[PATCH 03/23] tick/nohz: Make nohz_full parameter optional

Waiman Long
2026-04-21T03:05:29Z

To provide nohz_full tick support, there is a set of tick dependency
masks that need to be evaluated on every IRQ and context switch.
Switching on nohz_full tick support at runtime will be problematic
as some of the tick dependency masks may not be properly set causing
problem down the road.

Allow nohz_full boot option to be specified without any
parameter to force enable nohz_full tick support without any
CPU in the tick_nohz_full_mask yet. The context_tracking_key and
tick_nohz_full_running flag will be enabled in this case to make
tick_nohz_full_enabled() return true.

There is still a small performance overhead by force enable nohz_full
this way. So it should only be used if there is a chance that some
CPUs may become isolated later via the cpuset isolated partition
functionality and better CPU isolation closed to nohz_full is desired.

Signed-off-by: Waiman Long <longman@redhat.com>
---
Documentation/admin-guide/kernel-parameters.txt | 15 +++++++++------
include/linux/context_tracking.h | 7 ++++++-
kernel/context_tracking.c | 4 +++-
kernel/rcu/tree_nocb.h | 2 +-
kernel/sched/isolation.c | 13 ++++++++++++-
kernel/time/tick-sched.c | 11 +++++++++--
6 files changed, 40 insertions(+), 12 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 95f97ce487a4..f0eedaebe9d6 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4550,13 +4550,16 @@ Kernel parameters
Valid arguments: on, off
Default: on

- nohz_full= [KNL,BOOT,SMP,ISOL]
- The argument is a cpu list, as described above.
+ nohz_full[=cpu-list]
+ [KNL,BOOT,SMP,ISOL]
In kernels built with CONFIG_NO_HZ_FULL=y, set
- the specified list of CPUs whose tick will be stopped
- whenever possible. The boot CPU will be forced outside
- the range to maintain the timekeeping. Any CPUs
- in this list will have their RCU callbacks offloaded,
+ the specified list of CPUs whose tick will be
+ stopped whenever possible. If the argument is
+ not specified, nohz_full will be forced enabled
+ without any CPU in the nohz_full list yet.
+ The boot CPU will be forced outside the range
+ to maintain the timekeeping. Any CPUs in this
+ list will have their RCU callbacks offloaded,
just as if they had also been called out in the
rcu_nocbs= boot parameter.

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index af9fe87a0922..a3fea7f9fef6 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -9,8 +9,13 @@

#include <asm/ptrace.h>

-
#ifdef CONFIG_CONTEXT_TRACKING_USER
+/*
+ * Pass CONTEXT_TRACKING_FORCE_ENABLE to ct_cpu_track_user() to force enable
+ * user context tracking.
+ */
+#define CONTEXT_TRACKING_FORCE_ENABLE (-1)
+
extern void ct_cpu_track_user(int cpu);

/* Called with interrupts disabled. */
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index a743e7ffa6c0..925999de1a28 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -678,7 +678,9 @@ void __init ct_cpu_track_user(int cpu)
{
static __initdata bool initialized = false;

- if (!per_cpu(context_tracking.active, cpu)) {
+ if (cpu == CONTEXT_TRACKING_FORCE_ENABLE) {
+ static_branch_inc(&context_tracking_key);
+ } else if (!per_cpu(context_tracking.active, cpu)) {
per_cpu(context_tracking.active, cpu) = true;
static_branch_inc(&context_tracking_key);
}
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index b3337c7231cc..2d06dcb61f37 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1267,7 +1267,7 @@ void __init rcu_init_nohz(void)
struct shrinker * __maybe_unused lazy_rcu_shrinker;

#if defined(CONFIG_NO_HZ_FULL)
- if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
+ if (tick_nohz_full_running)
cpumask = tick_nohz_full_mask;
#endif

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 965d6f8fe344..c233d55a1e95 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -268,6 +268,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
}

alloc_bootmem_cpumask_var(&non_housekeeping_mask);
+
if (cpulist_parse(str, non_housekeeping_mask) < 0) {
pr_warn("Housekeeping: nohz_full= or isolcpus= incorrect CPU range\n");
goto free_non_housekeeping_mask;
@@ -277,6 +278,13 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
cpumask_andnot(housekeeping_staging,
cpu_possible_mask, non_housekeeping_mask);

+ /*
+ * Allow "nohz_full" without parameter to force enable nohz_full
+ * at boot time without any CPUs in the nohz_full list yet.
+ */
+ if ((flags & HK_FLAG_KERNEL_NOISE) && !*str)
+ goto setup_housekeeping_staging;
+
first_cpu = cpumask_first_and(cpu_present_mask, housekeeping_staging);
if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
__cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
@@ -290,6 +298,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
if (cpumask_empty(non_housekeeping_mask))
goto free_housekeeping_staging;

+setup_housekeeping_staging:
if (!housekeeping.flags) {
/* First setup call ("nohz_full=" or "isolcpus=") */
enum hk_type type;
@@ -357,10 +366,12 @@ static int __init housekeeping_nohz_full_setup(char *str)
unsigned long flags;

flags = HK_FLAG_KERNEL_NOISE | HK_FLAG_KERNEL_NOISE_BOOT;
+ if (*str == '=')
+ str++;

return housekeeping_setup(str, flags);
}
-__setup("nohz_full=", housekeeping_nohz_full_setup);
+__setup("nohz_full", housekeeping_nohz_full_setup);

static int __init housekeeping_isolcpus_setup(char *str)
{
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 9e5264458414..ed877b2c9040 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -676,8 +676,15 @@ void __init tick_nohz_init(void)
}
}

- for_each_cpu(cpu, tick_nohz_full_mask)
- ct_cpu_track_user(cpu);
+ /*
+ * Force enable context_tracking_key if tick_nohz_full_mask empty
+ */
+ if (cpumask_empty(tick_nohz_full_mask)) {
+ ct_cpu_track_user(CONTEXT_TRACKING_FORCE_ENABLE);
+ } else {
+ for_each_cpu(cpu, tick_nohz_full_mask)
+ ct_cpu_track_user(cpu);
+ }

ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
"kernel/nohz:predown", NULL,
--
2.53.0