[RESEND PATCH v13 6/9] sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any

Gabriele Monaco
2025-10-20T11:28:34Z

Currently the user can set up isolcpus and nohz_full in such a way that
leaves no housekeeping CPU (i.e. no CPU that is neither domain isolated
nor nohz full). This can be a problem for other subsystems (e.g. the
timer wheel imgration).

Prevent this configuration by invalidating the last setting in case the
union of isolcpus (domain) and nohz_full covers all CPUs.

Reviewed-by: Waiman Long <longman@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
kernel/sched/isolation.c | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index a4cf17b1fab0..3ad0d6df6a0a 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -167,6 +167,29 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
}
}

+ /*
+ * Check the combination of nohz_full and isolcpus=domain,
+ * necessary to avoid problems with the timer migration
+ * hierarchy. managed_irq is ignored by this check since it
+ * isn't considered in the timer migration logic.
+ */
+ iter_flags = housekeeping.flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
+ type = find_first_bit(&iter_flags, HK_TYPE_MAX);
+ /*
+ * Pass the check if none of these flags were previously set or
+ * are not in the current selection.
+ */
+ iter_flags = flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
+ first_cpu = (type == HK_TYPE_MAX || !iter_flags) ? 0 :
+ cpumask_first_and_and(cpu_present_mask,
+ housekeeping_staging, housekeeping.cpumasks[type]);
+ if (first_cpu >= min(nr_cpu_ids, setup_max_cpus)) {
+ pr_warn("Housekeeping: must include one present CPU "
+ "neither in nohz_full= nor in isolcpus=domain, "
+ "ignoring setting %s\n", str);
+ goto free_housekeeping_staging;
+ }
+
iter_flags = flags & ~housekeeping.flags;

for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
--
2.51.0