Re: [PATCH v2] sched/fair: Prefer fully idle cores for NOHZ balancing
From: Mete Durlu
Date: Fri Jul 31 2026 - 08:59:06 EST
On 29/07/2026 18:32, Andrea Righi wrote:
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.
Cc: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Cc: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
Changes in v2:
- Avoid repeated is_core_idle() checks on wide SMT systems by pruning
the remaining siblings of a partially busy core (Prateek Nayak)
- Link to v1: https://lore.kernel.org/r/20260728214442.1648483-1-arighi@xxxxxxxxxx/
kernel/sched/fair.c | 48 ++++++++++++++++++++++++++++++++++++---------
1 file changed, 39 insertions(+), 9 deletions(-)
Hi,
thank you for the interesting patch! I am testing this on s390
to see how it impacts our platform. After reading the discussion
on v1 and reviewing the code I got minor nit. See below;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 37001c63452e5..b9e26938fb12a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13965,28 +13965,58 @@ 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();
- hk_mask = housekeeping_cpumask(HK_TYPE_KERNEL_NOISE);
+ /*
+ * 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_and(ilb_cpu, nohz.idle_cpus_mask, hk_mask) {
+ for_each_cpu(ilb_cpu, ilb_cpus) {
if (ilb_cpu == this_cpu)
continue;
- if (idle_cpu(ilb_cpu))
- return ilb_cpu;
+ if (!idle_cpu(ilb_cpu))
+ continue;
+
+ /*
+ * 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;
+
+ /*
+ * 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));
Just a nit but;
I think the purpose here is to move between cores if smt is active but
current logic seems to be doing that only after an idle cpu is found.
Consider that we first found an idle cpu but the siblings are busy,
after we move to the next core we start traversing per cpu again.
Wouldn't it be better if we always move per core after a fallback is
found? How about sth like below?
...
for_each_cpu(ilb_cpu, ilb_cpus) {
if (ilb_cpu == this_cpu)
continue;
if (sched_smt_active()) {
if (fallback < 0) {
if (!idle_cpu(ilb_cpu))
continue;
fallback = ilb_cpu;
}
/*
* 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 (is_core_idle(ilb_cpu))
return ilb_cpu;
cpumask_andnot(ilb_cpus, ilb_cpus, cpu_smt_mask(ilb_cpu));
continue;
}
if (idle_cpu(ilb_cpu))
return ilb_cpu;
}
return fallback;