* [RFC PATCH v2 1/2] tick/nohz: Add a policy dependency for full dynticks
2026-10-17 13:13 [RFC PATCH v2 0/2] tick/nohz: Add per-CPU control of non-idle tick suppression Peng Wang
@ 2026-10-17 13:13 ` Peng Wang
2026-10-17 13:13 ` [RFC PATCH v2 2/2] cpu: Add per-CPU control of non-idle tick suppression Peng Wang
1 sibling, 0 replies; 3+ messages in thread
From: Peng Wang @ 2026-10-17 13:13 UTC (permalink / raw)
To: Anna-Maria Behnsen, Frederic Weisbecker, Ingo Molnar,
Thomas Gleixner
Cc: Peng Wang, Greg Kroah-Hartman, Rafael J . Wysocki,
Danilo Krummrich, Steven Rostedt, Masami Hiramatsu,
Mathieu Desnoyers, Peter Zijlstra, Waiman Long, Tejun Heo,
He Rongguang, linux-kernel, linux-api, driver-core,
linux-trace-kernel, cgroups
Cloud hosts may pin vCPU threads one-to-one to host CPUs. At full
allocation this includes the housekeeping CPUs, which always retain
their non-idle tick. Operators want comparable performance for otherwise
equivalent vCPUs. Retaining the non-idle tick on nohz_full CPUs as well
avoids one placement-dependent difference in host tick interference.
With spare capacity, selected CPUs may stop the tick to reduce host
scheduler-tick interference with vCPUs.
Allocation changes as VMs are created or destroyed while other VMs
remain running. A vCPU thread may be the only runnable task in either
state, so existing tick dependencies do not express this policy.
Add a per-CPU POLICY dependency within the boot-time nohz_full set.
Permission defaults to enabled. The policy can be changed without
taking CPUs offline. Revoking permission uses the existing asynchronous
kick; granting it leaves all other dependencies effective. Preserve
POLICY across CPU hotplug and report it in tick_stop tracing.
Idle tick suppression, nohz_full membership, context tracking, RCU
offloading and housekeeping configuration are unchanged.
Co-developed-by: He Rongguang <herongguang@linux.alibaba.com>
Signed-off-by: He Rongguang <herongguang@linux.alibaba.com>
Signed-off-by: Peng Wang <peng_wang@linux.alibaba.com>
---
include/linux/tick.h | 8 +++++--
include/trace/events/timer.h | 3 ++-
kernel/time/tick-sched.c | 42 ++++++++++++++++++++++++++++++++++++
3 files changed, 50 insertions(+), 3 deletions(-)
diff --git a/include/linux/tick.h b/include/linux/tick.h
index b121c5d53e308..8b250f245ff31 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -111,9 +111,10 @@ enum tick_dep_bits {
TICK_DEP_BIT_SCHED = 2,
TICK_DEP_BIT_CLOCK_UNSTABLE = 3,
TICK_DEP_BIT_RCU = 4,
- TICK_DEP_BIT_RCU_EXP = 5
+ TICK_DEP_BIT_RCU_EXP = 5,
+ TICK_DEP_BIT_POLICY = 6
};
-#define TICK_DEP_BIT_MAX TICK_DEP_BIT_RCU_EXP
+#define TICK_DEP_BIT_MAX TICK_DEP_BIT_POLICY
#define TICK_DEP_MASK_NONE 0
#define TICK_DEP_MASK_POSIX_TIMER (1 << TICK_DEP_BIT_POSIX_TIMER)
@@ -122,6 +123,7 @@ enum tick_dep_bits {
#define TICK_DEP_MASK_CLOCK_UNSTABLE (1 << TICK_DEP_BIT_CLOCK_UNSTABLE)
#define TICK_DEP_MASK_RCU (1 << TICK_DEP_BIT_RCU)
#define TICK_DEP_MASK_RCU_EXP (1 << TICK_DEP_BIT_RCU_EXP)
+#define TICK_DEP_MASK_POLICY (1 << TICK_DEP_BIT_POLICY)
#ifdef CONFIG_NO_HZ_COMMON
extern bool tick_nohz_enabled;
@@ -205,6 +207,8 @@ extern void tick_nohz_dep_set_signal(struct task_struct *tsk,
extern void tick_nohz_dep_clear_signal(struct signal_struct *signal,
enum tick_dep_bits bit);
extern bool tick_nohz_cpu_hotpluggable(unsigned int cpu);
+extern bool tick_nohz_full_get_allowed(unsigned int cpu);
+extern int tick_nohz_full_set_allowed(unsigned int cpu, bool allowed);
/*
* The below are tick_nohz_[set,clear]_dep() wrappers that optimize off-cases
diff --git a/include/trace/events/timer.h b/include/trace/events/timer.h
index 3aa0608c63611..dff9579cd90eb 100644
--- a/include/trace/events/timer.h
+++ b/include/trace/events/timer.h
@@ -435,7 +435,8 @@ TRACE_EVENT(itimer_expire,
tick_dep_name(SCHED) \
tick_dep_name(CLOCK_UNSTABLE) \
tick_dep_name(RCU) \
- tick_dep_name_end(RCU_EXP)
+ tick_dep_name(RCU_EXP) \
+ tick_dep_name_end(POLICY)
#undef tick_dep_name
#undef tick_dep_mask_name
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 87e2a7a875265..0d6fcc3736bae 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -375,6 +375,11 @@ static bool check_tick_dependency(atomic_t *dep)
return true;
}
+ if (val & TICK_DEP_MASK_POLICY) {
+ trace_tick_stop(0, TICK_DEP_MASK_POLICY);
+ return true;
+ }
+
return false;
}
@@ -561,6 +566,39 @@ void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit)
}
EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_cpu);
+/* Read policy, not whether a CPU currently has its tick stopped. */
+bool tick_nohz_full_get_allowed(unsigned int cpu)
+{
+ struct tick_sched *ts = per_cpu_ptr(&tick_cpu_sched, cpu);
+
+ /* CPU teardown resets and restores this bit under stop_machine(). */
+ return !(atomic_read(&ts->tick_dep_mask) & TICK_DEP_MASK_POLICY);
+}
+
+int tick_nohz_full_set_allowed(unsigned int cpu, bool allowed)
+{
+ struct tick_sched *ts = per_cpu_ptr(&tick_cpu_sched, cpu);
+
+ if (!tick_nohz_full_cpu(cpu))
+ return -EINVAL;
+
+ /* CPU device removal may wait for an active sysfs callback. */
+ if (!cpus_read_trylock())
+ return -EBUSY;
+
+ if (allowed) {
+ tick_nohz_dep_clear_cpu(cpu, TICK_DEP_BIT_POLICY);
+ } else if (cpu_online(cpu)) {
+ /* The existing kick requests an asynchronous restart. */
+ tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_POLICY);
+ } else {
+ /* Record policy without queuing irq_work on an offline CPU. */
+ atomic_or(TICK_DEP_MASK_POLICY, &ts->tick_dep_mask);
+ }
+ cpus_read_unlock();
+ return 0;
+}
+
/*
* Set a per-task tick dependency. RCU needs this. Also posix CPU timers
* in order to elapse per task timers.
@@ -1533,6 +1571,7 @@ void tick_sched_timer_dying(int cpu)
{
struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
unsigned long idle_calls, idle_sleeps;
+ int policy;
/* This must happen before hrtimers are migrated! */
if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES))
@@ -1540,7 +1579,10 @@ void tick_sched_timer_dying(int cpu)
idle_calls = ts->idle_calls;
idle_sleeps = ts->idle_sleeps;
+ /* Preserve only the policy dependency across CPU offline/online. */
+ policy = atomic_read(&ts->tick_dep_mask) & TICK_DEP_MASK_POLICY;
memset(ts, 0, sizeof(*ts));
+ atomic_set(&ts->tick_dep_mask, policy);
ts->idle_calls = idle_calls;
ts->idle_sleeps = idle_sleeps;
}
--
2.47.0
^ permalink raw reply related [flat|nested] 3+ messages in thread* [RFC PATCH v2 2/2] cpu: Add per-CPU control of non-idle tick suppression
2026-10-17 13:13 [RFC PATCH v2 0/2] tick/nohz: Add per-CPU control of non-idle tick suppression Peng Wang
2026-10-17 13:13 ` [RFC PATCH v2 1/2] tick/nohz: Add a policy dependency for full dynticks Peng Wang
@ 2026-10-17 13:13 ` Peng Wang
1 sibling, 0 replies; 3+ messages in thread
From: Peng Wang @ 2026-10-17 13:13 UTC (permalink / raw)
To: Anna-Maria Behnsen, Frederic Weisbecker, Ingo Molnar,
Thomas Gleixner
Cc: Peng Wang, Greg Kroah-Hartman, Rafael J . Wysocki,
Danilo Krummrich, Steven Rostedt, Masami Hiramatsu,
Mathieu Desnoyers, Peter Zijlstra, Waiman Long, Tejun Heo,
He Rongguang, linux-kernel, linux-api, driver-core,
linux-trace-kernel, cgroups
Expose cpuN/nohz_tick_allowed on CPUs in the boot-time nohz_full set.
The boolean defaults to 1: allow non-idle tick suppression, subject to
existing dependencies. Writing 0 prevents suppression through POLICY.
Readback reports permission, not the current tick state.
Each write affects only that CPU. Registered offline CPUs remain
configurable, and policy persists across offline/online transitions.
Retry writes when CPU hotplug is in progress so CPU device removal
can drain active sysfs callbacks without a lock dependency cycle.
Co-developed-by: He Rongguang <herongguang@linux.alibaba.com>
Signed-off-by: He Rongguang <herongguang@linux.alibaba.com>
Signed-off-by: Peng Wang <peng_wang@linux.alibaba.com>
---
.../ABI/testing/sysfs-devices-system-cpu | 27 ++++++++++
drivers/base/cpu.c | 53 +++++++++++++++++++
2 files changed, 80 insertions(+)
diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 82d10d556cc89..4f6ac6b41e2b6 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -767,6 +767,33 @@ Description: Preferred MTE tag checking mode
See also: Documentation/arch/arm64/memory-tagging-extension.rst
+What: /sys/devices/system/cpu/cpuX/nohz_tick_allowed
+Date: October 2026
+Contact: Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+ (RW) Boolean allowing this CPU to stop its non-idle scheduler
+ tick. Present only for registered CPUs in the boot-time
+ nohz_full set, with CONFIG_NO_HZ_FULL.
+
+ The default is 1 (allowed), subject to existing tick
+ dependencies. Writing 0 prevents non-idle tick suppression;
+ writing 1 allows it without forcing the tick to stop. Each
+ write affects one CPU. Readback reports permission, not the
+ current tick state.
+
+ Revoking permission requests an asynchronous restart of a
+ stopped non-idle tick on an online CPU. The write does not wait
+ for restart. Idle tick suppression is unchanged; the policy
+ applies when the CPU leaves idle.
+
+ Registered offline CPUs may be configured. Policy is retained
+ per logical CPU across offline/online and CPU device removal
+ and re-addition. CPUs without a registered cpuN device cannot
+ be configured through this interface.
+
+ This does not change nohz_full membership, context tracking,
+ RCU offloading or housekeeping configuration.
+
What: /sys/devices/system/cpu/nohz_full
Date: Apr 2015
Contact: Linux kernel mailing list <linux-kernel@vger.kernel.org>
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 69e52fed42415..6fad1b0227dac 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -7,6 +7,7 @@
#include <linux/module.h>
#include <linux/init.h>
#include <linux/sched.h>
+#include <linux/sched/signal.h>
#include <linux/cpu.h>
#include <linux/topology.h>
#include <linux/device.h>
@@ -189,7 +190,56 @@ static const struct attribute_group crash_note_cpu_attr_group = {
};
#endif
+#ifdef CONFIG_NO_HZ_FULL
+static ssize_t nohz_tick_allowed_show(struct device *dev,
+ struct device_attribute *attr, char *buf)
+{
+ return sysfs_emit(buf, "%u\n", tick_nohz_full_get_allowed(dev->id));
+}
+
+static ssize_t nohz_tick_allowed_store(struct device *dev,
+ struct device_attribute *attr,
+ const char *buf, size_t count)
+{
+ bool allowed;
+ int ret;
+
+ ret = kstrtobool(buf, &allowed);
+ if (ret)
+ return ret;
+
+ ret = tick_nohz_full_set_allowed(dev->id, allowed);
+ if (ret == -EBUSY) {
+ /* Retry after releasing the sysfs active reference. */
+ fsleep(5000);
+ return restart_syscall();
+ }
+
+ return ret ? ret : count;
+}
+static DEVICE_ATTR_RW(nohz_tick_allowed);
+
+static umode_t nohz_tick_is_visible(struct kobject *kobj,
+ struct attribute *attr, int unused)
+{
+ return tick_nohz_full_cpu(kobj_to_dev(kobj)->id) ? attr->mode : 0;
+}
+
+static struct attribute *nohz_tick_cpu_attrs[] = {
+ &dev_attr_nohz_tick_allowed.attr,
+ NULL
+};
+
+static const struct attribute_group nohz_tick_cpu_attr_group = {
+ .attrs = nohz_tick_cpu_attrs,
+ .is_visible = nohz_tick_is_visible,
+};
+#endif
+
static const struct attribute_group *common_cpu_attr_groups[] = {
+#ifdef CONFIG_NO_HZ_FULL
+ &nohz_tick_cpu_attr_group,
+#endif
#ifdef CONFIG_CRASH_DUMP
&crash_note_cpu_attr_group,
#endif
@@ -197,6 +247,9 @@ static const struct attribute_group *common_cpu_attr_groups[] = {
};
static const struct attribute_group *hotplugable_cpu_attr_groups[] = {
+#ifdef CONFIG_NO_HZ_FULL
+ &nohz_tick_cpu_attr_group,
+#endif
#ifdef CONFIG_CRASH_DUMP
&crash_note_cpu_attr_group,
#endif
--
2.47.0
^ permalink raw reply related [flat|nested] 3+ messages in thread