[PATCH-next 00/23] cgroup/cpuset: Enable runtime update of nohz_full and managed_irq CPUs

[PATCH-next 00/23] cgroup/cpuset: Enable runtime update of nohz_full and managed_irq CPUs

[Waiman Long]

The "isolcpus=domain" CPU list implied by the HK_TYPE_DOMAIN housekeeping
cpumask is being dynamically updated whenever the set of cpuset isolated
partitions change. This patch series extends the isolated partition code
to make dynamic changes to the "nohz_full" and "isolcpus=managed_irq"
CPU lists. These CPU lists correspoond to equivalent changes in the
HK_TYPE_KERNEL_NOISE and HK_TYPE_MANAGED_IRQ housekeeping cpumasks.

To facilitate the changing of these CPU lists, the CPU hotplug code
which is doing a lot of heavy lifting is now being used. For changing
the "nohz_full" and HK_TYPE_KERNEL_NOISE cpumasks, the affected CPUs
are torn down into offline mode first. Then the housekeeping cpumask
and the corresponding cpumasks in the RCU, tick and watchdog subsystems
are modified. After that, the affected CPUs are brought online again.

It is slightly different with the "managed_irq" HK_TYPE_MANAGED_IRQ
cpumask, the cpumask is updated first and the affected CPUs are torn
down and brought up to move the managed interrupts away from the newly
isolated CPUs.

Using CPU hotplug does have its drawback when multiple isolated
partitions are being managed in a system. The CPU offline process uses
the stop_machine mechanism to stop all the CPUs except the one being torn
down. This will cause latency spike for isolated CPUs in other isolated
partitions. It is not a problem if only one isolated partition is
needed. This is an issue we need to address in the near future.

Patches 1-7 enables runtime updates to the nohz_full/managed_irq
cpumasks in sched/isolation, tick, RCU and watchdog subsystems.

Patches 8-17 modifies various subsystems that need access to the
HK_TYPE_MANAGED_IRQ cpumask and HK_TYPE_KERNEL_NOISE cpumask and its
aliases. Like the runtime modifiable HK_TYPE_DOMAIN cpumask, RCU is used
to protect access to cpumasks to avoid potential UAF problem. Patch 17
updates the housekeeping_dereference_check() to print a WARNING when
lockdep is enabled if those housekeeping cpumasks are used without the
proper lock protection.

Patch 18 introduces a new cpuhp_offline_cb() API that enables the
shutting down of a given list of CPUs, run a callback function and then
brought those CPUs up again while disallowing any concurrent CPU hotplug
activity.

Patches 19-23 updates the cpuset code, selftest and documentation to
allow change in isolated partition configuration to be reflected in
the housekeeping and other cpumasks dynamically.

As there is a slight overhead in enabling dynamic update to the nohz_full
cpumask, this new nohz_full and managed_irq runtime update feature
has to be explicitly opted in by adding a nohz_full kernel command
line parameter with or without a CPU list to indicate a desire to use
this feature. It is also because a number of subsystems have explicit
check of nohz_full at boot time to adjust their behavior which may not
be easy to modify after boot.

Waiman Long (23):
  sched/isolation: Add HK_TYPE_KERNEL_NOISE_BOOT &
    HK_TYPE_MANAGED_IRQ_BOOT
  sched/isolation: Enhance housekeeping_update() to support updating
    more than one HK cpumask
  tick/nohz: Make nohz_full parameter optional
  tick/nohz: Allow runtime changes in full dynticks CPUs
  tick: Pass timer tick job to an online HK CPU in tick_cpu_dying()
  rcu/nocbs: Allow runtime changes in RCU NOCBS cpumask
  watchdog: Sync up with runtime change of isolated CPUs
  arm64: topology: Use RCU to protect access to HK_TYPE_TICK cpumask
  workqueue: Use RCU to protect access of HK_TYPE_TIMER cpumask
  cpu: Use RCU to protect access of HK_TYPE_TIMER cpumask
  hrtimer: Use RCU to protect access of HK_TYPE_TIMER cpumask
  net: Use boot time housekeeping cpumask settings for now
  sched/core: Use RCU to protect access of HK_TYPE_KERNEL_NOISE cpumask
  hwmon/coretemp: Use RCU to protect access of HK_TYPE_MISC cpumask
  Drivers: hv: Use RCU to protect access of HK_TYPE_MANAGED_IRQ cpumask
  genirq/cpuhotplug: Use RCU to protect access of HK_TYPE_MANAGED_IRQ
    cpumask
  sched/isolation: Extend housekeeping_dereference_check() to cover
    changes in nohz_full or manged_irqs           cpumasks
  cpu/hotplug: Add a new cpuhp_offline_cb() API
  cgroup/cpuset: Improve check for calling housekeeping_update()
  cgroup/cpuset: Enable runtime update of
    HK_TYPE_{KERNEL_NOISE,MANAGED_IRQ} cpumasks
  cgroup/cpuset: Limit the side effect of using CPU hotplug on isolated
    partition
  cgroup/cpuset: Prevent offline_disabled CPUs from being used in
    isolated partition
  cgroup/cpuset: Documentation and kselftest updates

 Documentation/admin-guide/cgroup-v2.rst       |  35 ++-
 .../admin-guide/kernel-parameters.txt         |  15 +-
 arch/arm64/kernel/topology.c                  |  17 +-
 drivers/hv/channel_mgmt.c                     |  15 +-
 drivers/hv/vmbus_drv.c                        |   7 +-
 drivers/hwmon/coretemp.c                      |   6 +-
 include/linux/context_tracking.h              |   8 +-
 include/linux/cpuhplock.h                     |   9 +
 include/linux/nmi.h                           |   2 +
 include/linux/rcupdate.h                      |   2 +
 include/linux/sched/isolation.h               |  18 +-
 include/linux/tick.h                          |   2 +
 kernel/cgroup/cpuset-internal.h               |   1 +
 kernel/cgroup/cpuset.c                        | 292 +++++++++++++++++-
 kernel/context_tracking.c                     |  19 +-
 kernel/cpu.c                                  |  72 +++++
 kernel/irq/cpuhotplug.c                       |   1 +
 kernel/irq/manage.c                           |   1 +
 kernel/rcu/tree_nocb.h                        |  24 +-
 kernel/sched/core.c                           |   4 +-
 kernel/sched/isolation.c                      | 135 +++++---
 kernel/time/hrtimer.c                         |   4 +-
 kernel/time/tick-common.c                     |  16 +-
 kernel/time/tick-sched.c                      |  48 ++-
 kernel/watchdog.c                             |  24 ++
 kernel/workqueue.c                            |   6 +-
 net/core/net-sysfs.c                          |   2 +-
 .../selftests/cgroup/test_cpuset_prs.sh       |  70 ++++-
 28 files changed, 747 insertions(+), 108 deletions(-)

-- 
2.53.0

[PATCH 01/23] sched/isolation: Add HK_TYPE_KERNEL_NOISE_BOOT & HK_TYPE_MANAGED_IRQ_BOOT

[Waiman Long]

Since commit 4fca0e550d50 ("sched/isolation: Save boot defined
domain flags"), HK_TYPE_DOMAIN_BOOT was added to record the boot
time "isolcpus{=domain}" setting. As we are going to make the
HK_TYPE_MANAGED_IRQ and HK_TYPE_KERNEL_NOISE housekeeping cpumasks
runtime modifiable, we need some additional cpumasks to record the boot
time settings to make sure that those housekeeping cpumasks will always
be a subset of their boot time equivalents.

Introduce the new HK_TYPE_KERNEL_NOISE_BOOT and HK_TYPE_MANAGED_IRQ_BOOT
housekeeping types to do that.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/sched/isolation.h | 16 ++++++++++++++--
 kernel/sched/isolation.c        | 16 +++++++++-------
 2 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
index dc3975ff1b2e..d1707f121e20 100644
--- a/include/linux/sched/isolation.h
+++ b/include/linux/sched/isolation.h
@@ -14,10 +14,22 @@ enum hk_type {
 	 * is always a subset of HK_TYPE_DOMAIN_BOOT.
 	 */
 	HK_TYPE_DOMAIN,
-	/* Inverse of boot-time isolcpus=managed_irq argument */
-	HK_TYPE_MANAGED_IRQ,
+
 	/* Inverse of boot-time nohz_full= or isolcpus=nohz arguments */
+	HK_TYPE_KERNEL_NOISE_BOOT,
+	/*
+	 * A subset of HK_TYPE_KERNEL_NOISE_BOOT as it may excludes some
+	 * additional isolated CPUs at run time.
+	 */
 	HK_TYPE_KERNEL_NOISE,
+
+	/* Inverse of boot-time isolcpus=managed_irq argument */
+	HK_TYPE_MANAGED_IRQ_BOOT,
+	/*
+	 * A subset of HK_TYPE_MANAGED_IRQ_BOOT as it may excludes some
+	 * additional isolated CPUs at run time.
+	 */
+	HK_TYPE_MANAGED_IRQ,
 	HK_TYPE_MAX,
 
 	/*
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index a947d75b43f1..9ec9ae510dc7 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -12,10 +12,12 @@
 #include "sched.h"
 
 enum hk_flags {
-	HK_FLAG_DOMAIN_BOOT	= BIT(HK_TYPE_DOMAIN_BOOT),
-	HK_FLAG_DOMAIN		= BIT(HK_TYPE_DOMAIN),
-	HK_FLAG_MANAGED_IRQ	= BIT(HK_TYPE_MANAGED_IRQ),
-	HK_FLAG_KERNEL_NOISE	= BIT(HK_TYPE_KERNEL_NOISE),
+	HK_FLAG_DOMAIN_BOOT	  = BIT(HK_TYPE_DOMAIN_BOOT),
+	HK_FLAG_DOMAIN		  = BIT(HK_TYPE_DOMAIN),
+	HK_FLAG_KERNEL_NOISE_BOOT = BIT(HK_TYPE_KERNEL_NOISE_BOOT),
+	HK_FLAG_KERNEL_NOISE	  = BIT(HK_TYPE_KERNEL_NOISE),
+	HK_FLAG_MANAGED_IRQ_BOOT  = BIT(HK_TYPE_MANAGED_IRQ_BOOT),
+	HK_FLAG_MANAGED_IRQ	  = BIT(HK_TYPE_MANAGED_IRQ),
 };
 
 DEFINE_STATIC_KEY_FALSE(housekeeping_overridden);
@@ -315,7 +317,7 @@ static int __init housekeeping_nohz_full_setup(char *str)
 {
 	unsigned long flags;
 
-	flags = HK_FLAG_KERNEL_NOISE;
+	flags = HK_FLAG_KERNEL_NOISE | HK_FLAG_KERNEL_NOISE_BOOT;
 
 	return housekeeping_setup(str, flags);
 }
@@ -334,7 +336,7 @@ static int __init housekeeping_isolcpus_setup(char *str)
 		 */
 		if (!strncmp(str, "nohz,", 5)) {
 			str += 5;
-			flags |= HK_FLAG_KERNEL_NOISE;
+			flags |= HK_FLAG_KERNEL_NOISE | HK_FLAG_KERNEL_NOISE_BOOT;
 			continue;
 		}
 
@@ -346,7 +348,7 @@ static int __init housekeeping_isolcpus_setup(char *str)
 
 		if (!strncmp(str, "managed_irq,", 12)) {
 			str += 12;
-			flags |= HK_FLAG_MANAGED_IRQ;
+			flags |= HK_FLAG_MANAGED_IRQ | HK_FLAG_MANAGED_IRQ_BOOT;
 			continue;
 		}
 
-- 
2.53.0

[PATCH 02/23] sched/isolation: Enhance housekeeping_update() to support updating more than one HK cpumask

[Waiman Long]

The housekeeping_update() function currently allows update to the
HK_TYPE_DOMAIN cpumask only. As we are going to enable dynamic
modification of the other housekeeping cpumasks, we need to extend
it to support passing in the information about the HK cpumask(s) to
be updated.  In cases where some HK cpumasks happen to be the same,
it will be more efficient to update multiple HK cpumasks in one single
call instead of calling it multiple times. Extend housekeeping_update()
to support that as well.

Also add the restriction that passed in isolated cpumask parameter
of housekeeping_update() must include all the CPUs isolated at boot
time. This is currently the case for cpuset anyway.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/sched/isolation.h |  2 +-
 kernel/cgroup/cpuset.c          |  2 +-
 kernel/sched/isolation.c        | 99 +++++++++++++++++++++++----------
 3 files changed, 71 insertions(+), 32 deletions(-)

diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
index d1707f121e20..a17f16e0156e 100644
--- a/include/linux/sched/isolation.h
+++ b/include/linux/sched/isolation.h
@@ -51,7 +51,7 @@ extern const struct cpumask *housekeeping_cpumask(enum hk_type type);
 extern bool housekeeping_enabled(enum hk_type type);
 extern void housekeeping_affine(struct task_struct *t, enum hk_type type);
 extern bool housekeeping_test_cpu(int cpu, enum hk_type type);
-extern int housekeeping_update(struct cpumask *isol_mask);
+extern int housekeeping_update(struct cpumask *isol_mask, unsigned long flags);
 extern void __init housekeeping_init(void);
 
 #else
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 1335e437098e..a4eccb0ec0d1 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1354,7 +1354,7 @@ static void cpuset_update_sd_hk_unlock(void)
 		 */
 		mutex_unlock(&cpuset_mutex);
 		cpus_read_unlock();
-		WARN_ON_ONCE(housekeeping_update(isolated_hk_cpus));
+		WARN_ON_ONCE(housekeeping_update(isolated_hk_cpus, BIT(HK_TYPE_DOMAIN)));
 		mutex_unlock(&cpuset_top_mutex);
 	} else {
 		cpuset_full_unlock();
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 9ec9ae510dc7..965d6f8fe344 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -120,48 +120,87 @@ bool housekeeping_test_cpu(int cpu, enum hk_type type)
 }
 EXPORT_SYMBOL_GPL(housekeeping_test_cpu);
 
-int housekeeping_update(struct cpumask *isol_mask)
-{
-	struct cpumask *trial, *old = NULL;
-	int err;
+/* HK type processing table */
+static struct {
+	int type;
+	int boot_type;
+} hk_types[] = {
+	{ HK_TYPE_DOMAIN,       HK_TYPE_DOMAIN_BOOT	  },
+	{ HK_TYPE_MANAGED_IRQ,  HK_TYPE_MANAGED_IRQ_BOOT  },
+	{ HK_TYPE_KERNEL_NOISE, HK_TYPE_KERNEL_NOISE_BOOT }
+};
 
-	trial = kmalloc(cpumask_size(), GFP_KERNEL);
-	if (!trial)
-		return -ENOMEM;
+#define HK_TYPE_CNT	ARRAY_SIZE(hk_types)
 
-	cpumask_andnot(trial, housekeeping_cpumask(HK_TYPE_DOMAIN_BOOT), isol_mask);
-	if (!cpumask_intersects(trial, cpu_online_mask)) {
-		kfree(trial);
-		return -EINVAL;
+int housekeeping_update(struct cpumask *isol_mask, unsigned long flags)
+{
+	struct cpumask *trial[HK_TYPE_CNT];
+	int i, err = 0;
+
+	for (i = 0; i < HK_TYPE_CNT; i++) {
+		int type = hk_types[i].type;
+		int boot = hk_types[i].boot_type;
+
+		trial[i] = NULL;
+		if (flags & BIT(type)) {
+			trial[i] = kmalloc(cpumask_size(), GFP_KERNEL);
+			if (!trial[i]) {
+				err = -ENOMEM;
+				goto out;
+			}
+			/*
+			 * The new HK cpumask must be a subset of its boot
+			 * cpumask.
+			 */
+			cpumask_andnot(trial[i], cpu_possible_mask, isol_mask);
+			if (!cpumask_intersects(trial[i], cpu_online_mask) ||
+			    !cpumask_subset(trial[i], housekeeping_cpumask(boot))) {
+				i++;
+				err = -EINVAL;
+				goto out;
+			}
+		}
 	}
 
 	if (!housekeeping.flags)
 		static_branch_enable(&housekeeping_overridden);
 
-	if (housekeeping.flags & HK_FLAG_DOMAIN)
-		old = housekeeping_cpumask_dereference(HK_TYPE_DOMAIN);
-	else
-		WRITE_ONCE(housekeeping.flags, housekeeping.flags | HK_FLAG_DOMAIN);
-	rcu_assign_pointer(housekeeping.cpumasks[HK_TYPE_DOMAIN], trial);
-
-	synchronize_rcu();
-
-	pci_probe_flush_workqueue();
-	mem_cgroup_flush_workqueue();
-	vmstat_flush_workqueue();
+	for (i = 0; i < HK_TYPE_CNT; i++) {
+		int type =  hk_types[i].type;
+		struct cpumask *old;
 
-	err = workqueue_unbound_housekeeping_update(housekeeping_cpumask(HK_TYPE_DOMAIN));
-	WARN_ON_ONCE(err < 0);
+		if (!trial[i])
+			continue;
+		old = NULL;
+		if (housekeeping.flags & BIT(type))
+			old = housekeeping_cpumask_dereference(type);
+		rcu_assign_pointer(housekeeping.cpumasks[type], trial[i]);
+		trial[i] = old;
+	}
 
-	err = tmigr_isolated_exclude_cpumask(isol_mask);
-	WARN_ON_ONCE(err < 0);
+	if ((housekeeping.flags & flags) != flags)
+		WRITE_ONCE(housekeeping.flags, housekeeping.flags | flags);
 
-	err = kthreads_update_housekeeping();
-	WARN_ON_ONCE(err < 0);
+	synchronize_rcu();
 
-	kfree(old);
+	if (flags & HK_FLAG_DOMAIN) {
+		/*
+		 * HK_TYPE_DOMAIN specific callbacks
+		 */
+		pci_probe_flush_workqueue();
+		mem_cgroup_flush_workqueue();
+		vmstat_flush_workqueue();
+
+		WARN_ON_ONCE(workqueue_unbound_housekeeping_update(
+				housekeeping_cpumask(HK_TYPE_DOMAIN)) < 0);
+		WARN_ON_ONCE(tmigr_isolated_exclude_cpumask(isol_mask) < 0);
+		WARN_ON_ONCE(kthreads_update_housekeeping() < 0);
+	}
 
-	return 0;
+out:
+	while (--i >= 0)
+		kfree(trial[i]);
+	return err;
 }
 
 void __init housekeeping_init(void)
-- 
2.53.0

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

[Waiman Long]

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

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

[Thomas Gleixner]

On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
> 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.

s/IRQ/interrupt/

This is a changelog and not a SMS service.

> 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.

That's useless blurb with zero content.

> 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.

I kinda can crystal-ball what you are trying to say here, but that does
not make it qualified as a proper change log.

> 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.

Why has this key to be enabled on boot if there are no CPUs in the
isolated mask?

If you want to manage this dynamically at runtime then enable the key
once CPUs are isolated. Yes, it's more work, but that avoids the "should
only be used" nonsense and makes this more robust down the road.

Thanks,

        tglx

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

[Waiman Long]

On 4/21/26 4:32 AM, Thomas Gleixner wrote:
> On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
>> 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.
> s/IRQ/interrupt/
>
> This is a changelog and not a SMS service.
>> 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.
> That's useless blurb with zero content.
>
>> 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.
> I kinda can crystal-ball what you are trying to say here, but that does
> not make it qualified as a proper change log.
>
>> 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.
> Why has this key to be enabled on boot if there are no CPUs in the
> isolated mask?
>
> If you want to manage this dynamically at runtime then enable the key
> once CPUs are isolated. Yes, it's more work, but that avoids the "should
> only be used" nonsense and makes this more robust down the road.

OK, I will try to make it fully dynamic. Of course, it will be more work.

Cheers,
Longman

> Thanks,
>
>          tglx
>
>

[PATCH 04/23] tick/nohz: Allow runtime changes in full dynticks CPUs

[Waiman Long]

Full dynticks can only be enabled if "nohz_full" boot option has been
been specified with or without parameter. Any change in the list of
nohz_full CPUs have to be reflected in tick_nohz_full_mask. Introduce
a new tick_nohz_full_update_cpus() helper that can be called to update
the tick_nohz_full_mask at run time. The housekeeping_update() function
is modified to call the new helper when the HK_TYPE_KERNEL_NOSIE cpumask
is going to be changed.

We also need to enable CPU context tracking for those CPUs that
are in tick_nohz_full_mask. So remove __init from tick_nohz_init()
and ct_cpu_track_user() so that they be called later when an isolated
cpuset partition is being created. The __ro_after_init attribute is
taken away from context_tracking_key as well.

Also add a new ct_cpu_untrack_user() function to reverse the action of
ct_cpu_track_user() in case we need to disable the nohz_full mode of
a CPU.

With nohz_full enabled, the boot CPU (typically CPU 0) will be the
tick CPU which cannot be shut down easily. So the boot CPU should not
be used in an isolated cpuset partition.

With runtime modification of nohz_full CPUs, tick_do_timer_cpu can become
TICK_DO_TIMER_NONE. So remove the two TICK_DO_TIMER_NONE WARN_ON_ONCE()
checks in tick-sched.c to avoid unnecessary warnings.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/context_tracking.h |  1 +
 include/linux/tick.h             |  2 ++
 kernel/context_tracking.c        | 15 ++++++++++---
 kernel/sched/isolation.c         |  3 +++
 kernel/time/tick-sched.c         | 37 ++++++++++++++++++++++++++------
 5 files changed, 48 insertions(+), 10 deletions(-)

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index a3fea7f9fef6..1a6b816f1ad6 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -17,6 +17,7 @@
 #define CONTEXT_TRACKING_FORCE_ENABLE	(-1)
 
 extern void ct_cpu_track_user(int cpu);
+extern void ct_cpu_untrack_user(int cpu);
 
 /* Called with interrupts disabled.  */
 extern void __ct_user_enter(enum ctx_state state);
diff --git a/include/linux/tick.h b/include/linux/tick.h
index 738007d6f577..05586f14461c 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -274,6 +274,7 @@ static inline void tick_dep_clear_signal(struct signal_struct *signal,
 extern void tick_nohz_full_kick_cpu(int cpu);
 extern void __tick_nohz_task_switch(void);
 extern void __init tick_nohz_full_setup(cpumask_var_t cpumask);
+extern void tick_nohz_full_update_cpus(struct cpumask *cpumask);
 #else
 static inline bool tick_nohz_full_enabled(void) { return false; }
 static inline bool tick_nohz_full_cpu(int cpu) { return false; }
@@ -299,6 +300,7 @@ static inline void tick_dep_clear_signal(struct signal_struct *signal,
 static inline void tick_nohz_full_kick_cpu(int cpu) { }
 static inline void __tick_nohz_task_switch(void) { }
 static inline void tick_nohz_full_setup(cpumask_var_t cpumask) { }
+static inline void tick_nohz_full_update_cpus(struct cpumask *cpumask) { }
 #endif
 
 static inline void tick_nohz_task_switch(void)
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 925999de1a28..394e432630a3 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -411,7 +411,7 @@ static __always_inline void ct_kernel_enter(bool user, int offset) { }
 #define CREATE_TRACE_POINTS
 #include <trace/events/context_tracking.h>
 
-DEFINE_STATIC_KEY_FALSE_RO(context_tracking_key);
+DEFINE_STATIC_KEY_FALSE(context_tracking_key);
 EXPORT_SYMBOL_GPL(context_tracking_key);
 
 static noinstr bool context_tracking_recursion_enter(void)
@@ -674,9 +674,9 @@ void user_exit_callable(void)
 }
 NOKPROBE_SYMBOL(user_exit_callable);
 
-void __init ct_cpu_track_user(int cpu)
+void ct_cpu_track_user(int cpu)
 {
-	static __initdata bool initialized = false;
+	static bool initialized;
 
 	if (cpu == CONTEXT_TRACKING_FORCE_ENABLE) {
 		static_branch_inc(&context_tracking_key);
@@ -700,6 +700,15 @@ void __init ct_cpu_track_user(int cpu)
 	initialized = true;
 }
 
+void ct_cpu_untrack_user(int cpu)
+{
+	if (!per_cpu(context_tracking.active, cpu))
+		return;
+
+	per_cpu(context_tracking.active, cpu) = false;
+	static_branch_dec(&context_tracking_key);
+}
+
 #ifdef CONFIG_CONTEXT_TRACKING_USER_FORCE
 void __init context_tracking_init(void)
 {
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index c233d55a1e95..48b155e0b290 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -181,6 +181,9 @@ int housekeeping_update(struct cpumask *isol_mask, unsigned long flags)
 	if ((housekeeping.flags & flags) != flags)
 		WRITE_ONCE(housekeeping.flags, housekeeping.flags | flags);
 
+	if (flags & HK_FLAG_KERNEL_NOISE)
+		tick_nohz_full_update_cpus(isol_mask);
+
 	synchronize_rcu();
 
 	if (flags & HK_FLAG_DOMAIN) {
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index ed877b2c9040..7baa757ca45f 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -241,9 +241,6 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
 	tick_cpu = READ_ONCE(tick_do_timer_cpu);
 
 	if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && unlikely(tick_cpu == TICK_DO_TIMER_NONE)) {
-#ifdef CONFIG_NO_HZ_FULL
-		WARN_ON_ONCE(tick_nohz_full_running);
-#endif
 		WRITE_ONCE(tick_do_timer_cpu, cpu);
 		tick_cpu = cpu;
 	}
@@ -629,6 +626,36 @@ void __init tick_nohz_full_setup(cpumask_var_t cpumask)
 	tick_nohz_full_running = true;
 }
 
+/* Get the new set of run-time nohz CPU list & update accordingly */
+void tick_nohz_full_update_cpus(struct cpumask *cpumask)
+{
+	int cpu;
+
+	if (!tick_nohz_full_running) {
+		pr_warn_once("Full dynticks cannot be enabled without the nohz_full kernel boot parameter!\n");
+		return;
+	}
+
+	/*
+	 * To properly enable/disable nohz_full dynticks for the affected CPUs,
+	 * the new nohz_full CPUs have to be copied to tick_nohz_full_mask and
+	 * ct_cpu_track_user/ct_cpu_untrack_user() will have to be called
+	 * for those CPUs that have their states changed. Those CPUs should be
+	 * in an offline state.
+	 */
+	for_each_cpu_andnot(cpu, cpumask, tick_nohz_full_mask) {
+		WARN_ON_ONCE(cpu_online(cpu));
+		ct_cpu_track_user(cpu);
+		cpumask_set_cpu(cpu, tick_nohz_full_mask);
+	}
+
+	for_each_cpu_andnot(cpu, tick_nohz_full_mask, cpumask) {
+		WARN_ON_ONCE(cpu_online(cpu));
+		ct_cpu_untrack_user(cpu);
+		cpumask_clear_cpu(cpu, tick_nohz_full_mask);
+	}
+}
+
 bool tick_nohz_cpu_hotpluggable(unsigned int cpu)
 {
 	/*
@@ -1238,10 +1265,6 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
 		 */
 		if (tick_cpu == cpu)
 			return false;
-
-		/* Should not happen for nohz-full */
-		if (WARN_ON_ONCE(tick_cpu == TICK_DO_TIMER_NONE))
-			return false;
 	}
 
 	return true;
-- 
2.53.0

Re: [PATCH 04/23] tick/nohz: Allow runtime changes in full dynticks CPUs

[Thomas Gleixner]

On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
> Full dynticks can only be enabled if "nohz_full" boot option has been
> been specified with or without parameter. Any change in the list of
> nohz_full CPUs have to be reflected in tick_nohz_full_mask. Introduce
> a new tick_nohz_full_update_cpus() helper that can be called to update
> the tick_nohz_full_mask at run time. The housekeeping_update() function
> is modified to call the new helper when the HK_TYPE_KERNEL_NOSIE cpumask
> is going to be changed.
>
> We also need to enable CPU context tracking for those CPUs that

We need nothing. Use passive voice for change logs as requested in
documentation.

> are in tick_nohz_full_mask. So remove __init from tick_nohz_init()
> and ct_cpu_track_user() so that they be called later when an isolated
> cpuset partition is being created. The __ro_after_init attribute is
> taken away from context_tracking_key as well.
>
> Also add a new ct_cpu_untrack_user() function to reverse the action of
> ct_cpu_track_user() in case we need to disable the nohz_full mode of
> a CPU.
>
> With nohz_full enabled, the boot CPU (typically CPU 0) will be the
> tick CPU which cannot be shut down easily. So the boot CPU should not
> be used in an isolated cpuset partition.
>
> With runtime modification of nohz_full CPUs, tick_do_timer_cpu can become
> TICK_DO_TIMER_NONE. So remove the two TICK_DO_TIMER_NONE WARN_ON_ONCE()
> checks in tick-sched.c to avoid unnecessary warnings.

in tick-sched.c? Describe the functions which contain that.

>  static inline void tick_nohz_task_switch(void)
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index 925999de1a28..394e432630a3 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -411,7 +411,7 @@ static __always_inline void ct_kernel_enter(bool user, int offset) { }
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/context_tracking.h>
>  
> -DEFINE_STATIC_KEY_FALSE_RO(context_tracking_key);
> +DEFINE_STATIC_KEY_FALSE(context_tracking_key);
>  EXPORT_SYMBOL_GPL(context_tracking_key);
>  
>  static noinstr bool context_tracking_recursion_enter(void)
> @@ -674,9 +674,9 @@ void user_exit_callable(void)
>  }
>  NOKPROBE_SYMBOL(user_exit_callable);
>  
> -void __init ct_cpu_track_user(int cpu)
> +void ct_cpu_track_user(int cpu)
>  {
> -	static __initdata bool initialized = false;
> +	static bool initialized;
>  
>  	if (cpu == CONTEXT_TRACKING_FORCE_ENABLE) {
>  		static_branch_inc(&context_tracking_key);
> @@ -700,6 +700,15 @@ void __init ct_cpu_track_user(int cpu)
>  	initialized = true;
>  }
>  
> +void ct_cpu_untrack_user(int cpu)
> +{
> +	if (!per_cpu(context_tracking.active, cpu))
> +		return;
> +
> +	per_cpu(context_tracking.active, cpu) = false;
> +	static_branch_dec(&context_tracking_key);
> +}
> +

Why is this in a patch which makes tick/nohz related changes? This is a
preparatory change, so make it that way and do not bury it inside
something else.

> +/* Get the new set of run-time nohz CPU list & update accordingly */
> +void tick_nohz_full_update_cpus(struct cpumask *cpumask)
> +{
> +	int cpu;
> +
> +	if (!tick_nohz_full_running) {
> +		pr_warn_once("Full dynticks cannot be enabled without the nohz_full kernel boot parameter!\n");

That's the result of this enforced enable hackery. Make this work
properly.

> +		return;
> +	}
> +
> +	/*
> +	 * To properly enable/disable nohz_full dynticks for the affected CPUs,
> +	 * the new nohz_full CPUs have to be copied to tick_nohz_full_mask and
> +	 * ct_cpu_track_user/ct_cpu_untrack_user() will have to be called
> +	 * for those CPUs that have their states changed. Those CPUs should be
> +	 * in an offline state.
> +	 */
> +	for_each_cpu_andnot(cpu, cpumask, tick_nohz_full_mask) {
> +		WARN_ON_ONCE(cpu_online(cpu));
> +		ct_cpu_track_user(cpu);
> +		cpumask_set_cpu(cpu, tick_nohz_full_mask);
> +	}
> +
> +	for_each_cpu_andnot(cpu, tick_nohz_full_mask, cpumask) {
> +		WARN_ON_ONCE(cpu_online(cpu));
> +		ct_cpu_untrack_user(cpu);
> +		cpumask_clear_cpu(cpu, tick_nohz_full_mask);
> +	}
> +}

So this writes to tick_nohz_full_mask while other CPUs can access
it. That's just wrong and I'm not at all interested in the resulting
KCSAN warnings.

tick_nohz_full_mask needs to become a RCU protected pointer, which is
updated once the new mask is established in a separately allocated one.

Thanks,

        tglx

Re: [PATCH 04/23] tick/nohz: Allow runtime changes in full dynticks CPUs

[Waiman Long]

On 4/21/26 4:50 AM, Thomas Gleixner wrote:
> On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
>> Full dynticks can only be enabled if "nohz_full" boot option has been
>> been specified with or without parameter. Any change in the list of
>> nohz_full CPUs have to be reflected in tick_nohz_full_mask. Introduce
>> a new tick_nohz_full_update_cpus() helper that can be called to update
>> the tick_nohz_full_mask at run time. The housekeeping_update() function
>> is modified to call the new helper when the HK_TYPE_KERNEL_NOSIE cpumask
>> is going to be changed.
>>
>> We also need to enable CPU context tracking for those CPUs that
> We need nothing. Use passive voice for change logs as requested in
> documentation.
>
>> are in tick_nohz_full_mask. So remove __init from tick_nohz_init()
>> and ct_cpu_track_user() so that they be called later when an isolated
>> cpuset partition is being created. The __ro_after_init attribute is
>> taken away from context_tracking_key as well.
>>
>> Also add a new ct_cpu_untrack_user() function to reverse the action of
>> ct_cpu_track_user() in case we need to disable the nohz_full mode of
>> a CPU.
>>
>> With nohz_full enabled, the boot CPU (typically CPU 0) will be the
>> tick CPU which cannot be shut down easily. So the boot CPU should not
>> be used in an isolated cpuset partition.
>>
>> With runtime modification of nohz_full CPUs, tick_do_timer_cpu can become
>> TICK_DO_TIMER_NONE. So remove the two TICK_DO_TIMER_NONE WARN_ON_ONCE()
>> checks in tick-sched.c to avoid unnecessary warnings.
> in tick-sched.c? Describe the functions which contain that.
>
>>   static inline void tick_nohz_task_switch(void)
>> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
>> index 925999de1a28..394e432630a3 100644
>> --- a/kernel/context_tracking.c
>> +++ b/kernel/context_tracking.c
>> @@ -411,7 +411,7 @@ static __always_inline void ct_kernel_enter(bool user, int offset) { }
>>   #define CREATE_TRACE_POINTS
>>   #include <trace/events/context_tracking.h>
>>   
>> -DEFINE_STATIC_KEY_FALSE_RO(context_tracking_key);
>> +DEFINE_STATIC_KEY_FALSE(context_tracking_key);
>>   EXPORT_SYMBOL_GPL(context_tracking_key);
>>   
>>   static noinstr bool context_tracking_recursion_enter(void)
>> @@ -674,9 +674,9 @@ void user_exit_callable(void)
>>   }
>>   NOKPROBE_SYMBOL(user_exit_callable);
>>   
>> -void __init ct_cpu_track_user(int cpu)
>> +void ct_cpu_track_user(int cpu)
>>   {
>> -	static __initdata bool initialized = false;
>> +	static bool initialized;
>>   
>>   	if (cpu == CONTEXT_TRACKING_FORCE_ENABLE) {
>>   		static_branch_inc(&context_tracking_key);
>> @@ -700,6 +700,15 @@ void __init ct_cpu_track_user(int cpu)
>>   	initialized = true;
>>   }
>>   
>> +void ct_cpu_untrack_user(int cpu)
>> +{
>> +	if (!per_cpu(context_tracking.active, cpu))
>> +		return;
>> +
>> +	per_cpu(context_tracking.active, cpu) = false;
>> +	static_branch_dec(&context_tracking_key);
>> +}
>> +
> Why is this in a patch which makes tick/nohz related changes? This is a
> preparatory change, so make it that way and do not bury it inside
> something else.
Sure, I will break out the context tracking change into its own patch.
>> +/* Get the new set of run-time nohz CPU list & update accordingly */
>> +void tick_nohz_full_update_cpus(struct cpumask *cpumask)
>> +{
>> +	int cpu;
>> +
>> +	if (!tick_nohz_full_running) {
>> +		pr_warn_once("Full dynticks cannot be enabled without the nohz_full kernel boot parameter!\n");
> That's the result of this enforced enable hackery. Make this work
> properly.
>
>> +		return;
>> +	}
>> +
>> +	/*
>> +	 * To properly enable/disable nohz_full dynticks for the affected CPUs,
>> +	 * the new nohz_full CPUs have to be copied to tick_nohz_full_mask and
>> +	 * ct_cpu_track_user/ct_cpu_untrack_user() will have to be called
>> +	 * for those CPUs that have their states changed. Those CPUs should be
>> +	 * in an offline state.
>> +	 */
>> +	for_each_cpu_andnot(cpu, cpumask, tick_nohz_full_mask) {
>> +		WARN_ON_ONCE(cpu_online(cpu));
>> +		ct_cpu_track_user(cpu);
>> +		cpumask_set_cpu(cpu, tick_nohz_full_mask);
>> +	}
>> +
>> +	for_each_cpu_andnot(cpu, tick_nohz_full_mask, cpumask) {
>> +		WARN_ON_ONCE(cpu_online(cpu));
>> +		ct_cpu_untrack_user(cpu);
>> +		cpumask_clear_cpu(cpu, tick_nohz_full_mask);
>> +	}
>> +}
> So this writes to tick_nohz_full_mask while other CPUs can access
> it. That's just wrong and I'm not at all interested in the resulting
> KCSAN warnings.
>
> tick_nohz_full_mask needs to become a RCU protected pointer, which is
> updated once the new mask is established in a separately allocated one.

I will work on that.

Cheers,
Longman

>
> Thanks,
>
>          tglx
>
>

[PATCH 05/23] tick: Pass timer tick job to an online HK CPU in tick_cpu_dying()

[Waiman Long]

In tick_cpu_dying(), if the dying CPU is the current timekeeper,
it has to pass the job over to another CPU. The current code passes
it to another online CPU. However, that CPU may not be a timer tick
housekeeping CPU.  If that happens, another CPU will have to manually
take it over again later. Avoid this unnecessary work by directly
assigning an online housekeeping CPU.

Use READ_ONCE/WRITE_ONCE() to access tick_do_timer_cpu in case the
non-HK CPUs may not be in stop machine in the future.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/time/tick-common.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index d305d8521896..4834a1b9044b 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -17,6 +17,7 @@
 #include <linux/profile.h>
 #include <linux/sched.h>
 #include <linux/module.h>
+#include <linux/sched/isolation.h>
 #include <trace/events/power.h>
 
 #include <asm/irq_regs.h>
@@ -394,12 +395,19 @@ int tick_cpu_dying(unsigned int dying_cpu)
 {
 	/*
 	 * If the current CPU is the timekeeper, it's the only one that can
-	 * safely hand over its duty. Also all online CPUs are in stop
-	 * machine, guaranteed not to be idle, therefore there is no
+	 * safely hand over its duty. Also all online housekeeping CPUs are
+	 * in stop machine, guaranteed not to be idle, therefore there is no
 	 * concurrency and it's safe to pick any online successor.
 	 */
-	if (tick_do_timer_cpu == dying_cpu)
-		tick_do_timer_cpu = cpumask_first(cpu_online_mask);
+	if (READ_ONCE(tick_do_timer_cpu) == dying_cpu) {
+		unsigned int new_cpu;
+
+		guard(rcu)();
+		new_cpu = cpumask_first_and(cpu_online_mask, housekeeping_cpumask(HK_TYPE_TICK));
+		if (WARN_ON_ONCE(new_cpu >= nr_cpu_ids))
+			new_cpu = cpumask_first(cpu_online_mask);
+		WRITE_ONCE(tick_do_timer_cpu, new_cpu);
+	}
 
 	/* Make sure the CPU won't try to retake the timekeeping duty */
 	tick_sched_timer_dying(dying_cpu);
-- 
2.53.0

Re: [PATCH 05/23] tick: Pass timer tick job to an online HK CPU in tick_cpu_dying()

[Thomas Gleixner]

On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
> In tick_cpu_dying(), if the dying CPU is the current timekeeper,
> it has to pass the job over to another CPU. The current code passes
> it to another online CPU. However, that CPU may not be a timer tick
> housekeeping CPU.  If that happens, another CPU will have to manually
> take it over again later. Avoid this unnecessary work by directly
> assigning an online housekeeping CPU.
>
> Use READ_ONCE/WRITE_ONCE() to access tick_do_timer_cpu in case the
> non-HK CPUs may not be in stop machine in the future.

'may not be in the future' is yet more handwaving without
content. Please write your change logs in a way so that people who have
not spent months on this can follow.

> @@ -394,12 +395,19 @@ int tick_cpu_dying(unsigned int dying_cpu)
>  {
>  	/*
>  	 * If the current CPU is the timekeeper, it's the only one that can
> -	 * safely hand over its duty. Also all online CPUs are in stop
> -	 * machine, guaranteed not to be idle, therefore there is no
> +	 * safely hand over its duty. Also all online housekeeping CPUs are
> +	 * in stop machine, guaranteed not to be idle, therefore there is no
>  	 * concurrency and it's safe to pick any online successor.
>  	 */
> -	if (tick_do_timer_cpu == dying_cpu)
> -		tick_do_timer_cpu = cpumask_first(cpu_online_mask);
> +	if (READ_ONCE(tick_do_timer_cpu) == dying_cpu) {
> +		unsigned int new_cpu;
> +
> +		guard(rcu)();

What's this guard for?

> +		new_cpu = cpumask_first_and(cpu_online_mask, housekeeping_cpumask(HK_TYPE_TICK));

Why has this to use housekeeping_cpumask() and does not use
tick_nohz_full_mask?

Thanks,

        tglx

Re: [PATCH 05/23] tick: Pass timer tick job to an online HK CPU in tick_cpu_dying()

[Waiman Long]

On 4/21/26 4:55 AM, Thomas Gleixner wrote:
> On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
>> In tick_cpu_dying(), if the dying CPU is the current timekeeper,
>> it has to pass the job over to another CPU. The current code passes
>> it to another online CPU. However, that CPU may not be a timer tick
>> housekeeping CPU.  If that happens, another CPU will have to manually
>> take it over again later. Avoid this unnecessary work by directly
>> assigning an online housekeeping CPU.
>>
>> Use READ_ONCE/WRITE_ONCE() to access tick_do_timer_cpu in case the
>> non-HK CPUs may not be in stop machine in the future.
> 'may not be in the future' is yet more handwaving without
> content. Please write your change logs in a way so that people who have
> not spent months on this can follow.
>
>> @@ -394,12 +395,19 @@ int tick_cpu_dying(unsigned int dying_cpu)
>>   {
>>   	/*
>>   	 * If the current CPU is the timekeeper, it's the only one that can
>> -	 * safely hand over its duty. Also all online CPUs are in stop
>> -	 * machine, guaranteed not to be idle, therefore there is no
>> +	 * safely hand over its duty. Also all online housekeeping CPUs are
>> +	 * in stop machine, guaranteed not to be idle, therefore there is no
>>   	 * concurrency and it's safe to pick any online successor.
>>   	 */
>> -	if (tick_do_timer_cpu == dying_cpu)
>> -		tick_do_timer_cpu = cpumask_first(cpu_online_mask);
>> +	if (READ_ONCE(tick_do_timer_cpu) == dying_cpu) {
>> +		unsigned int new_cpu;
>> +
>> +		guard(rcu)();
> What's this guard for?
>
>> +		new_cpu = cpumask_first_and(cpu_online_mask, housekeeping_cpumask(HK_TYPE_TICK));
> Why has this to use housekeeping_cpumask() and does not use
> tick_nohz_full_mask?

The RCU guard is for accessing the HK_TYPE_TICK(HK_TYPE_KERNEL_NOISE) 
cpumask. tick_nohz_full_mask cpumask is actually the inverse of 
HK_TYPE_TICK cpumask. Yes, I could use cpumask_first_andnot() with 
tick_nohz_full_mask. If we make tick_nohz_full_mask an RCU protected 
pointer, we still need the guard.

Cheers,
Longman

[PATCH 06/23] rcu/nocbs: Allow runtime changes in RCU NOCBS cpumask

[Waiman Long]

We can make use of the rcu_nocb_cpu_offload()/rcu_nocb_cpu_deoffload()
APIs to enable RCU NO-CB CPU offloading of newly isolated CPUs and
deoffloading of de-isolated CPUs.

Add a new rcu_nocb_update_cpus() helper to do that and call it directly
from housekeeping_update() when the HK_TYPE_KERNEL_NOISE cpumask is
being changed.

This dynamic RCU NO-CB CPU offloading feature can only used if either
"rcs_nocbs" or the "nohz_full" boot command parameters are used with or
without parameter so that the proper RCU NO-CB resources are properly
initialized at boot time.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/rcupdate.h |  2 ++
 kernel/rcu/tree_nocb.h   | 22 ++++++++++++++++++++++
 kernel/sched/isolation.c |  4 +++-
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 04f3f86a4145..987e3d1d413e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -150,6 +150,7 @@ void rcu_init_nohz(void);
 int rcu_nocb_cpu_offload(int cpu);
 int rcu_nocb_cpu_deoffload(int cpu);
 void rcu_nocb_flush_deferred_wakeup(void);
+void rcu_nocb_update_cpus(struct cpumask *cpumask);
 
 #define RCU_NOCB_LOCKDEP_WARN(c, s) RCU_LOCKDEP_WARN(c, s)
 
@@ -159,6 +160,7 @@ static inline void rcu_init_nohz(void) { }
 static inline int rcu_nocb_cpu_offload(int cpu) { return -EINVAL; }
 static inline int rcu_nocb_cpu_deoffload(int cpu) { return 0; }
 static inline void rcu_nocb_flush_deferred_wakeup(void) { }
+static inline void rcu_nocb_update_cpus(struct cpumask *cpumask) { }
 
 #define RCU_NOCB_LOCKDEP_WARN(c, s)
 
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 2d06dcb61f37..b2daba1e5cb9 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1173,6 +1173,28 @@ int rcu_nocb_cpu_offload(int cpu)
 }
 EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
 
+void rcu_nocb_update_cpus(struct cpumask *cpumask)
+{
+	int cpu, ret;
+
+	if (!rcu_state.nocb_is_setup) {
+		pr_warn_once("Dynamic RCU NOCB cannot be enabled without nohz_full/rcu_nocbs kernel boot parameter!\n");
+		return;
+	}
+
+	for_each_cpu_andnot(cpu, cpumask, rcu_nocb_mask) {
+		ret = rcu_nocb_cpu_offload(cpu);
+		if (WARN_ON_ONCE(ret))
+			return;
+	}
+
+	for_each_cpu_andnot(cpu, rcu_nocb_mask, cpumask) {
+		ret = rcu_nocb_cpu_deoffload(cpu);
+		if (WARN_ON_ONCE(ret))
+			return;
+	}
+}
+
 #ifdef CONFIG_RCU_LAZY
 static unsigned long
 lazy_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 48b155e0b290..b5635484ec69 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -181,8 +181,10 @@ int housekeeping_update(struct cpumask *isol_mask, unsigned long flags)
 	if ((housekeeping.flags & flags) != flags)
 		WRITE_ONCE(housekeeping.flags, housekeeping.flags | flags);
 
-	if (flags & HK_FLAG_KERNEL_NOISE)
+	if (flags & HK_FLAG_KERNEL_NOISE) {
 		tick_nohz_full_update_cpus(isol_mask);
+		rcu_nocb_update_cpus(isol_mask);
+	}
 
 	synchronize_rcu();
 
-- 
2.53.0

[PATCH 07/23] watchdog: Sync up with runtime change of isolated CPUs

[Waiman Long]

At bootup, watchdog will exclude nohz_full CPUs specified at boot time.
As we are now enabling runtime changes to nohz_full CPUs, the list of
CPUs with watchdog timer running should be updated to exclude the
current set of isolated CPUs.

Add a new watchdog_cpumask_update() helper to be invoked
by housekeeping_update() when the HK_TYPE_KERNEL_NOISE
(HK_TYPE_TIMER) cpumask is being updated to update watchdog_cpumask and
watchdog_allowed_mask for soft lockup detector. The cpumask updates will
be done when the affected CPUs are in the offline state. When those
CPUs are brought up later, the new cpumask will be used to determine
if any hard/soft watchdog should be enabled again.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/nmi.h      |  2 ++
 kernel/sched/isolation.c |  1 +
 kernel/watchdog.c        | 24 ++++++++++++++++++++++++
 3 files changed, 27 insertions(+)

diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index bc1162895f35..5bf941d2b168 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -17,6 +17,7 @@
 void lockup_detector_init(void);
 void lockup_detector_retry_init(void);
 void lockup_detector_soft_poweroff(void);
+void watchdog_cpumask_update(struct cpumask *mask);
 
 extern int watchdog_user_enabled;
 extern int watchdog_thresh;
@@ -37,6 +38,7 @@ extern int sysctl_hardlockup_all_cpu_backtrace;
 static inline void lockup_detector_init(void) { }
 static inline void lockup_detector_retry_init(void) { }
 static inline void lockup_detector_soft_poweroff(void) { }
+static inline void watchdog_cpumask_update(struct cpumask *mask) { }
 #endif /* !CONFIG_LOCKUP_DETECTOR */
 
 #ifdef CONFIG_SOFTLOCKUP_DETECTOR
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index b5635484ec69..1f3f1c83dd12 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -184,6 +184,7 @@ int housekeeping_update(struct cpumask *isol_mask, unsigned long flags)
 	if (flags & HK_FLAG_KERNEL_NOISE) {
 		tick_nohz_full_update_cpus(isol_mask);
 		rcu_nocb_update_cpus(isol_mask);
+		watchdog_cpumask_update(isol_mask);
 	}
 
 	synchronize_rcu();
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 87dd5e0f6968..498c1463b843 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -1071,6 +1071,30 @@ static inline void lockup_detector_setup(void)
 }
 #endif /* !CONFIG_SOFTLOCKUP_DETECTOR */
 
+/**
+ * watchdog_cpumask_update - update watchdog_cpumask & watchdog_allowed_mask
+ * @isol_mask: cpumask of isolated CPUs
+ *
+ * Update watchdog_cpumask and watchdog_allowed_mask to be inverse of the
+ * given isolated cpumask to disable watchdog activities on isolated CPUs.
+ * It should be called with the affected CPUs in offline state which will be
+ * brought up online later.
+ *
+ * Any changes made in watchdog_cpumask by users via the sysctl parameter will
+ * be overridden. However, proc_watchdog_update() isn't called. So change will
+ * only happens on CPUs that will brought up later on to minimize changes to
+ * the existing watchdog configuration.
+ */
+void watchdog_cpumask_update(struct cpumask *isol_mask)
+{
+	mutex_lock(&watchdog_mutex);
+	cpumask_andnot(&watchdog_cpumask, cpu_possible_mask, isol_mask);
+#ifdef CONFIG_SOFTLOCKUP_DETECTOR
+	cpumask_copy(&watchdog_allowed_mask, &watchdog_cpumask);
+#endif
+	mutex_unlock(&watchdog_mutex);
+}
+
 /**
  * lockup_detector_soft_poweroff - Interface to stop lockup detector(s)
  *
-- 
2.53.0

[PATCH 08/23] arm64: topology: Use RCU to protect access to HK_TYPE_TICK cpumask

[Waiman Long]

As the HK_TYPE_TICK cpumask is going to be changeable at run time, we
need to use RCU to protect access to the cpumask to prevent it from
going away in the middle of the operation.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 arch/arm64/kernel/topology.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index b32f13358fbb..48f150801689 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -173,6 +173,7 @@ void arch_cpu_idle_enter(void)
 	if (!amu_fie_cpu_supported(cpu))
 		return;
 
+	guard(rcu)();
 	/* Kick in AMU update but only if one has not happened already */
 	if (housekeeping_cpu(cpu, HK_TYPE_TICK) &&
 	    time_is_before_jiffies(per_cpu(cpu_amu_samples.last_scale_update, cpu)))
@@ -187,11 +188,16 @@ int arch_freq_get_on_cpu(int cpu)
 	unsigned int start_cpu = cpu;
 	unsigned long last_update;
 	unsigned int freq = 0;
+	bool hk_cpu;
 	u64 scale;
 
 	if (!amu_fie_cpu_supported(cpu) || !arch_scale_freq_ref(cpu))
 		return -EOPNOTSUPP;
 
+	scoped_guard(rcu) {
+		hk_cpu = housekeeping_cpu(cpu, HK_TYPE_TICK);
+	}
+
 	while (1) {
 
 		amu_sample = per_cpu_ptr(&cpu_amu_samples, cpu);
@@ -204,16 +210,21 @@ int arch_freq_get_on_cpu(int cpu)
 		 * (and thus freq scale), if available, for given policy: this boils
 		 * down to identifying an active cpu within the same freq domain, if any.
 		 */
-		if (!housekeeping_cpu(cpu, HK_TYPE_TICK) ||
+		if (!hk_cpu ||
 		    time_is_before_jiffies(last_update + msecs_to_jiffies(AMU_SAMPLE_EXP_MS))) {
 			struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
+			bool hk_intersects;
 			int ref_cpu;
 
 			if (!policy)
 				return -EINVAL;
 
-			if (!cpumask_intersects(policy->related_cpus,
-						housekeeping_cpumask(HK_TYPE_TICK))) {
+			scoped_guard(rcu) {
+				hk_intersects = cpumask_intersects(policy->related_cpus,
+							housekeeping_cpumask(HK_TYPE_TICK));
+			}
+
+			if (!hk_intersects) {
 				cpufreq_cpu_put(policy);
 				return -EOPNOTSUPP;
 			}
-- 
2.53.0

[PATCH 09/23] workqueue: Use RCU to protect access of HK_TYPE_TIMER cpumask

[Waiman Long]

As HK_TYPE_TIMER cpumask is going to be changeable at run time, use
RCU to protect access to the cpumask.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/workqueue.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 08b1c786b463..2dab3872281a 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2557,8 +2557,10 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
 	if (housekeeping_enabled(HK_TYPE_TIMER)) {
 		/* If the current cpu is a housekeeping cpu, use it. */
 		cpu = smp_processor_id();
-		if (!housekeeping_test_cpu(cpu, HK_TYPE_TIMER))
-			cpu = housekeeping_any_cpu(HK_TYPE_TIMER);
+		scoped_guard(rcu) {
+			if (!housekeeping_test_cpu(cpu, HK_TYPE_TIMER))
+				cpu = housekeeping_any_cpu(HK_TYPE_TIMER);
+		}
 		add_timer_on(timer, cpu);
 	} else {
 		if (likely(cpu == WORK_CPU_UNBOUND))
-- 
2.53.0

[PATCH 10/23] cpu: Use RCU to protect access of HK_TYPE_TIMER cpumask

[Waiman Long]

As HK_TYPE_TIMER cpumask is going to be changeable at run time, use
RCU to protect access to the cpumask.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cpu.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index bc4f7a9ba64e..0d02b5d7a7ba 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1890,6 +1890,8 @@ int freeze_secondary_cpus(int primary)
 	cpu_maps_update_begin();
 	if (primary == -1) {
 		primary = cpumask_first(cpu_online_mask);
+
+		guard(rcu)();
 		if (!housekeeping_cpu(primary, HK_TYPE_TIMER))
 			primary = housekeeping_any_cpu(HK_TYPE_TIMER);
 	} else {
-- 
2.53.0

Re: [PATCH 10/23] cpu: Use RCU to protect access of HK_TYPE_TIMER cpumask

[Thomas Gleixner]

On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
> As HK_TYPE_TIMER cpumask is going to be changeable at run time, use
> RCU to protect access to the cpumask.
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cpu.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index bc4f7a9ba64e..0d02b5d7a7ba 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1890,6 +1890,8 @@ int freeze_secondary_cpus(int primary)
>  	cpu_maps_update_begin();
>  	if (primary == -1) {
>  		primary = cpumask_first(cpu_online_mask);
> +
> +		guard(rcu)();
>  		if (!housekeeping_cpu(primary, HK_TYPE_TIMER))
>  			primary = housekeeping_any_cpu(HK_TYPE_TIMER);

housekeeping_cpu() and housekeeping_any_cpu() can operate on two
different CPU masks once the runtime update is enabled.

Seriously?

Re: [PATCH 10/23] cpu: Use RCU to protect access of HK_TYPE_TIMER cpumask

[Waiman Long]

On 4/21/26 4:57 AM, Thomas Gleixner wrote:
> On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
>> As HK_TYPE_TIMER cpumask is going to be changeable at run time, use
>> RCU to protect access to the cpumask.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   kernel/cpu.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/kernel/cpu.c b/kernel/cpu.c
>> index bc4f7a9ba64e..0d02b5d7a7ba 100644
>> --- a/kernel/cpu.c
>> +++ b/kernel/cpu.c
>> @@ -1890,6 +1890,8 @@ int freeze_secondary_cpus(int primary)
>>   	cpu_maps_update_begin();
>>   	if (primary == -1) {
>>   		primary = cpumask_first(cpu_online_mask);
>> +
>> +		guard(rcu)();
>>   		if (!housekeeping_cpu(primary, HK_TYPE_TIMER))
>>   			primary = housekeeping_any_cpu(HK_TYPE_TIMER);
> housekeeping_cpu() and housekeeping_any_cpu() can operate on two
> different CPU masks once the runtime update is enabled.
>
> Seriously?

Good point, will fix that in the next version.

Cheers,
Longman

[PATCH 11/23] hrtimer: Use RCU to protect access of HK_TYPE_TIMER cpumask

[Waiman Long]

As HK_TYPE_TIMER cpumask is going to be changeable at run time, use
RCU to protect access to the cpumask.

The access of HK_TYPE_TIMER cpumask within hrtimers_cpu_dying() is
protected as interrupt is disabled and all the other CPUs are stopped
when this function is invoked as part of the CPU tear down process.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/time/hrtimer.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 000fb6ba7d74..85495400a193 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -242,8 +242,10 @@ static bool hrtimer_suitable_target(struct hrtimer *timer, struct hrtimer_clock_
 static inline struct hrtimer_cpu_base *get_target_base(struct hrtimer_cpu_base *base, bool pinned)
 {
 	if (!hrtimer_base_is_online(base)) {
-		int cpu = cpumask_any_and(cpu_online_mask, housekeeping_cpumask(HK_TYPE_TIMER));
+		int cpu;
 
+		guard(rcu)();
+		cpu = cpumask_any_and(cpu_online_mask, housekeeping_cpumask(HK_TYPE_TIMER));
 		return &per_cpu(hrtimer_bases, cpu);
 	}
 
-- 
2.53.0

Re: [PATCH 11/23] hrtimer: Use RCU to protect access of HK_TYPE_TIMER cpumask

[Thomas Gleixner]

On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
> As HK_TYPE_TIMER cpumask is going to be changeable at run time, use

As the ...

> RCU to protect access to the cpumask.
>
> The access of HK_TYPE_TIMER cpumask within hrtimers_cpu_dying() is
> protected as interrupt is disabled and all the other CPUs are stopped

interrupts are disabled

> when this function is invoked as part of the CPU tear down process.

[PATCH 12/23] net: Use boot time housekeeping cpumask settings for now

[Waiman Long]

Checking of the housekeeping cpumasks was originally added to the
store_rps_map() method in commit 07bbecb34106 ("net: Restrict receive
packets queuing to housekeeping CPUs"). This store method ensures
that only CPUs in housekeeping cpumask can be set in the "rps_cpus"
of the individual network devices. Later, commit 662ff1aac854 ("net:
Keep ignoring isolated cpuset change") replaced HK_TYPE_DOMAIN by
HK_TYPE_DOMAIN_BOOT as HK_TYPE_DOMAIN cpumask was going to be made
changeable at run time.

Now we are going to make the nohz_full housekeeping cpumask to be
changeable at run time too. Ideally we should update the housekeeping
cpumasks changes should be propagated to the RPS cpumasks, but we
need more thoughts to properly synchronize them. Ignore the run time
housekeeping cpumask changes and only use the boot time "isolcpus="
and "nohz_full=" values for now.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 net/core/net-sysfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index e430645748a7..9e666cc3b34c 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -1023,7 +1023,7 @@ int rps_cpumask_housekeeping(struct cpumask *mask)
 {
 	if (!cpumask_empty(mask)) {
 		cpumask_and(mask, mask, housekeeping_cpumask(HK_TYPE_DOMAIN_BOOT));
-		cpumask_and(mask, mask, housekeeping_cpumask(HK_TYPE_WQ));
+		cpumask_and(mask, mask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE_BOOT));
 		if (cpumask_empty(mask))
 			return -EINVAL;
 	}
-- 
2.53.0

[PATCH 13/23] sched/core: Use RCU to protect access of HK_TYPE_KERNEL_NOISE cpumask

[Waiman Long]

As HK_TYPE_KERNEL_NOISE is going to be changeable at run time, use
RCU to protect access to the cpumask when needed. Some access of the
HK_TYPE_KERNEL_NOISE cpumask is done inside the tick code that has
interrupt disabled which is a rcu_read_lock() critical section. Anyway,
housekeeping_cpumask() will warn if they are used in invalid context.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/sched/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8952f5764517..6ae00c23d8a7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1261,6 +1261,8 @@ int get_nohz_timer_target(void)
 	struct sched_domain *sd;
 	const struct cpumask *hk_mask;
 
+	guard(rcu)();
+
 	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE)) {
 		if (!idle_cpu(cpu))
 			return cpu;
@@ -1269,8 +1271,6 @@ int get_nohz_timer_target(void)
 
 	hk_mask = housekeeping_cpumask(HK_TYPE_KERNEL_NOISE);
 
-	guard(rcu)();
-
 	for_each_domain(cpu, sd) {
 		for_each_cpu_and(i, sched_domain_span(sd), hk_mask) {
 			if (cpu == i)
-- 
2.53.0

[PATCH 14/23] hwmon/coretemp: Use RCU to protect access of HK_TYPE_MISC cpumask

[Waiman Long]

As HK_TYPE_MISC cpumask is going to be changeable at run time, use RCU
to protect access to the cpumask.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 drivers/hwmon/coretemp.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/coretemp.c b/drivers/hwmon/coretemp.c
index 6a0d94711ead..0284ea6a3b79 100644
--- a/drivers/hwmon/coretemp.c
+++ b/drivers/hwmon/coretemp.c
@@ -563,8 +563,10 @@ static int create_core_data(struct platform_device *pdev, unsigned int cpu,
 	u32 eax, edx;
 	int err;
 
-	if (!housekeeping_cpu(cpu, HK_TYPE_MISC))
-		return 0;
+	scoped_guard(rcu) {
+		if (!housekeeping_cpu(cpu, HK_TYPE_MISC))
+			return 0;
+	}
 
 	tdata = init_temp_data(pdata, cpu, pkg_flag);
 	if (!tdata)
-- 
2.53.0

[PATCH 15/23] Drivers: hv: Use RCU to protect access of HK_TYPE_MANAGED_IRQ cpumask

[Waiman Long]

As HK_TYPE_MANAGED_IRQ cpumask is going to be changeable at run time,
use RCU to protect access to the cpumask. The memory allocation
alloc_cpumask_var() call is done before taking the rcu_read_lock()
as this call can be sleepable.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 drivers/hv/channel_mgmt.c | 15 +++++++++------
 drivers/hv/vmbus_drv.c    |  7 +++++--
 2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 84eb0a6a0b54..44441dafed90 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -752,13 +752,16 @@ static void init_vp_index(struct vmbus_channel *channel)
 	u32 i, ncpu = num_online_cpus();
 	cpumask_var_t available_mask;
 	struct cpumask *allocated_mask;
-	const struct cpumask *hk_mask = housekeeping_cpumask(HK_TYPE_MANAGED_IRQ);
+	const struct cpumask *hk_mask;
 	u32 target_cpu;
 	int numa_node;
+	bool alloc_ok;
 
-	if (!perf_chn ||
-	    !alloc_cpumask_var(&available_mask, GFP_KERNEL) ||
-	    cpumask_empty(hk_mask)) {
+	alloc_ok = alloc_cpumask_var(&available_mask, GFP_KERNEL);
+	guard(rcu)();
+	hk_mask = housekeeping_cpumask(HK_TYPE_MANAGED_IRQ);
+
+	if (!perf_chn || !alloc_ok || cpumask_empty(hk_mask)) {
 		/*
 		 * If the channel is not a performance critical
 		 * channel, bind it to VMBUS_CONNECT_CPU.
@@ -770,7 +773,7 @@ static void init_vp_index(struct vmbus_channel *channel)
 		channel->target_cpu = VMBUS_CONNECT_CPU;
 		if (perf_chn)
 			hv_set_allocated_cpu(VMBUS_CONNECT_CPU);
-		return;
+		goto out_free;
 	}
 
 	for (i = 1; i <= ncpu + 1; i++) {
@@ -808,7 +811,7 @@ static void init_vp_index(struct vmbus_channel *channel)
 	}
 
 	channel->target_cpu = target_cpu;
-
+out_free:
 	free_cpumask_var(available_mask);
 }
 
diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 3faa74e49a6b..60c7a5ac15c0 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1763,8 +1763,11 @@ int vmbus_channel_set_cpu(struct vmbus_channel *channel, u32 target_cpu)
 	if (target_cpu >= nr_cpumask_bits)
 		return -EINVAL;
 
-	if (!cpumask_test_cpu(target_cpu, housekeeping_cpumask(HK_TYPE_MANAGED_IRQ)))
-		return -EINVAL;
+	scoped_guard(rcu) {
+		if (!cpumask_test_cpu(target_cpu,
+				      housekeeping_cpumask(HK_TYPE_MANAGED_IRQ)))
+			return -EINVAL;
+	}
 
 	if (!cpu_online(target_cpu))
 		return -EINVAL;
-- 
2.53.0

[PATCH 16/23] genirq/cpuhotplug: Use RCU to protect access of HK_TYPE_MANAGED_IRQ cpumask

[Waiman Long]

As HK_TYPE_MANAGED_IRQ cpumask is going to be changeable at run time,
use RCU to protect access to the cpumask.

To enable the new HK_TYPE_MANAGED_IRQ cpumask to take effect, the
following steps can be done.

 1) Update the HK_TYPE_MANAGED_IRQ cpumask to take out the newly isolated
    CPUs and add back the de-isolated CPUs.
 2) Tear down the affected CPUs to cause irq_migrate_all_off_this_cpu()
    to be called on the affected CPUs to migrate the irqs to other
    HK_TYPE_MANAGED_IRQ housekeeping CPUs.
 3) Bring up the previously offline CPUs to invoke
    irq_affinity_online_cpu() to allow the newly de-isolated CPUs to
    be used for managed irqs.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/irq/cpuhotplug.c | 1 +
 kernel/irq/manage.c     | 1 +
 2 files changed, 2 insertions(+)

diff --git a/kernel/irq/cpuhotplug.c b/kernel/irq/cpuhotplug.c
index cd5689e383b0..86437c78f1f2 100644
--- a/kernel/irq/cpuhotplug.c
+++ b/kernel/irq/cpuhotplug.c
@@ -196,6 +196,7 @@ static bool hk_should_isolate(struct irq_data *data, unsigned int cpu)
 	if (!housekeeping_enabled(HK_TYPE_MANAGED_IRQ))
 		return false;
 
+	guard(rcu)();
 	hk_mask = housekeeping_cpumask(HK_TYPE_MANAGED_IRQ);
 	if (cpumask_subset(irq_data_get_effective_affinity_mask(data), hk_mask))
 		return false;
diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 2e8072437826..8270c4de260b 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -263,6 +263,7 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask, bool
 	    housekeeping_enabled(HK_TYPE_MANAGED_IRQ)) {
 		const struct cpumask *hk_mask;
 
+		guard(rcu)();
 		hk_mask = housekeeping_cpumask(HK_TYPE_MANAGED_IRQ);
 
 		cpumask_and(tmp_mask, mask, hk_mask);
-- 
2.53.0

Re: [PATCH 16/23] genirq/cpuhotplug: Use RCU to protect access of HK_TYPE_MANAGED_IRQ cpumask

[Thomas Gleixner]

On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:

> As HK_TYPE_MANAGED_IRQ cpumask is going to be changeable at run time,
> use RCU to protect access to the cpumask.
>
> To enable the new HK_TYPE_MANAGED_IRQ cpumask to take effect, the
> following steps can be done.

Can be done?

>  1) Update the HK_TYPE_MANAGED_IRQ cpumask to take out the newly isolated
>     CPUs and add back the de-isolated CPUs.
>  2) Tear down the affected CPUs to cause irq_migrate_all_off_this_cpu()
>     to be called on the affected CPUs to migrate the irqs to other
>     HK_TYPE_MANAGED_IRQ housekeeping CPUs.
>  3) Bring up the previously offline CPUs to invoke
>     irq_affinity_online_cpu() to allow the newly de-isolated CPUs to
>     be used for managed irqs.

Which previously offline CPUs?

> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index 2e8072437826..8270c4de260b 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -263,6 +263,7 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask, bool
>  	    housekeeping_enabled(HK_TYPE_MANAGED_IRQ)) {
>  		const struct cpumask *hk_mask;
>  
> +		guard(rcu)();
>  		hk_mask = housekeeping_cpumask(HK_TYPE_MANAGED_IRQ);
>  
>  		cpumask_and(tmp_mask, mask, hk_mask);

How is this hunk related to $Subject?

Thanks,

        tglx

Re: [PATCH 16/23] genirq/cpuhotplug: Use RCU to protect access of HK_TYPE_MANAGED_IRQ cpumask

[Waiman Long]

On 4/21/26 5:02 AM, Thomas Gleixner wrote:
> On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
>
>> As HK_TYPE_MANAGED_IRQ cpumask is going to be changeable at run time,
>> use RCU to protect access to the cpumask.
>>
>> To enable the new HK_TYPE_MANAGED_IRQ cpumask to take effect, the
>> following steps can be done.
> Can be done?
>
>>   1) Update the HK_TYPE_MANAGED_IRQ cpumask to take out the newly isolated
>>      CPUs and add back the de-isolated CPUs.
>>   2) Tear down the affected CPUs to cause irq_migrate_all_off_this_cpu()
>>      to be called on the affected CPUs to migrate the irqs to other
>>      HK_TYPE_MANAGED_IRQ housekeeping CPUs.
>>   3) Bring up the previously offline CPUs to invoke
>>      irq_affinity_online_cpu() to allow the newly de-isolated CPUs to
>>      be used for managed irqs.
> Which previously offline CPUs?
This part should go into another patch.
>
>> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
>> index 2e8072437826..8270c4de260b 100644
>> --- a/kernel/irq/manage.c
>> +++ b/kernel/irq/manage.c
>> @@ -263,6 +263,7 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask, bool
>>   	    housekeeping_enabled(HK_TYPE_MANAGED_IRQ)) {
>>   		const struct cpumask *hk_mask;
>>   
>> +		guard(rcu)();
>>   		hk_mask = housekeeping_cpumask(HK_TYPE_MANAGED_IRQ);
>>   
>>   		cpumask_and(tmp_mask, mask, hk_mask);
> How is this hunk related to $Subject?

The subject is actually about using RCU to protect access to 
housekeeping cpumask. There are extra info in the commit  log that 
should go to another patch.

Cheers,
Longman

>

[PATCH 17/23] sched/isolation: Extend housekeeping_dereference_check() to cover changes in nohz_full or manged_irqs cpumasks

[Waiman Long]

As we are going to make HK_TYPE_KERNEL_NOISE and
HK_TYPE_MANAGED_IRQ housekeeping cpumasks run time changeable, extend
housekeeping_dereference_check() to cover changes to those cpumasks
as well.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/sched/isolation.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 1f3f1c83dd12..1647e7b08bac 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -38,7 +38,8 @@ EXPORT_SYMBOL_GPL(housekeeping_enabled);
 
 static bool housekeeping_dereference_check(enum hk_type type)
 {
-	if (IS_ENABLED(CONFIG_LOCKDEP) && type == HK_TYPE_DOMAIN) {
+	if (IS_ENABLED(CONFIG_LOCKDEP) &&
+	   (BIT(type) & (HK_FLAG_DOMAIN | HK_FLAG_KERNEL_NOISE | HK_FLAG_MANAGED_IRQ))) {
 		/* Cpuset isn't even writable yet? */
 		if (system_state <= SYSTEM_SCHEDULING)
 			return true;
-- 
2.53.0

[PATCH 18/23] cpu/hotplug: Add a new cpuhp_offline_cb() API

[Waiman Long]

Add a new cpuhp_offline_cb() API that allows us to offline a set of
CPUs one-by-one, run the given callback function and then bring those
CPUs back online again while inhibiting any concurrent CPU hotplug
operations from happening.

This new API can be used to enable runtime adjustment of nohz_full and
isolcpus boot command line options. A new cpuhp_offline_cb_mode flag
is also added to signal that the system is in this offline callback
transient state so that some hotplug operations can be optimized out
if we choose to.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/cpuhplock.h |  9 +++++
 kernel/cpu.c              | 70 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 79 insertions(+)

diff --git a/include/linux/cpuhplock.h b/include/linux/cpuhplock.h
index 286b3ab92e15..37637baa32eb 100644
--- a/include/linux/cpuhplock.h
+++ b/include/linux/cpuhplock.h
@@ -9,7 +9,9 @@
 
 #include <linux/cleanup.h>
 #include <linux/errno.h>
+#include <linux/cpumask_types.h>
 
+typedef int (*cpuhp_cb_t)(void *arg);
 struct device;
 
 extern int lockdep_is_cpus_held(void);
@@ -29,6 +31,8 @@ void clear_tasks_mm_cpumask(int cpu);
 int remove_cpu(unsigned int cpu);
 int cpu_device_down(struct device *dev);
 void smp_shutdown_nonboot_cpus(unsigned int primary_cpu);
+int cpuhp_offline_cb(struct cpumask *mask, cpuhp_cb_t func, void *arg);
+extern bool cpuhp_offline_cb_mode;
 
 #else /* CONFIG_HOTPLUG_CPU */
 
@@ -43,6 +47,11 @@ static inline void cpu_hotplug_disable(void) { }
 static inline void cpu_hotplug_enable(void) { }
 static inline int remove_cpu(unsigned int cpu) { return -EPERM; }
 static inline void smp_shutdown_nonboot_cpus(unsigned int primary_cpu) { }
+static inline int cpuhp_offline_cb(struct cpumask *mask, cpuhp_cb_t func, void *arg)
+{
+	return -EPERM;
+}
+#define cpuhp_offline_cb_mode	false
 #endif	/* !CONFIG_HOTPLUG_CPU */
 
 DEFINE_LOCK_GUARD_0(cpus_read_lock, cpus_read_lock(), cpus_read_unlock())
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 0d02b5d7a7ba..9b32f742cd1d 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1520,6 +1520,76 @@ int remove_cpu(unsigned int cpu)
 }
 EXPORT_SYMBOL_GPL(remove_cpu);
 
+bool cpuhp_offline_cb_mode;
+
+/**
+ * cpuhp_offline_cb - offline CPUs, invoke callback function & online CPUs afterward
+ * @mask: A mask of CPUs to be taken offline and then online
+ * @func: A callback function to be invoked while the given CPUs are offline
+ * @arg:  Argument to be passed back to the callback function
+ *
+ * Return: 0 if successful, an error code otherwise
+ */
+int cpuhp_offline_cb(struct cpumask *mask, cpuhp_cb_t func, void *arg)
+{
+	int off_cpu, on_cpu, ret, ret2 = 0;
+
+	if (WARN_ON_ONCE(cpumask_empty(mask) ||
+	   !cpumask_subset(mask, cpu_online_mask)))
+		return -EINVAL;
+
+	pr_debug("%s: begin (CPU list = %*pbl)\n", __func__, cpumask_pr_args(mask));
+	lock_device_hotplug();
+	cpuhp_offline_cb_mode = true;
+	/*
+	 * If all offline operations succeed, off_cpu should become nr_cpu_ids.
+	 */
+	for_each_cpu(off_cpu, mask) {
+		ret = device_offline(get_cpu_device(off_cpu));
+		if (unlikely(ret))
+			break;
+	}
+	if (!ret)
+		ret = func(arg);
+
+	/* Bring previously offline CPUs back online */
+	for_each_cpu(on_cpu, mask) {
+		int retries = 0;
+
+		if (on_cpu == off_cpu)
+			break;
+
+retry:
+		ret2 = device_online(get_cpu_device(on_cpu));
+
+		/*
+		 * With the unlikely event that CPU hotplug is disabled while
+		 * this operation is in progress, we will need to wait a bit
+		 * for hotplug to hopefully be re-enabled again. If not, print
+		 * a warning and return the error.
+		 *
+		 * cpu_hotplug_disabled is supposed to be accessed while
+		 * holding the cpu_add_remove_lock mutex. So we need to
+		 * use the data_race() macro to access it here.
+		 */
+		while ((ret2 == -EBUSY) && data_race(cpu_hotplug_disabled) &&
+		       (++retries <= 5)) {
+			msleep(20);
+			if (!data_race(cpu_hotplug_disabled))
+				goto retry;
+		}
+		if (ret2) {
+			pr_warn("%s: Failed to bring CPU %d back online!\n",
+				__func__, on_cpu);
+			break;
+		}
+	}
+	cpuhp_offline_cb_mode = false;
+	unlock_device_hotplug();
+	pr_debug("%s: end\n", __func__);
+	return ret ? ret : ret2;
+}
+
 void smp_shutdown_nonboot_cpus(unsigned int primary_cpu)
 {
 	unsigned int cpu;
-- 
2.53.0

Re: [PATCH 18/23] cpu/hotplug: Add a new cpuhp_offline_cb() API

[Thomas Gleixner]

On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
> Add a new cpuhp_offline_cb() API that allows us to offline a set of
> CPUs one-by-one, run the given callback function and then bring those
> CPUs back online again while inhibiting any concurrent CPU hotplug
> operations from happening.

Please provide a properly structured change log which explains the
context, the problem and the solution in separate paragraphs and this
order. This is not new. It's documented...

> This new API can be used to enable runtime adjustment of nohz_full and
> isolcpus boot command line options. A new cpuhp_offline_cb_mode flag
> is also added to signal that the system is in this offline callback
> transient state so that some hotplug operations can be optimized out
> if we choose to.

We chose nothing.

> +#include <linux/cpumask_types.h>

What for? This header only needs a 'struct cpumask' forward declaration
so that the compiler can handle the pointer argument, no?

> +typedef int (*cpuhp_cb_t)(void *arg);

You couldn't come up with a more generic name for this, right?

>  struct device;
>  
>  extern int lockdep_is_cpus_held(void);
> @@ -29,6 +31,8 @@ void clear_tasks_mm_cpumask(int cpu);
>  int remove_cpu(unsigned int cpu);
>  int cpu_device_down(struct device *dev);
>  void smp_shutdown_nonboot_cpus(unsigned int primary_cpu);
> +int cpuhp_offline_cb(struct cpumask *mask, cpuhp_cb_t func, void *arg);

Ditto.

> +extern bool cpuhp_offline_cb_mode;

Groan. The only users are in the cpusets code which invokes this muck
and should therefore know what's going on, no?

>  #else /* CONFIG_HOTPLUG_CPU */
>  
> @@ -43,6 +47,11 @@ static inline void cpu_hotplug_disable(void) { }
>  static inline void cpu_hotplug_enable(void) { }
>  static inline int remove_cpu(unsigned int cpu) { return -EPERM; }
>  static inline void smp_shutdown_nonboot_cpus(unsigned int primary_cpu) { }
> +static inline int cpuhp_offline_cb(struct cpumask *mask, cpuhp_cb_t func, void *arg)
> +{
> +	return -EPERM;

-EPERM?

> +/**
> + * cpuhp_offline_cb - offline CPUs, invoke callback function & online CPUs afterward
> + * @mask: A mask of CPUs to be taken offline and then online
> + * @func: A callback function to be invoked while the given CPUs are offline
> + * @arg:  Argument to be passed back to the callback function
> + *
> + * Return: 0 if successful, an error code otherwise
> + */
> +int cpuhp_offline_cb(struct cpumask *mask, cpuhp_cb_t func, void *arg)
> +{
> +	int off_cpu, on_cpu, ret, ret2 = 0;
> +
> +	if (WARN_ON_ONCE(cpumask_empty(mask) ||
> +	   !cpumask_subset(mask, cpu_online_mask)))
> +		return -EINVAL;

No line break required. You have 100 characters.

But what's worse is that the access to cpu_online_mask is not protected
against a concurrent CPU hotplug operation.

> +
> +	pr_debug("%s: begin (CPU list = %*pbl)\n", __func__, cpumask_pr_args(mask));

Tracing?

> +	lock_device_hotplug();
> +	cpuhp_offline_cb_mode = true;
> +	/*
> +	 * If all offline operations succeed, off_cpu should become nr_cpu_ids.
> +	 */
> +	for_each_cpu(off_cpu, mask) {
> +		ret = device_offline(get_cpu_device(off_cpu));
> +		if (unlikely(ret))
> +			break;
> +	}
> +	if (!ret)
> +		ret = func(arg);
> +
> +	/* Bring previously offline CPUs back online */
> +	for_each_cpu(on_cpu, mask) {
> +		int retries = 0;
> +
> +		if (on_cpu == off_cpu)
> +			break;
> +
> +retry:
> +		ret2 = device_online(get_cpu_device(on_cpu));
> +
> +		/*
> +		 * With the unlikely event that CPU hotplug is disabled while
> +		 * this operation is in progress, we will need to wait a bit
> +		 * for hotplug to hopefully be re-enabled again. If not, print
> +		 * a warning and return the error.
> +		 *
> +		 * cpu_hotplug_disabled is supposed to be accessed while
> +		 * holding the cpu_add_remove_lock mutex. So we need to
> +		 * use the data_race() macro to access it here.
> +		 */
> +		while ((ret2 == -EBUSY) && data_race(cpu_hotplug_disabled) &&
> +		       (++retries <= 5)) {
> +			msleep(20);
> +			if (!data_race(cpu_hotplug_disabled))
> +				goto retry;
> +		}
> +		if (ret2) {
> +			pr_warn("%s: Failed to bring CPU %d back online!\n",
> +				__func__, on_cpu);

Provide a proper text and not this silly __func__ thing.

> +			break;
> +		}
> +	}

TBH. This is unreviewable gunk and the whole 'unlikely event that CPU
hotplug is disabled' is just a lazy hack.

All of this can be avoided including this made up callback function.

It's not rocket science to provide:

     1) A function which serializes against any other CPU hotplug
        related action.

     2) A function which brings the CPUs in a given CPU mask down

     3) A function which brings the CPUs in a given CPU mask up

     4) A function which undoes #1

Yeah I know, it's more work and not convoluted enough. But see below.

That brings me to that other hack namely cpuhp_offline_cb_mode, which
you self described as such in patch 21/23:

> +	/*
> +	 * Hack: In cpuhp_offline_cb_mode, pretend all partitions are empty
> +	 * to prevent unnecessary partition invalidation.
> +	 */
> +	if (cpuhp_offline_cb_mode)
> +		return false;
> +

We are not merging hacks. End of story. But you knew that already, no?

Let's take a step back and see what you really need to achieve:

  1) Update tick_nohz_full_mask
  2) Update the managed interrupt mask
  3) Update CPU sets

Independent of the direction of this update you need to ensure that the
affected functionality keeps working correctly.

You achieve that by bulk offlining the affected CPUs, invoking a magic
callback and then bulk onlining the affected CPUs again, which requires
that ill defined cpuhp_offline_cb_mode hackery and probably some more
hacks all over the place.

You can achieve the same by doing CPU by CPU operations in the right
order without this mode hack, when you establish proper limitations for
this:

  At no point in time it's allowed to empty a CPU set or a affected CPU
  mask, except when you completely undo the isolation of CPUs.

  That can be computed upfront w/o changing anything at all. Once the
  validity is established, the update can proceed. Or you can leave it
  to user space which can keep the pieces if it gets it wrong.

That's a reasonable limitation as there is absolutely zero justification
to support something like:

       housekeeping_cpus = [CPU 0], isolated_cpus = [CPU 1]
  ---> housekeeping_cpus = [CPU 1], isolated_cpus = [CPU 0]

just because we can with enough horrible hacks.

If you get that out of the way, then a CPU by CPU update becomes the
obvious and simplest solution. The ordering constraints can be computed
in user space upfront and there is no reason to do any of this in the
kernel itself except for an eventual validation step. It might be a tad
slower, but this is all but a hotpath operation.

Just for the record. I suggested exactly this more than a year ago and
it's still the right thing to do.

And of course neither your cover letter nor any of the patches give a
proper rationale why you think that your bulk hackery is better. For the
very simple reason that there is no rationale at all.

This bulk muck is doomed when your ultimate goal is to avoid the stop
machine dance. With a per CPU update it is actually doable without more
ill defined hacks all over the place.

   1) Bring down the CPU to CPUHP_AP_SCHED_WAIT_EMPTY, which is the last
      state before stop machine is invoked.

      At that point:

         - no user space thread is running on the CPU anymore

         - everything related to this CPU has been shut down or moved
           elsewhere

         - interrupt managed device queues are quiesced if the CPU was
           the last online one in the queue affinity mask. If not the
           interrupt might still be affine to the CPU, but there is at
           least one other CPU available in the mask.

   2) Update the tick NOHZ handover

      This can be done without going into stop machine by providing a
      hotplug callback right between CPUHP_AP_SMPBOOT_THREADS and
      CPUHP_AP_IRQ_AFFINITY_ONLINE.

      That's trivial enough to achieve and can work independently of
      NOHZ full.

   3) Rework the affinity management, so that interrupt affinities can
      be reassigned in the CPUHP_AP_IRQ_AFFINITY_ONLINE state.

      That needs a lot of thoughts, but there is no real reason why it
      can't work.

   4) Flip the housekeeping CPU masks in sched_cpu_wait_empty() after
      balance_hotplug_wait().

   5) Bring the CPU online again.

For #2 and #3 to work you need a separate CPU mask which avoids touching
CPU online mask. For #3 this needs some more work to avoid reassigning the
interrupts once sparse_irq_lock is dropped, but the bulk is achieved
with the separate CPU mask.

No?

Thanks,

        tglx

[PATCH 19/23] cgroup/cpuset: Improve check for calling housekeeping_update()

[Waiman Long]

By making sure that isolated_hk_cpus matches isolated_cpus at boot time,
we can more accurately determine if calling housekeeping_update()
is needed by comparing if the two cpumasks are equal. The
update_housekeeping flag still have a use in cpuset_handle_hotplug()
to determine if a work function should be queued to invoke
cpuset_update_sd_hk_unlock() as it is not supposed to look at
isolated_hk_cpus without holding cpuset_top_mutex.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 36 ++++++++++++++++++++----------------
 1 file changed, 20 insertions(+), 16 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index a4eccb0ec0d1..1b0c50b46a49 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1339,26 +1339,29 @@ static void cpuset_update_sd_hk_unlock(void)
 	__releases(&cpuset_mutex)
 	__releases(&cpuset_top_mutex)
 {
+	update_housekeeping = false;
+
 	/* force_sd_rebuild will be cleared in rebuild_sched_domains_locked() */
 	if (force_sd_rebuild)
 		rebuild_sched_domains_locked();
 
-	if (update_housekeeping) {
-		update_housekeeping = false;
-		cpumask_copy(isolated_hk_cpus, isolated_cpus);
-
-		/*
-		 * housekeeping_update() is now called without holding
-		 * cpus_read_lock and cpuset_mutex. Only cpuset_top_mutex
-		 * is still being held for mutual exclusion.
-		 */
-		mutex_unlock(&cpuset_mutex);
-		cpus_read_unlock();
-		WARN_ON_ONCE(housekeeping_update(isolated_hk_cpus, BIT(HK_TYPE_DOMAIN)));
-		mutex_unlock(&cpuset_top_mutex);
-	} else {
+	if (cpumask_equal(isolated_hk_cpus, isolated_cpus)) {
+		/* No housekeeping cpumask update needed */
 		cpuset_full_unlock();
+		return;
 	}
+
+	cpumask_copy(isolated_hk_cpus, isolated_cpus);
+
+	/*
+	 * housekeeping_update() is now called without holding
+	 * cpus_read_lock and cpuset_mutex. Only cpuset_top_mutex
+	 * is still being held for mutual exclusion.
+	 */
+	mutex_unlock(&cpuset_mutex);
+	cpus_read_unlock();
+	WARN_ON_ONCE(housekeeping_update(isolated_hk_cpus, BIT(HK_TYPE_DOMAIN)));
+	mutex_unlock(&cpuset_top_mutex);
 }
 
 /*
@@ -3692,10 +3695,11 @@ int __init cpuset_init(void)
 
 	BUG_ON(!alloc_cpumask_var(&cpus_attach, GFP_KERNEL));
 
-	if (housekeeping_enabled(HK_TYPE_DOMAIN_BOOT))
+	if (housekeeping_enabled(HK_TYPE_DOMAIN_BOOT)) {
 		cpumask_andnot(isolated_cpus, cpu_possible_mask,
 			       housekeeping_cpumask(HK_TYPE_DOMAIN_BOOT));
-
+		cpumask_copy(isolated_hk_cpus, isolated_cpus);
+	}
 	return 0;
 }
 
-- 
2.53.0

[PATCH 20/23] cgroup/cpuset: Enable runtime update of HK_TYPE_{KERNEL_NOISE,MANAGED_IRQ} cpumasks

[Waiman Long]

One simple way to enable runtime update of HK_TYPE_KERNEL_NOISE
(nohz_full) and HK_TYPE_MANAGED_IRQ cpumasks is to make use of the CPU
hotplug to facilitate the transition of those CPUs that are changing
states as long as CONFIG_HOTPLUG_CPU is enabled and a nohz_full boot
parameter is provided. Otherwise, only HK_TYPE_DOMAIN cpumask will be
updated at run time.

For changes in HK_TYPE_DOMAIN cpumask, it can be done without using CPU
hotplug. For changes in HK_TYPE_MANAGED_IRQ cpumask, we have to update
the cpumask first and then tear down and bring up the newly isolated
CPUs to migrate the managed irqs in those CPUs to other housekeeping
CPUs.

For changes in HK_TYPE_KERNEL_NOISE, we have to tear down all the newly
isolated and de-isolated CPUs, change the cpumask and then bring all the
offline CPUs back online.

As it is possible that the various boot versions of the housekeeping
cpumasks are different resulting in the use of different set of isolated
cpumasks for calling housekeeping_update(), we may need to pre-allocate
these cpumasks if necessary.

Note that the use of CPU hotplug to facilitate the changing of
HK_TYPE_KERNEL_NOISE and HK_TYPE_MANAGED_IRQ housekeeping cpumasks has
the drawback that during the tear down of a CPU from CPUHP_TEARDOWN_CPU
state to CPUHP_AP_OFFLINE, the stop_machine code will be invoked to stop
all the other CPUs including all the pre-existing isolated CPUs. That
will cause latency spikes on those isolated CPUs. That latency spike
should only happen when the cpuset isolated partition setting is changed
resulting in changes in those housekeeping cpumasks.

One possible workaround that is being used right now is to pre-allocate
a set of nohz_full and managed_irq CPUs at boot time. The semi-isolated
CPUs are then used to create cpuset isolated partitions when needed
to enable full isolation. This will likely continue even if we made
the nohz_full and managed_irq CPUs runtime changeable if they can't
tolerate these latency spikes.

This is a problem we need to address in a future patch series.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 199 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 182 insertions(+), 17 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 1b0c50b46a49..a927b9cd4f71 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -152,9 +152,17 @@ static cpumask_var_t	isolated_cpus;		/* CSCB */
 static bool		update_housekeeping;	/* RWCS */
 
 /*
- * Copy of isolated_cpus to be passed to housekeeping_update()
+ * Cpumasks to be passed to housekeeping_update()
+ * isolated_hk_cpus - copy of isolated_cpus for HK_TYPE_DOMAIN
+ * isolated_nohz_cpus - for HK_TYPE_KERNEL_NOISE
+ * isolated_mirq_cpus - for HK_TYPE_MANAGED_IRQ
  */
 static cpumask_var_t	isolated_hk_cpus;	/* T */
+static cpumask_var_t	isolated_nohz_cpus;	/* T */
+static cpumask_var_t	isolated_mirq_cpus;	/* T */
+
+static bool		boot_nohz_le_domain __ro_after_init;
+static bool		boot_mirq_le_domain __ro_after_init;
 
 /*
  * A flag to force sched domain rebuild at the end of an operation.
@@ -1328,29 +1336,67 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
 	return false;
 }
 
+static int cpuset_nohz_update_cbfunc(void *arg)
+{
+	struct cpumask *isol_cpus = (struct cpumask *)arg;
+
+	if (isol_cpus)
+		housekeeping_update(isol_cpus, BIT(HK_TYPE_KERNEL_NOISE));
+	return 0;
+}
+
 /*
- * cpuset_update_sd_hk_unlock - Rebuild sched domains, update HK & unlock
- *
- * Update housekeeping cpumasks and rebuild sched domains if necessary and
- * then do a cpuset_full_unlock().
- * This should be called at the end of cpuset operation.
  */
-static void cpuset_update_sd_hk_unlock(void)
-	__releases(&cpuset_mutex)
-	__releases(&cpuset_top_mutex)
+static void cpuset_update_housekeeping_unlock(void)
 {
-	update_housekeeping = false;
+	bool update_nohz, update_mirq;
+	cpumask_var_t cpus;
+	int ret;
 
-	/* force_sd_rebuild will be cleared in rebuild_sched_domains_locked() */
-	if (force_sd_rebuild)
-		rebuild_sched_domains_locked();
+	if (!tick_nohz_full_enabled())
+		return;
 
-	if (cpumask_equal(isolated_hk_cpus, isolated_cpus)) {
-		/* No housekeeping cpumask update needed */
+	update_nohz = boot_nohz_le_domain;
+	update_mirq = boot_mirq_le_domain;
+
+	if (WARN_ON_ONCE(!alloc_cpumask_var(&cpus, GFP_KERNEL))) {
 		cpuset_full_unlock();
 		return;
 	}
 
+	/*
+	 * Update isolated_nohz_cpus/isolated_mirq_cpus if necessary
+	 */
+	if (!boot_nohz_le_domain) {
+		cpumask_andnot(cpus, cpu_possible_mask,
+			       housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
+		cpumask_or(cpus, cpus, isolated_cpus);
+		update_nohz = !cpumask_equal(isolated_nohz_cpus, cpus);
+		if (update_nohz)
+			cpumask_copy(isolated_nohz_cpus, cpus);
+	}
+	if (!boot_mirq_le_domain) {
+		cpumask_andnot(cpus, cpu_possible_mask,
+			       housekeeping_cpumask(HK_TYPE_MANAGED_IRQ));
+		cpumask_or(cpus, cpus, isolated_cpus);
+		update_mirq = !cpumask_equal(isolated_mirq_cpus, cpus);
+		if (update_mirq)
+			cpumask_copy(isolated_mirq_cpus, cpus);
+	}
+
+	/*
+	 * Compute list of CPUs to be brought offline into "cpus"
+	 * isolated_hk_cpus - old cpumask
+	 * isolated_cpus    - new cpumask
+	 *
+	 * With update_nohz, we need to offline both the newly isolated
+	 * and de-isolated CPUs. With only update_mirq, we only need to
+	 * offline the new isolated CPUs.
+	 */
+	if (update_nohz)
+		cpumask_xor(cpus, isolated_hk_cpus, isolated_cpus);
+	else if (update_mirq)
+		cpumask_andnot(cpus, isolated_cpus, isolated_hk_cpus);
 	cpumask_copy(isolated_hk_cpus, isolated_cpus);
 
 	/*
@@ -1360,10 +1406,103 @@ static void cpuset_update_sd_hk_unlock(void)
 	 */
 	mutex_unlock(&cpuset_mutex);
 	cpus_read_unlock();
-	WARN_ON_ONCE(housekeeping_update(isolated_hk_cpus, BIT(HK_TYPE_DOMAIN)));
+
+	if (!update_mirq) {
+		ret = housekeeping_update(isolated_hk_cpus, BIT(HK_TYPE_DOMAIN));
+	} else if (boot_mirq_le_domain) {
+		ret = housekeeping_update(isolated_hk_cpus,
+					  BIT(HK_TYPE_DOMAIN)|BIT(HK_TYPE_MANAGED_IRQ));
+	} else {
+		ret = housekeeping_update(isolated_hk_cpus, BIT(HK_TYPE_DOMAIN));
+		if (!ret)
+			ret = housekeeping_update(isolated_mirq_cpus,
+						  BIT(HK_TYPE_MANAGED_IRQ));
+	}
+
+	if (WARN_ON_ONCE(ret))
+		goto out_free;
+
+	/*
+	 * Calling cpuhp_offline_cb() is only needed if either
+	 * HK_TYPE_KERNEL_NOISE and/or HK_TYPE_MANAGED_IRQ cpumasks
+	 * needed to be updated.
+	 *
+	 * TODO: When tearing down a CPU from CPUHP_TEARDOWN_CPU state
+	 * downward to CPUHP_AP_OFFLINE, the stop_machine code will be
+	 * invoked to stop all the other CPUs in the system. This will
+	 * cause latency spikes on existing isolated CPUs. We will need
+	 * some new mechanism to enable us to not stop the existing
+	 * isolated CPUs whenever possible. A possible workaround is
+	 * to pre-allocate a set of nohz_full and manged_irq CPUs at
+	 * boot time and use them to create isolated cpuset partitions
+	 * so that CPU hotplug won't need to be used.
+	 */
+	if (update_mirq || update_nohz) {
+		struct cpumask *nohz_cpus;
+
+		/*
+		 * Calling housekeeping_update() is only needed if
+		 * update_nohz is set.
+		 */
+		nohz_cpus = !update_nohz ? NULL : boot_nohz_le_domain
+					 ? isolated_hk_cpus
+					 : isolated_nohz_cpus;
+		/*
+		 * Mask out offline CPUs in cpus
+		 * If there is no online CPUs, we can call
+		 * housekeeping_update() directly if needed.
+		 */
+		cpumask_and(cpus, cpus, cpu_online_mask);
+		if (!cpumask_empty(cpus))
+			ret = cpuhp_offline_cb(cpus, cpuset_nohz_update_cbfunc,
+					       nohz_cpus);
+		else if (nohz_cpus)
+			ret = housekeeping_update(nohz_cpus, BIT(HK_TYPE_KERNEL_NOISE));
+	}
+	WARN_ON_ONCE(ret);
+out_free:
+	free_cpumask_var(cpus);
 	mutex_unlock(&cpuset_top_mutex);
 }
 
+/*
+ * cpuset_update_sd_hk_unlock - Rebuild sched domains, update HK & unlock
+ *
+ * Update housekeeping cpumasks and rebuild sched domains if necessary and
+ * then do a cpuset_full_unlock().
+ * This should be called at the end of cpuset operation.
+ */
+static void cpuset_update_sd_hk_unlock(void)
+	__releases(&cpuset_mutex)
+	__releases(&cpuset_top_mutex)
+{
+	/* force_sd_rebuild will be cleared in rebuild_sched_domains_locked() */
+	if (force_sd_rebuild)
+		rebuild_sched_domains_locked();
+
+	update_housekeeping = false;
+
+	if (cpumask_equal(isolated_cpus, isolated_hk_cpus)) {
+		cpuset_full_unlock();
+		return;
+	}
+
+	if (!tick_nohz_full_enabled()) {
+		/*
+		 * housekeeping_update() is now called without holding
+		 * cpus_read_lock and cpuset_mutex. Only cpuset_top_mutex
+		 * is still being held for mutual exclusion.
+		 */
+		mutex_unlock(&cpuset_mutex);
+		cpus_read_unlock();
+		WARN_ON_ONCE(housekeeping_update(isolated_hk_cpus,
+						 BIT(HK_TYPE_DOMAIN)));
+		mutex_unlock(&cpuset_top_mutex);
+	} else {
+		cpuset_update_housekeeping_unlock();
+	}
+}
+
 /*
  * Work function to invoke cpuset_update_sd_hk_unlock()
  */
@@ -3700,6 +3839,29 @@ int __init cpuset_init(void)
 			       housekeeping_cpumask(HK_TYPE_DOMAIN_BOOT));
 		cpumask_copy(isolated_hk_cpus, isolated_cpus);
 	}
+
+	/*
+	 * If nohz_full and/or managed_irq cpu list, if present, is a subset
+	 * of the domain cpu list, i.e. HK_TYPE_DOMAIN_BOOT cpumask is a subset
+	 * of HK_TYPE_KERNEL_NOISE_BOOT/HK_TYPE_MANAGED_IRQ_BOOT cpumask, the
+	 * corresponding non-boot housekeeping cpumasks will follow changes
+	 * in the HK_TYPE_DOMAIN cpumask. So we don't need to use separate
+	 * cpumasks to isolate them.
+	 */
+	boot_nohz_le_domain = cpumask_subset(housekeeping_cpumask(HK_TYPE_DOMAIN_BOOT),
+					     housekeeping_cpumask(HK_TYPE_KERNEL_NOISE_BOOT));
+	boot_mirq_le_domain = cpumask_subset(housekeeping_cpumask(HK_TYPE_DOMAIN_BOOT),
+					     housekeeping_cpumask(HK_TYPE_MANAGED_IRQ_BOOT));
+	if (!boot_nohz_le_domain) {
+		BUG_ON(!alloc_cpumask_var(&isolated_nohz_cpus, GFP_KERNEL));
+		cpumask_andnot(isolated_nohz_cpus, cpu_possible_mask,
+			       housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
+	}
+	if (!boot_mirq_le_domain) {
+		BUG_ON(!alloc_cpumask_var(&isolated_mirq_cpus, GFP_KERNEL));
+		cpumask_andnot(isolated_mirq_cpus, cpu_possible_mask,
+			       housekeeping_cpumask(HK_TYPE_MANAGED_IRQ));
+	}
 	return 0;
 }
 
@@ -3954,7 +4116,10 @@ static void cpuset_handle_hotplug(void)
 	 */
 	if (force_sd_rebuild)
 		rebuild_sched_domains_cpuslocked();
-	if (update_housekeeping)
+	/*
+	 * Don't need to update housekeeping cpumasks in cpuhp_offline_cb mode.
+	 */
+	if (update_housekeeping && !cpuhp_offline_cb_mode)
 		queue_work(system_dfl_wq, &hk_sd_work);
 
 	free_tmpmasks(ptmp);
-- 
2.53.0

[PATCH 21/23] cgroup/cpuset: Limit the side effect of using CPU hotplug on isolated partition

[Waiman Long]

CPU hotplug is used to facilitate the modification of the
HK_TYPE_KERNEL_NOISE and HK_TYPE_MANAGED_IRQ cpumasks. However, tearing
down and bringing up CPUs can impact the cpuset partition states
as well. For instance, tearing down the last CPU of a partition can
invalidate the partition with active tasks which will not happen if
CPU hotplug isn't used.

A workaround of this issue is disable the invalidation by pretending that
the partition has no task, and making the tasks within the partition
to the effective CPUs of its parent for a short while during the short
transition process where the CPUs will be teared down and the brought
up again.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index a927b9cd4f71..5f6b4e67748f 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -434,6 +434,13 @@ static inline bool partition_is_populated(struct cpuset *cs,
 	struct cpuset *cp;
 	struct cgroup_subsys_state *pos_css;
 
+	/*
+	 * Hack: In cpuhp_offline_cb_mode, pretend all partitions are empty
+	 * to prevent unnecessary partition invalidation.
+	 */
+	if (cpuhp_offline_cb_mode)
+		return false;
+
 	/*
 	 * We cannot call cs_is_populated(cs) directly, as
 	 * nr_populated_domain_children may include populated
@@ -3881,6 +3888,17 @@ hotplug_update_tasks(struct cpuset *cs,
 	cs->effective_mems = *new_mems;
 	spin_unlock_irq(&callback_lock);
 
+	/*
+	 * When cpuhp_offline_cb_mode is active, valid isolated partition
+	 * with tasks may have no online CPUs available for a short while.
+	 * In that case, we fall back to parent's effective CPUs temporarily
+	 * which will be reset back to their rightful value once the affected
+	 * CPUs are online again.
+	 */
+	if (cpuhp_offline_cb_mode && cpumask_empty(new_cpus) &&
+	   (cs->partition_root_state == PRS_ISOLATED))
+		cpumask_copy(new_cpus, parent_cs(cs)->effective_cpus);
+
 	if (cpus_updated)
 		cpuset_update_tasks_cpumask(cs, new_cpus);
 	if (mems_updated)
-- 
2.53.0

[PATCH 22/23] cgroup/cpuset: Prevent offline_disabled CPUs from being used in isolated partition

[Waiman Long]

If tick_nohz_full_enabled() is true, we are going to use CPU
hotplug to enable runtime changes to the HK_TYPE_KERNEL_NOISE and
HK_TYPE_MANAGED_IRQ cpumasks. However, for some architectures, one
or maybe more CPUs will have the offline_disabled flag set in their
cpu devices. For instance, x64-64 will set this flag for its boot CPU
(typically CPU 0) to disable it from going offline. Those CPUs can't
be used in cpuset isolated partition, or we are going to have problem
in the CPU offline process.

Find out the set of CPUs with offline_disabled set in a
new cpuset_init_late() helper, set the corresponding bits in
offline_disabled_cpus cpumask and check it when isolated partitions are
being created or modified to ensure that we will not use any of those
offline disabled CPUs in an isolated partition.

An error message mentioning those offline disabled CPUs will be
constructed in cpuset_init_late() and shown in "cpuset.cpus.partition"
when isolated creation or modification fails.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset-internal.h |  1 +
 kernel/cgroup/cpuset.c          | 89 ++++++++++++++++++++++++++++++++-
 2 files changed, 88 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
index fd7d19842ded..87b7411540ff 100644
--- a/kernel/cgroup/cpuset-internal.h
+++ b/kernel/cgroup/cpuset-internal.h
@@ -35,6 +35,7 @@ enum prs_errcode {
 	PERR_HKEEPING,
 	PERR_ACCESS,
 	PERR_REMOTE,
+	PERR_OL_DISABLED,
 };
 
 /* bits in struct cpuset flags field */
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 5f6b4e67748f..f3af8ef6c64e 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -20,6 +20,8 @@
  */
 #include "cpuset-internal.h"
 
+#include <linux/cpu.h>
+#include <linux/device.h>
 #include <linux/init.h>
 #include <linux/interrupt.h>
 #include <linux/kernel.h>
@@ -48,7 +50,7 @@ DEFINE_STATIC_KEY_FALSE(cpusets_enabled_key);
  */
 DEFINE_STATIC_KEY_FALSE(cpusets_insane_config_key);
 
-static const char * const perr_strings[] = {
+static const char *perr_strings[] __ro_after_init = {
 	[PERR_INVCPUS]   = "Invalid cpu list in cpuset.cpus.exclusive",
 	[PERR_INVPARENT] = "Parent is an invalid partition root",
 	[PERR_NOTPART]   = "Parent is not a partition root",
@@ -59,6 +61,7 @@ static const char * const perr_strings[] = {
 	[PERR_HKEEPING]  = "partition config conflicts with housekeeping setup",
 	[PERR_ACCESS]    = "Enable partition not permitted",
 	[PERR_REMOTE]    = "Have remote partition underneath",
+	[PERR_OL_DISABLED] = "", /* To be set up later */
 };
 
 /*
@@ -164,6 +167,12 @@ static cpumask_var_t	isolated_mirq_cpus;	/* T */
 static bool		boot_nohz_le_domain __ro_after_init;
 static bool		boot_mirq_le_domain __ro_after_init;
 
+/*
+ * Cpumask of CPUs with offline_disabled set
+ * The cpumask is effectively __ro_after_init.
+ */
+static cpumask_var_t	offline_disabled_cpus;
+
 /*
  * A flag to force sched domain rebuild at the end of an operation.
  * It can be set in
@@ -1649,6 +1658,8 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
 	 * The effective_xcpus mask can contain offline CPUs, but there must
 	 * be at least one or more online CPUs present before it can be enabled.
 	 *
+	 * An isolated partition cannot contain CPUs with offline disabled.
+	 *
 	 * Note that creating a remote partition with any local partition root
 	 * above it or remote partition root underneath it is not allowed.
 	 */
@@ -1661,6 +1672,9 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
 	     !isolated_cpus_can_update(tmp->new_cpus, NULL)) ||
 	    prstate_housekeeping_conflict(new_prs, tmp->new_cpus))
 		return PERR_HKEEPING;
+	if (tick_nohz_full_enabled() && (new_prs == PRS_ISOLATED) &&
+	    cpumask_intersects(tmp->new_cpus, offline_disabled_cpus))
+		return PERR_OL_DISABLED;
 
 	spin_lock_irq(&callback_lock);
 	partition_xcpus_add(new_prs, NULL, tmp->new_cpus);
@@ -1746,6 +1760,16 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
 		goto invalidate;
 	}
 
+	/*
+	 * Isolated partition cannot contains CPUs with offline_disabled
+	 * bit set.
+	 */
+	if (tick_nohz_full_enabled() && (prs == PRS_ISOLATED) &&
+	    cpumask_intersects(excpus, offline_disabled_cpus)) {
+		cs->prs_err = PERR_OL_DISABLED;
+		goto invalidate;
+	}
+
 	adding   = cpumask_andnot(tmp->addmask, excpus, cs->effective_xcpus);
 	deleting = cpumask_andnot(tmp->delmask, cs->effective_xcpus, excpus);
 
@@ -1913,6 +1937,11 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 		if (tasks_nocpu_error(parent, cs, xcpus))
 			return PERR_NOCPUS;
 
+		if (tick_nohz_full_enabled() &&
+		    (new_prs == PRS_ISOLATED) &&
+		    cpumask_intersects(xcpus, offline_disabled_cpus))
+			return PERR_OL_DISABLED;
+
 		/*
 		 * This function will only be called when all the preliminary
 		 * checks have passed. At this point, the following condition
@@ -1979,12 +2008,21 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 					       parent->effective_xcpus);
 		}
 
+		/*
+		 * Isolated partition cannot contain CPUs with offline_disabled
+		 * bit set.
+		 */
+		if (tick_nohz_full_enabled() &&
+		   ((old_prs == PRS_ISOLATED) ||
+		    (old_prs == PRS_INVALID_ISOLATED)) &&
+		    cpumask_intersects(newmask, offline_disabled_cpus)) {
+			part_error = PERR_OL_DISABLED;
 		/*
 		 * TBD: Invalidate a currently valid child root partition may
 		 * still break isolated_cpus_can_update() rule if parent is an
 		 * isolated partition.
 		 */
-		if (is_partition_valid(cs) && (old_prs != parent_prs)) {
+		} else if (is_partition_valid(cs) && (old_prs != parent_prs)) {
 			if ((parent_prs == PRS_ROOT) &&
 			    /* Adding to parent means removing isolated CPUs */
 			    !isolated_cpus_can_update(tmp->delmask, tmp->addmask))
@@ -1995,6 +2033,19 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 				part_error = PERR_HKEEPING;
 		}
 
+		if (part_error) {
+			deleting = false;
+			/*
+			 * For a previously valid partition, we need to move
+			 * the exclusive CPUs back to its parent.
+			 */
+			if (is_partition_valid(cs) &&
+			    !cpumask_empty(cs->effective_xcpus)) {
+				cpumask_copy(tmp->addmask, cs->effective_xcpus);
+				adding = true;
+			}
+		}
+
 		/*
 		 * The new CPUs to be removed from parent's effective CPUs
 		 * must be present.
@@ -3829,6 +3880,7 @@ int __init cpuset_init(void)
 	BUG_ON(!zalloc_cpumask_var(&subpartitions_cpus, GFP_KERNEL));
 	BUG_ON(!zalloc_cpumask_var(&isolated_cpus, GFP_KERNEL));
 	BUG_ON(!zalloc_cpumask_var(&isolated_hk_cpus, GFP_KERNEL));
+	BUG_ON(!zalloc_cpumask_var(&offline_disabled_cpus, GFP_KERNEL));
 
 	cpumask_setall(top_cpuset.cpus_allowed);
 	nodes_setall(top_cpuset.mems_allowed);
@@ -4188,6 +4240,39 @@ void __init cpuset_init_smp(void)
 	BUG_ON(!cpuset_migrate_mm_wq);
 }
 
+/**
+ * cpuset_init_late - initialize the list of CPUs with offline_disabled set
+ *
+ * Description: Initialize a cpumask with CPUs that have the offline_disabled
+ *		bit set. It is done in a separate initcall as cpuset_init_smp()
+ *		is called before driver_init() where the CPU devices will be
+ *		set up.
+ */
+static int __init cpuset_init_late(void)
+{
+	int cpu;
+
+	if (!tick_nohz_full_enabled())
+		return 0;
+	/*
+	 * Iterate all the possible CPUs to see which one has offline disabled.
+	 */
+	for_each_possible_cpu(cpu) {
+		if (get_cpu_device(cpu)->offline_disabled)
+			__cpumask_set_cpu(cpu, offline_disabled_cpus);
+	}
+	if (!cpumask_empty(offline_disabled_cpus)) {
+		char buf[128];
+
+		snprintf(buf, sizeof(buf),
+			 "CPU %*pbl with offline disabled not allowed in isolated partition",
+			 cpumask_pr_args(offline_disabled_cpus));
+		perr_strings[PERR_OL_DISABLED] = kstrdup(buf, GFP_KERNEL);
+	}
+	return 0;
+}
+pure_initcall(cpuset_init_late);
+
 /*
  * Return cpus_allowed mask from a task's cpuset.
  */
-- 
2.53.0

[PATCH 23/23] cgroup/cpuset: Documentation and kselftest updates

[Waiman Long]

As CPU hotplug is now being used to enable runtime update to the list
of nohz_full and managed_irq CPUs, we should avoid using CPU 0 in the
formation of isolated partition as CPU 0 may not be able to be brought
offline like in the case of x86-64 architecture. So a number of the
test cases in test_cpuset_prs.sh will have to be updated accordingly.

A new test will also be run in offline isn't allowed in CPU 0 to verify
that using CPU 0 as part of an isolated partition will fail.

The cgroup-v2.rst is also updated to reflect the new capability of using
CPU hotplug to enable run time change to the nohz_full and managed_irq
CPU lists.

Since there is a slight performance overhead to enable runtime changes
to nohz_full CPU list, users have to explicitly opt in by adding a
"nohz_ful" kernel command line parameter with or without a CPU list.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 Documentation/admin-guide/cgroup-v2.rst       | 35 +++++++---
 .../selftests/cgroup/test_cpuset_prs.sh       | 70 +++++++++++++++++--
 2 files changed, 92 insertions(+), 13 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 8ad0b2781317..e97fc031eb86 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2604,11 +2604,12 @@ Cpuset Interface Files
 
 	It accepts only the following input values when written to.
 
-	  ==========	=====================================
+	  ==========	===============================================
 	  "member"	Non-root member of a partition
 	  "root"	Partition root
-	  "isolated"	Partition root without load balancing
-	  ==========	=====================================
+	  "isolated"	Partition root without load balancing and other
+		        OS noises
+	  ==========	===============================================
 
 	A cpuset partition is a collection of cpuset-enabled cgroups with
 	a partition root at the top of the hierarchy and its descendants
@@ -2652,11 +2653,29 @@ Cpuset Interface Files
 	partition or scheduling domain.  The set of exclusive CPUs is
 	determined by the value of its "cpuset.cpus.exclusive.effective".
 
-	When set to "isolated", the CPUs in that partition will be in
-	an isolated state without any load balancing from the scheduler
-	and excluded from the unbound workqueues.  Tasks placed in such
-	a partition with multiple CPUs should be carefully distributed
-	and bound to each of the individual CPUs for optimal performance.
+	When set to "isolated", the CPUs in that partition will be in an
+	isolated state without any load balancing from the scheduler and
+	excluded from the unbound workqueues as well as other OS noises.
+	Tasks placed in such a partition with multiple CPUs should be
+	carefully distributed and bound to each of the individual CPUs
+	for optimal performance.
+
+	As CPU hotplug, if supported, is used to improve the degree of
+	CPU isolation close to the "nohz_full" kernel boot parameter.
+	In some architectures, like x86-64, the boot CPU (typically CPU
+	0) cannot be brought offline, so the boot CPU should not be used
+	for forming isolated partitions.  The "nohz_full" kernel boot
+	parameter needs to be present to enable full dynticks support
+	and RCU no-callback CPU mode for CPUs in isolated partitions
+	even if the optional cpu list isn't provided.
+
+	Using CPU hotplug for creating or destroying an isolated
+	partition can cause latency spike in applications running
+	in other isolated partitions.  A reserved list of CPUs can
+	optionally be put in the "nohz_full" kernel boot parameter to
+	alleviate this problem.  When these reserved CPUs are used for
+	isolated partitions, CPU hotplug won't need to be invoked and
+	so there won't be latency spike in other isolated partitions.
 
 	A partition root ("root" or "isolated") can be in one of the
 	two possible states - valid or invalid.  An invalid partition
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index a56f4153c64d..eebb4122b581 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -67,6 +67,12 @@ then
 	echo Y > /sys/kernel/debug/sched/verbose
 fi
 
+# Enable dynamic debug message if available
+DYN_DEBUG=/proc/dynamic_debug/control
+[[ -f $DYN_DEBUG ]] && {
+	echo "file kernel/cpu.c +p" > $DYN_DEBUG
+}
+
 cd $CGROUP2
 echo +cpuset > cgroup.subtree_control
 
@@ -84,6 +90,15 @@ echo member > test/cpuset.cpus.partition
 echo "" > test/cpuset.cpus
 [[ $RESULT -eq 0 ]] && skip_test "Child cgroups are using cpuset!"
 
+#
+# If nohz_full parameter is specified and nohz_full file exists, CPU hotplug
+# will be used to modify nohz_full cpumask to include all the isolated CPUs
+# in cpuset isolated partitions.
+#
+NOHZ_FULL=/sys/devices/system/cpu/nohz_full
+BOOT_NOHZ_FULL=$(fmt -1 /proc/cmdline | grep "^nohz_full")
+[[ "$BOOT_NOHZ_FULL" = nohz_full ]] && CHK_NOHZ_FULL=1
+
 #
 # If isolated CPUs have been reserved at boot time (as shown in
 # cpuset.cpus.isolated), these isolated CPUs should be outside of CPUs 0-8
@@ -318,8 +333,8 @@ TEST_MATRIX=(
 	# Invalid to valid local partition direct transition tests
 	" C1-3:P2  X4:P2    .      .      .      .      .      .     0 A1:1-3|XA1:1-3|A2:1-3:XA2: A1:P2|A2:P-2 1-3"
 	" C1-3:P2  X4:P2    .      .      .    X3:P2    .      .     0 A1:1-2|XA1:1-3|A2:3:XA2:3 A1:P2|A2:P2 1-3"
-	" C0-3:P2    .      .    C4-6   C0-4     .      .      .     0 A1:0-4|B1:5-6 A1:P2|B1:P0"
-	" C0-3:P2    .      .    C4-6 C0-4:C0-3  .      .      .     0 A1:0-3|B1:4-6 A1:P2|B1:P0 0-3"
+	" C1-3:P2    .      .    C4-6   C1-4     .      .      .     0 A1:1-4|B1:5-6 A1:P2|B1:P0"
+	" C1-3:P2    .      .    C4-6 C1-4:C1-3  .      .      .     0 A1:1-3|B1:4-6 A1:P2|B1:P0 1-3"
 
 	# Local partition invalidation tests
 	" C0-3:X1-3:P2 C1-3:X2-3:P2 C2-3:X3:P2 \
@@ -329,8 +344,8 @@ TEST_MATRIX=(
 	" C0-3:X1-3:P2 C1-3:X2-3:P2 C2-3:X3:P2 \
 				   .      .    C4:X     .      .     0 A1:1-3|A2:1-3|A3:2-3|XA2:|XA3: A1:P2|A2:P-2|A3:P-2 1-3"
 	# Local partition CPU change tests
-	" C0-5:P2  C4-5:P1  .      .      .    C3-5     .      .     0 A1:0-2|A2:3-5 A1:P2|A2:P1 0-2"
-	" C0-5:P2  C4-5:P1  .      .    C1-5     .      .      .     0 A1:1-3|A2:4-5 A1:P2|A2:P1 1-3"
+	" C1-5:P2  C4-5:P1  .      .      .    C3-5     .      .     0 A1:1-2|A2:3-5 A1:P2|A2:P1 1-2"
+	" C1-5:P2  C4-5:P1  .      .    C2-5     .      .      .     0 A1:2-3|A2:4-5 A1:P2|A2:P1 2-3"
 
 	# cpus_allowed/exclusive_cpus update tests
 	" C0-3:X2-3 C1-3:X2-3 C2-3:X2-3 \
@@ -442,6 +457,21 @@ TEST_MATRIX=(
 	"   C0-3     .      .    C4-5   X3-5     .      .      .     1 A1:0-3|B1:4-5"
 )
 
+#
+# Test matrix to verify that using CPU 0 in isolated (local or remote) partition
+# will fail when offline isn't allowed for CPU 0.
+#
+CPU0_ISOLCPUS_MATRIX=(
+	#  old-A1 old-A2 old-A3 old-B1 new-A1 new-A2 new-A3 new-B1 fail ECPUs Pstate ISOLCPUS
+	#  ------ ------ ------ ------ ------ ------ ------ ------ ---- ----- ------ --------
+	"   C0-3     .      .    C4-5     P2     .      .      .     0 A1:0-3|B1:4-5 A1:P-2"
+	"   C1-3     .      .      .      P2     .      .      .     0 A1:1-3 A1:P2"
+	"   C1-3     .      .      .    P2:C0-3  .      .      .     0 A1:0-3 A1:P-2"
+	"  CX0-3   C0-3     .      .       .     P2     .      .     0 A1:0-3|A2:0-3 A2:P-2"
+	"  CX0-3 C0-3:X1-3  .      .       .     P2     .      .     0 A1:0|A2:1-3 A2:P2"
+	"  CX0-3 C0-3:X1-3  .      .       .   P2:X0-3  .      .     0 A1:0-3|A2:0-3 A2:P-2"
+)
+
 #
 # Cpuset controller remote partition test matrix.
 #
@@ -513,7 +543,7 @@ write_cpu_online()
 		}
 	fi
 	echo $VAL > $CPUFILE
-	pause 0.05
+	pause 0.10
 }
 
 #
@@ -654,6 +684,8 @@ dump_states()
 		[[ -e $PCPUS  ]] && echo "$PCPUS: $(cat $PCPUS)"
 		[[ -e $ISCPUS ]] && echo "$ISCPUS: $(cat $ISCPUS)"
 	done
+	# Dump nohz_full
+	[[ -f $NOHZ_FULL ]] && echo "nohz_full: $(cat $NOHZ_FULL)"
 }
 
 #
@@ -789,6 +821,18 @@ check_isolcpus()
 		EXPECTED_SDOMAIN=$EXPECTED_ISOLCPUS
 	fi
 
+	#
+	# Check if nohz_full match cpuset.cpus.isolated if nohz_boot parameter
+	# specified with no parameter.
+	#
+	[[ -f $NOHZ_FULL && "$BOOT_NOHZ_FULL" = nohz_full ]] && {
+		NOHZ_FULL_CPUS=$(cat $NOHZ_FULL)
+		[[ "$ISOLCPUS" != "$NOHZ_FULL_CPUS" ]] && {
+			echo "nohz_full ($NOHZ_FULL_CPUS) does not match cpuset.cpus.isolated ($ISOLCPUS)"
+			return 1
+		}
+	}
+
 	#
 	# Appending pre-isolated CPUs
 	# Even though CPU #8 isn't used for testing, it can't be pre-isolated
@@ -1070,6 +1114,21 @@ run_remote_state_test()
 	echo "All $I tests of $TEST PASSED."
 }
 
+#
+# Testing CPU 0 isolated partition test when offline is disabled
+#
+run_cpu0_isol_test()
+{
+	# Skip the test if CPU0 offline is allowed or if nohz_full kernel
+	# boot parameter is missing.
+	CPU0_ONLINE=/sys/devices/system/cpu/cpu0/online
+	[[ -f $CPU0_ONLINE ]] && return
+	grep -q -w nohz_full /proc/cmdline
+	[[ $? -ne 0 ]] && return
+
+	run_state_test CPU0_ISOLCPUS_MATRIX
+}
+
 #
 # Testing the new "isolated" partition root type
 #
@@ -1207,6 +1266,7 @@ test_inotify()
 trap cleanup 0 2 3 6
 run_state_test TEST_MATRIX
 run_remote_state_test REMOTE_TEST_MATRIX
+run_cpu0_isol_test
 test_isolated
 test_inotify
 echo "All tests PASSED."
-- 
2.53.0