[tip: sched/core] sched/fair: Simplify the entry condition for update_idle_cpu_scan()

[tip-bot2 for K Prateek Nayak]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     f1320a8dd8ba6518ddb53ea4e3efcb49dc41d257
Gitweb:        https://git.kernel.org/tip/f1320a8dd8ba6518ddb53ea4e3efcb49dc41d257
Author:        K Prateek Nayak <kprateek.nayak@xxxxxxx>
AuthorDate:    Thu, 12 Mar 2026 04:44:33 
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 18 Mar 2026 09:06:50 +01:00

sched/fair: Simplify the entry condition for update_idle_cpu_scan()

Only the topmost SD_SHARE_LLC domain has the "sd->shared" assigned.
Simply use "sd->shared" as an indicator for load balancing at the highest
SD_SHARE_LLC domain in update_idle_cpu_scan() instead of relying on
llc_size.

Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Tested-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Link: https://patch.msgid.link/20260312044434.1974-9-kprateek.nayak@xxxxxxx
---
 kernel/sched/fair.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3e24d3e..85c22f0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11234,6 +11234,7 @@ static void update_idle_cpu_scan(struct lb_env *env,
 				 unsigned long sum_util)
 {
 	struct sched_domain_shared *sd_share;
+	struct sched_domain *sd = env->sd;
 	int llc_weight, pct;
 	u64 x, y, tmp;
 	/*
@@ -11247,11 +11248,7 @@ static void update_idle_cpu_scan(struct lb_env *env,
 	if (!sched_feat(SIS_UTIL) || env->idle == CPU_NEWLY_IDLE)
 		return;
 
-	llc_weight = per_cpu(sd_llc_size, env->dst_cpu);
-	if (env->sd->span_weight != llc_weight)
-		return;
-
-	sd_share = rcu_dereference_all(per_cpu(sd_llc_shared, env->dst_cpu));
+	sd_share = sd->shared;
 	if (!sd_share)
 		return;
 
@@ -11285,10 +11282,11 @@ static void update_idle_cpu_scan(struct lb_env *env,
 	 */
 	/* equation [3] */
 	x = sum_util;
+	llc_weight = sd->span_weight;
 	do_div(x, llc_weight);
 
 	/* equation [4] */
-	pct = env->sd->imbalance_pct;
+	pct = sd->imbalance_pct;
 	tmp = x * x * pct * pct;
 	do_div(tmp, 10000 * SCHED_CAPACITY_SCALE);
 	tmp = min_t(long, tmp, SCHED_CAPACITY_SCALE);