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In this case, tick_nohz_idle_stop_tick() observes TS_FLAG_STOPPED already set and skips nohz_balance_enter_idle() because the !was_stopped condition assumes tick-stop and idle-entry are coupled. This leaves a tickless idle CPU absent from nohz.idle_cpus_mask, making it invisible to NOHZ idle load balancing while periodic balancing is also suppressed. The patch fixes this by decoupling tick-stop transition accounting from scheduler bookkeeping. idle_jiffies remains updated only on the tick-stop transition, while nohz_balance_enter_idle() is invoked whenever a CPU enters idle with the tick already stopped, relying on its existing idempotent gaurd to avoid duplicate registration. Tested on Ampere Altra on 6.19.0-rc8 with CONFIG_NO_HZ_FULL enabled: - This change improves load distribution by ensuring that tickless idle CPUs are visible to NOHZ idle load balancing. In llama-batched-bench, throughput improves by up to ~14% across multiple thread counts. - Hackbench single-process results improve by 5% and multi-process results improve by up to ~26%, consistent with reduced scheduler jitter and earlier utilization of fully idle cores. No regressions observed. Signed-off-by: Shubhang Kaushik Signed-off-by: Adam Li Reviewed-by: Christoph Lameter (Ampere) Reviewed-by: Shubhang Kaushik --- This is a resend of the original patch to ensure visibility. Previous resend: https://lkml.org/lkml/2025/8/21/170 Original thread: https://lkml.org/lkml/2025/8/21/171 The patch addresses a performance regression in NOHZ idle load balancing=20 observed under CONFIG_NO_HZ_FULL, where idle CPUs were becoming=20 invisible to the balancer. --- kernel/time/tick-sched.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 2f8a7923fa279409ffe950f770ff2eac868f6ece..eee6fcebe78c2f8d93464a55fe3= 32e12fe9c164e 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -1250,8 +1250,9 @@ void tick_nohz_idle_stop_tick(void) ts->idle_sleeps++; ts->idle_expires =3D expires; =20 - if (!was_stopped && tick_sched_flag_test(ts, TS_FLAG_STOPPED)) { - ts->idle_jiffies =3D ts->last_jiffies; + if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) { + if (!was_stopped) + ts->idle_jiffies =3D ts->last_jiffies; nohz_balance_enter_idle(cpu); } } else { --- base-commit: 18f7fcd5e69a04df57b563360b88be72471d6b62 change-id: 20260203-fix-nohz-idle-b2838276cb91 Best regards, --=20 Shubhang Kaushik