[tip: sched/core] sched/fair: Prefer fully idle cores for NOHZ balancing

[tip-bot2 for Andrea Righi]

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

Commit-ID:     293f9611ae73564febc553935830074f0f300694
Gitweb:        https://git.kernel.org/tip/293f9611ae73564febc553935830074f0f300694
Author:        Andrea Righi <arighi@xxxxxxxxxx>
AuthorDate:    Tue, 04 Aug 2026 17:13:24 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 07 Aug 2026 18:27:09 +02:00

sched/fair: Prefer fully idle cores for NOHZ balancing

find_new_ilb() selects the first idle housekeeping CPU without
considering whether another thread is running on the same physical core.
On an SMT system, the idle load balancer can therefore activate both
siblings even when another housekeeping CPU has an entirely idle core.

On most SMT systems, this is not problematic because the idle load
balancer is a short-lived activity and the transient wakeup of a sibling
has negligible performance impact.

However, this can be particularly costly on NVIDIA Olympus cores used in
Vera. Briefly activating an otherwise idle sibling can reduce the
performance available to the other sibling and this effect does not
necessarily end once the activated sibling becomes idle: after the ILB
finishes and its CPU enters WFI, full single-thread performance is
restored only after the sibling has remained idle for a qualification
interval (10 Ki cycles on the tested Vera system). Repeated short
sibling wakeups can therefore sustain the interference even with little
actual overlap.

Prevent this by preferring an idle housekeeping CPU whose entire SMT
core is idle. Retain the first idle CPU as a fallback when no fully idle
core is available, so NOHZ balancing continues to make forward progress.
Once a partially busy core has been examined, skip its remaining SMT
siblings to avoid repeating the core-idle check on wide SMT systems.

Tests performed using an ad hoc GEMM benchmark running one CPU-intensive
task per SMT core within its CPU affinity mask improved from
approximately 6.2 TFLOP/s to 9.4 TFLOP/s.

Note that this preference may wake a fully idle physical core instead of
using an idle sibling of an active core, potentially increasing ILB
wakeup latency or energy consumption on some architectures. It may also
scan additional CPUs before selecting the one to run the ILB. The
selection falls back to the first idle CPU when no fully idle SMT core
is available. Non-SMT systems continue to select the first idle
housekeeping CPU.

Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Mete Durlu <meted@xxxxxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Link: https://patch.msgid.link/20260804151324.918020-1-arighi@xxxxxxxxxx
---
 kernel/sched/fair.c | 55 +++++++++++++++++++++++++++++++++++---------
 1 file changed, 44 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index df8c9c2..a24dd20 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13962,29 +13962,62 @@ static inline int on_null_domain(struct rq *rq)
  */
 static inline int find_new_ilb(void)
 {
-	int this_cpu = smp_processor_id();
-	const struct cpumask *hk_mask;
-	int ilb_cpu;
+	struct cpumask *ilb_cpus;
+	int ilb_cpu, fallback = -1;
+
+	lockdep_assert_irqs_disabled();
+
+	/*
+	 * Reuse the per-CPU select_rq_mask, which is protected from concurrent
+	 * use on this CPU by having interrupts disabled.
+	 */
+	ilb_cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
+	cpumask_and(ilb_cpus, nohz.idle_cpus_mask,
+		    housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
+
+	for_each_cpu(ilb_cpu, ilb_cpus) {
+		if (!idle_cpu(ilb_cpu)) {
+			/*
+			 * Once an idle fallback exists, a busy CPU proves that
+			 * this core cannot be fully idle. Skip its siblings.
+			 */
+			if (sched_smt_active() && fallback >= 0)
+				cpumask_andnot(ilb_cpus, ilb_cpus, cpu_smt_mask(ilb_cpu));
+			continue;
+		}
 
-	hk_mask = housekeeping_cpumask(HK_TYPE_KERNEL_NOISE);
+		/*
+		 * Running the idle load balancer on an idle sibling of a busy
+		 * SMT core can reduce the capacity available to its sibling. Prefer
+		 * a CPU whose entire core is idle, but retain the first idle CPU as
+		 * a fallback so idle balancing can still make progress when no fully
+		 * idle core exists.
+		 */
+		if (sched_smt_active() && !is_core_idle(ilb_cpu)) {
+			if (fallback < 0)
+				fallback = ilb_cpu;
 
-	for_each_cpu_and(ilb_cpu, nohz.idle_cpus_mask, hk_mask) {
-		if (ilb_cpu == this_cpu)
+			/*
+			 * The core is not idle, so there is no need to check
+			 * any of its other SMT siblings.
+			 */
+			cpumask_andnot(ilb_cpus, ilb_cpus,
+				       cpu_smt_mask(ilb_cpu));
 			continue;
+		}
 
-		if (idle_cpu(ilb_cpu))
-			return ilb_cpu;
+		return ilb_cpu;
 	}
 
-	return -1;
+	return fallback;
 }
 
 /*
  * Kick a CPU to do the NOHZ balancing, if it is time for it, via a cross-CPU
  * SMP function call (IPI).
  *
- * We pick the first idle CPU in the HK_TYPE_KERNEL_NOISE housekeeping set
- * (if there is one).
+ * Prefer a CPU on a fully idle core in the HK_TYPE_KERNEL_NOISE housekeeping
+ * set. Fall back to the first idle CPU when no fully idle core exists.
  */
 static void kick_ilb(unsigned int flags)
 {