost_find_lca() to locate the lowest common ancestor
(LCA) in the cgroup hierarchy for EEVDF-aware yield operations.

The LCA represents the appropriate hierarchy level where vruntime
adjustments should be applied to ensure fairness is maintained across
cgroup boundaries. This is critical for virtualization workloads where
vCPUs may be organized in nested cgroups.

Key aspects:
- For CONFIG_FAIR_GROUP_SCHED: Walk up both entity hierarchies by
  aligning depths, then ascending together until common cfs_rq found
- For flat hierarchy: Simply verify both entities share the same cfs_rq
- Validate that meaningful contention exists (h_nr_queued > 1)
- Ensure yielding entity has non-zero slice for safe penalty calculation

Function operates under rq->lock protection. Static helper integrated
in subsequent patches.

v1 -> v2:
- Change nr_queued to h_nr_queued for accurate hierarchical task
  counting that includes tasks in child cgroups
- Improve comments to clarify the LCA algorithm

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 kernel/sched/fair.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2f327882bf4d..39dbdd222687 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9102,6 +9102,36 @@ yield_deboost_validate_tasks(struct rq *rq, struct task_struct *p_target)
 	return p_yielding;
 }
 
+/*
+ * Find the lowest common ancestor (LCA) in the cgroup hierarchy.
+ * Uses find_matching_se() to locate sibling entities at the same level,
+ * then returns their common cfs_rq for vruntime adjustments.
+ *
+ * Returns true if a valid LCA with meaningful contention (h_nr_queued > 1)
+ * is found, storing the LCA entities and common cfs_rq in output parameters.
+ */
+static bool __maybe_unused
+yield_deboost_find_lca(struct sched_entity *se_y, struct sched_entity *se_t,
+		       struct sched_entity **se_y_lca_out,
+		       struct sched_entity **se_t_lca_out,
+		       struct cfs_rq **cfs_rq_out)
+{
+	struct sched_entity *se_y_lca = se_y;
+	struct sched_entity *se_t_lca = se_t;
+	struct cfs_rq *cfs_rq;
+
+	find_matching_se(&se_y_lca, &se_t_lca);
+
+	cfs_rq = cfs_rq_of(se_y_lca);
+	if (cfs_rq->h_nr_queued <= 1)
+		return false;
+
+	*se_y_lca_out = se_y_lca;
+	*se_t_lca_out = se_t_lca;
+	*cfs_rq_out = cfs_rq;
+	return true;
+}
+
 /*
  * sched_yield() is very simple
  */
-- 
2.43.0