[PATCH v16 0/7] timers/migration: Exclude isolated cpus from hierarchy

[PATCH v16 0/7] timers/migration: Exclude isolated cpus from hierarchy

[Gabriele Monaco]

The timer migration mechanism allows active CPUs to pull timers from
idle ones to improve the overall idle time. This is however undesired
when CPU intensive workloads run on isolated cores, as the algorithm
would move the timers from housekeeping to isolated cores, negatively
affecting the isolation.

Exclude isolated cores from the timer migration algorithm, extend the
concept of unavailable cores, currently used for offline ones, to
isolated ones:
* A core is unavailable if isolated or offline;
* A core is available if non isolated and online;

A core is considered unavailable as isolated if it belongs to:
* the isolcpus (domain) list
* an isolated cpuset
Except if it is:
* in the nohz_full list (already idle for the hierarchy)
* the nohz timekeeper core (must be available to handle global timers)

CPUs are added to the hierarchy during late boot, excluding isolated
ones, the hierarchy is also adapted when the cpuset isolation changes.

Due to how the timer migration algorithm works, any CPU part of the
hierarchy can have their global timers pulled by remote CPUs and have to
pull remote timers, only skipping pulling remote timers would break the
logic.
For this reason, prevent isolated CPUs from pulling remote global
timers, but also the other way around: any global timer started on an
isolated CPU will run there. This does not break the concept of
isolation (global timers don't come from outside the CPU) and, if
considered inappropriate, can usually be mitigated with other isolation
techniques (e.g. IRQ pinning).

This effect was noticed on a 128 cores machine running oslat on the
isolated cores (1-31,33-63,65-95,97-127). The tool monopolises CPUs,
and the CPU with lowest count in a timer migration hierarchy (here 1
and 65) appears as always active and continuously pulls global timers,
from the housekeeping CPUs. This ends up moving driver work (e.g.
delayed work) to isolated CPUs and causes latency spikes:

before the change:

 # oslat -c 1-31,33-63,65-95,97-127 -D 62s
 ...
  Maximum:     1203 10 3 4 ... 5 (us)

after the change:

 # oslat -c 1-31,33-63,65-95,97-127 -D 62s
 ...
  Maximum:      10 4 3 4 3 ... 5 (us)

The same behaviour was observed on a machine with as few as 20 cores /
40 threads with isocpus set to: 1-9,11-39 with rtla-osnoise-top.

The first 4 patches are preparatory work to change the concept of
online/offline to available/unavailable, keep track of those in a
separate cpumask cleanup the setting/clearing functions and change a
function name in cpuset code.

Patch 5 adapt isolation to prevent domain isolated and nohz_full from
covering all CPUs not leaving any housekeeping one. This can lead to
problems with the changes introduced in this series because no CPU would
remain to handle global timers.
(The corresponding change for cpuset was removed from this version of
the series and is present in [2]).

Patch 7 extends the unavailable status to domain isolated CPUs, which
is the main contribution of the series.

Changes since v15:
* Improve comments consistency and general cleanup
* Bring back tmigr_available_cpumask changes in the set/clear functions
* Add global mutex to serialise set/clear available operations
* Rearrange operations in tmigr_isolated_exclude_cpumask() and use
  single cpumask
* Take hotplug lock in tmigr_init_isolation() and remove wrapper

Changes since v14:
* Rebase on tip/timers/core, rename one more ->online field
* Mark the static key as static
* Share code between tmigr_init_isolation and tmigr_isolated_exclude_cpumask

Changes since v13:
* Remove tmigr late initialisation and restore late isolation (as in v8 [1])
* Use workqueues in initialisation just like tmigr_available_cpumask()
* Use static key for tmigr_exclude_isolated
* Remove cpuset patch checking for HK conflict (included in [2])
* Rename cpuset helper to update_isolation_cpumasks as in [2]

Changes since v12:
* Pick and adapt patch by Yury Norov to initialise cpumasks
* Reorganise accesses to tmigr_available_cpumask to avoid races

Changes since v11:
* Rename isolcpus_nohz_conflict() to isolated_cpus_can_update()
* Move tick_nohz_cpu_hotpluggable() check to tmigr_is_isolated()
* Use workqueues in tmigr_isolated_exclude_cpumask() to avoid sleeping
  while atomic
* Add cpumask initialiser to safely use cpumask cleanup helpers

Changes since v10:
* Simplify housekeeping conflict condition
* Reword commit (Frederic Weisbecker)

Changes since v9:
* Fix total housekeeping enforcement to focus only on nohz and domain
* Avoid out of bound access in the housekeeping array if no flag is set
* Consider isolated_cpus while checking for nohz conflicts in cpuset
* Improve comment about why nohz CPUs are not excluded by tmigr

Changes since v8 [1]:
* Postpone hotplug registration to late initcall (Frederic Weisbecker)
* Move main activation logic in _tmigr_set_cpu_available() and call it
  after checking for isolation on hotplug and cpusets changes
* Call _tmigr_set_cpu_available directly to force enable tick CPU if
  required (this saves checking for that on every hotplug change).

Changes since v7:
* Move tmigr_available_cpumask out of tmc lock and specify conditions.
* Initialise tmigr isolation despite the state of isolcpus.
* Move tick CPU check to condition to run SMP call.
* Fix descriptions.

Changes since v6 [3]:
* Prevent isolation checks from running during early boot
* Prevent double (de)activation while setting cpus (un)available
* Use synchronous smp calls from the isolation path
* General cleanup

Changes since v5:
* Remove fallback if no housekeeping is left by isolcpus and nohz_full
* Adjust condition not to activate CPUs in the migration hierarchy
* Always force the nohz tick CPU active in the hierarchy

Changes since v4 [4]:
* use on_each_cpu_mask() with changes on isolated CPUs to avoid races
* keep nohz_full CPUs included in the timer migration hierarchy
* prevent domain isolated and nohz_full to cover all CPUs

Changes since v3:
* add parameter to function documentation
* split into multiple straightforward patches

Changes since v2:
* improve comments about handling CPUs isolated at boot
* minor cleanup

Changes since v1 [5]:
* split into smaller patches
* use available mask instead of unavailable
* simplification and cleanup

[1] - https://lore.kernel.org/lkml/20250714133050.193108-9-gmonaco@redhat.com
[2] - https://lore.kernel.org/lkml/20251104013037.296013-1-longman@redhat.com
[3] - https://lore.kernel.org/lkml/20250530142031.215594-1-gmonaco@redhat.com
[4] - https://lore.kernel.org/lkml/20250506091534.42117-7-gmonaco@redhat.com
[5] - https://lore.kernel.org/lkml/20250410065446.57304-2-gmonaco@redhat.com

Gabriele Monaco (6):
  timers/migration: Rename 'online' bit to 'available'
  timers/migration: Add mask for CPUs available in the hierarchy
  timers/migration: Use scoped_guard on available flag set/clear
  cgroup/cpuset: Rename update_unbound_workqueue_cpumask() to
    update_isolation_cpumasks()
  sched/isolation: Force housekeeping if isolcpus and nohz_full don't
    leave any
  timers/migration: Exclude isolated cpus from hierarchy

Yury Norov (1):
  cpumask: Add initialiser to use cleanup helpers

 include/linux/cpumask.h                |   2 +
 include/linux/timer.h                  |   9 ++
 include/trace/events/timer_migration.h |   4 +-
 kernel/cgroup/cpuset.c                 |  15 +-
 kernel/sched/isolation.c               |  23 +++
 kernel/time/timer_migration.c          | 207 +++++++++++++++++++++----
 kernel/time/timer_migration.h          |   2 +-
 7 files changed, 227 insertions(+), 35 deletions(-)


base-commit: 308bc2e33885df9288d3f1ed946a2b212e37db62
-- 
2.51.1

[PATCH v16 1/7] timers/migration: Rename 'online' bit to 'available'

[Gabriele Monaco]

The timer migration hierarchy excludes offline CPUs via the
tmigr_is_not_available function, which is essentially checking the
online bit for the CPU.

Rename the online bit to available and all references in function names
and tracepoint to generalise the concept of available CPUs.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 include/trace/events/timer_migration.h |  4 ++--
 kernel/time/timer_migration.c          | 24 ++++++++++++------------
 kernel/time/timer_migration.h          |  2 +-
 3 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/include/trace/events/timer_migration.h b/include/trace/events/timer_migration.h
index 47db5eaf2f9a..61171b13c687 100644
--- a/include/trace/events/timer_migration.h
+++ b/include/trace/events/timer_migration.h
@@ -173,14 +173,14 @@ DEFINE_EVENT(tmigr_cpugroup, tmigr_cpu_active,
 	TP_ARGS(tmc)
 );
 
-DEFINE_EVENT(tmigr_cpugroup, tmigr_cpu_online,
+DEFINE_EVENT(tmigr_cpugroup, tmigr_cpu_available,
 
 	TP_PROTO(struct tmigr_cpu *tmc),
 
 	TP_ARGS(tmc)
 );
 
-DEFINE_EVENT(tmigr_cpugroup, tmigr_cpu_offline,
+DEFINE_EVENT(tmigr_cpugroup, tmigr_cpu_unavailable,
 
 	TP_PROTO(struct tmigr_cpu *tmc),
 
diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 57e38674e56e..2cfebed35e22 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -429,7 +429,7 @@ static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
 
 static inline bool tmigr_is_not_available(struct tmigr_cpu *tmc)
 {
-	return !(tmc->tmgroup && tmc->online);
+	return !(tmc->tmgroup && tmc->available);
 }
 
 /*
@@ -926,7 +926,7 @@ static void tmigr_handle_remote_cpu(unsigned int cpu, u64 now,
 	 * updated the event takes care when hierarchy is completely
 	 * idle. Otherwise the migrator does it as the event is enqueued.
 	 */
-	if (!tmc->online || tmc->remote || tmc->cpuevt.ignore ||
+	if (!tmc->available || tmc->remote || tmc->cpuevt.ignore ||
 	    now < tmc->cpuevt.nextevt.expires) {
 		raw_spin_unlock_irq(&tmc->lock);
 		return;
@@ -973,7 +973,7 @@ static void tmigr_handle_remote_cpu(unsigned int cpu, u64 now,
 	 * (See also section "Required event and timerqueue update after a
 	 * remote expiry" in the documentation at the top)
 	 */
-	if (!tmc->online || !tmc->idle) {
+	if (!tmc->available || !tmc->idle) {
 		timer_unlock_remote_bases(cpu);
 		goto unlock;
 	}
@@ -1422,19 +1422,19 @@ static long tmigr_trigger_active(void *unused)
 {
 	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
 
-	WARN_ON_ONCE(!tmc->online || tmc->idle);
+	WARN_ON_ONCE(!tmc->available || tmc->idle);
 
 	return 0;
 }
 
-static int tmigr_cpu_offline(unsigned int cpu)
+static int tmigr_clear_cpu_available(unsigned int cpu)
 {
 	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
 	int migrator;
 	u64 firstexp;
 
 	raw_spin_lock_irq(&tmc->lock);
-	tmc->online = false;
+	tmc->available = false;
 	WRITE_ONCE(tmc->wakeup, KTIME_MAX);
 
 	/*
@@ -1442,7 +1442,7 @@ static int tmigr_cpu_offline(unsigned int cpu)
 	 * offline; Therefore nextevt value is set to KTIME_MAX
 	 */
 	firstexp = __tmigr_cpu_deactivate(tmc, KTIME_MAX);
-	trace_tmigr_cpu_offline(tmc);
+	trace_tmigr_cpu_unavailable(tmc);
 	raw_spin_unlock_irq(&tmc->lock);
 
 	if (firstexp != KTIME_MAX) {
@@ -1453,7 +1453,7 @@ static int tmigr_cpu_offline(unsigned int cpu)
 	return 0;
 }
 
-static int tmigr_cpu_online(unsigned int cpu)
+static int tmigr_set_cpu_available(unsigned int cpu)
 {
 	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
 
@@ -1462,11 +1462,11 @@ static int tmigr_cpu_online(unsigned int cpu)
 		return -EINVAL;
 
 	raw_spin_lock_irq(&tmc->lock);
-	trace_tmigr_cpu_online(tmc);
+	trace_tmigr_cpu_available(tmc);
 	tmc->idle = timer_base_is_idle();
 	if (!tmc->idle)
 		__tmigr_cpu_activate(tmc);
-	tmc->online = true;
+	tmc->available = true;
 	raw_spin_unlock_irq(&tmc->lock);
 	return 0;
 }
@@ -1758,7 +1758,7 @@ static int tmigr_add_cpu(unsigned int cpu)
 		 * The (likely) current CPU is expected to be online in the hierarchy,
 		 * otherwise the old root may not be active as expected.
 		 */
-		WARN_ON_ONCE(!per_cpu_ptr(&tmigr_cpu, raw_smp_processor_id())->online);
+		WARN_ON_ONCE(!per_cpu_ptr(&tmigr_cpu, raw_smp_processor_id())->available);
 		ret = tmigr_setup_groups(-1, old_root->numa_node, old_root, true);
 	}
 
@@ -1854,7 +1854,7 @@ static int __init tmigr_init(void)
 		goto err;
 
 	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
-				tmigr_cpu_online, tmigr_cpu_offline);
+				tmigr_set_cpu_available, tmigr_clear_cpu_available);
 	if (ret)
 		goto err;
 
diff --git a/kernel/time/timer_migration.h b/kernel/time/timer_migration.h
index ae19f70f8170..70879cde6fdd 100644
--- a/kernel/time/timer_migration.h
+++ b/kernel/time/timer_migration.h
@@ -97,7 +97,7 @@ struct tmigr_group {
  */
 struct tmigr_cpu {
 	raw_spinlock_t		lock;
-	bool			online;
+	bool			available;
 	bool			idle;
 	bool			remote;
 	struct tmigr_group	*tmgroup;
-- 
2.51.1

[PATCH v16 2/7] timers/migration: Add mask for CPUs available in the hierarchy

[Gabriele Monaco]

Keep track of the CPUs available for timer migration in a cpumask. This
prepares the ground to generalise the concept of unavailable CPUs.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 kernel/time/timer_migration.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 2cfebed35e22..3325ca7989b3 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -424,6 +424,12 @@ static struct tmigr_group *tmigr_root;
 
 static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
 
+/*
+ * CPUs available for timer migration.
+ * Protected by cpuset_mutex (with cpus_read_lock held) or cpus_write_lock.
+ */
+static cpumask_var_t tmigr_available_cpumask;
+
 #define TMIGR_NONE	0xFF
 #define BIT_CNT		8
 
@@ -1433,6 +1439,7 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	int migrator;
 	u64 firstexp;
 
+	cpumask_clear_cpu(cpu, tmigr_available_cpumask);
 	raw_spin_lock_irq(&tmc->lock);
 	tmc->available = false;
 	WRITE_ONCE(tmc->wakeup, KTIME_MAX);
@@ -1446,7 +1453,7 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	raw_spin_unlock_irq(&tmc->lock);
 
 	if (firstexp != KTIME_MAX) {
-		migrator = cpumask_any_but(cpu_online_mask, cpu);
+		migrator = cpumask_any(tmigr_available_cpumask);
 		work_on_cpu(migrator, tmigr_trigger_active, NULL);
 	}
 
@@ -1461,6 +1468,7 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	if (WARN_ON_ONCE(!tmc->tmgroup))
 		return -EINVAL;
 
+	cpumask_set_cpu(cpu, tmigr_available_cpumask);
 	raw_spin_lock_irq(&tmc->lock);
 	trace_tmigr_cpu_available(tmc);
 	tmc->idle = timer_base_is_idle();
@@ -1805,6 +1813,11 @@ static int __init tmigr_init(void)
 	if (ncpus == 1)
 		return 0;
 
+	if (!zalloc_cpumask_var(&tmigr_available_cpumask, GFP_KERNEL)) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
 	/*
 	 * Calculate the required hierarchy levels. Unfortunately there is no
 	 * reliable information available, unless all possible CPUs have been
-- 
2.51.1

[PATCH v16 3/7] timers/migration: Use scoped_guard on available flag set/clear

[Gabriele Monaco]

Cleanup tmigr_clear_cpu_available() and tmigr_set_cpu_available() to
prepare for easier checks on the available flag.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 kernel/time/timer_migration.c | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 3325ca7989b3..a01c7f8bdf52 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1440,17 +1440,17 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	u64 firstexp;
 
 	cpumask_clear_cpu(cpu, tmigr_available_cpumask);
-	raw_spin_lock_irq(&tmc->lock);
-	tmc->available = false;
-	WRITE_ONCE(tmc->wakeup, KTIME_MAX);
+	scoped_guard(raw_spinlock_irq, &tmc->lock) {
+		tmc->available = false;
+		WRITE_ONCE(tmc->wakeup, KTIME_MAX);
 
-	/*
-	 * CPU has to handle the local events on his own, when on the way to
-	 * offline; Therefore nextevt value is set to KTIME_MAX
-	 */
-	firstexp = __tmigr_cpu_deactivate(tmc, KTIME_MAX);
-	trace_tmigr_cpu_unavailable(tmc);
-	raw_spin_unlock_irq(&tmc->lock);
+		/*
+		 * CPU has to handle the local events on his own, when on the way to
+		 * offline; Therefore nextevt value is set to KTIME_MAX
+		 */
+		firstexp = __tmigr_cpu_deactivate(tmc, KTIME_MAX);
+		trace_tmigr_cpu_unavailable(tmc);
+	}
 
 	if (firstexp != KTIME_MAX) {
 		migrator = cpumask_any(tmigr_available_cpumask);
@@ -1469,13 +1469,13 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 		return -EINVAL;
 
 	cpumask_set_cpu(cpu, tmigr_available_cpumask);
-	raw_spin_lock_irq(&tmc->lock);
-	trace_tmigr_cpu_available(tmc);
-	tmc->idle = timer_base_is_idle();
-	if (!tmc->idle)
-		__tmigr_cpu_activate(tmc);
-	tmc->available = true;
-	raw_spin_unlock_irq(&tmc->lock);
+	scoped_guard(raw_spinlock_irq, &tmc->lock) {
+		trace_tmigr_cpu_available(tmc);
+		tmc->idle = timer_base_is_idle();
+		if (!tmc->idle)
+			__tmigr_cpu_activate(tmc);
+		tmc->available = true;
+	}
 	return 0;
 }
 
-- 
2.51.1

[PATCH v16 4/7] cgroup/cpuset: Rename update_unbound_workqueue_cpumask() to update_isolation_cpumasks()

[Gabriele Monaco]

update_unbound_workqueue_cpumask() updates unbound workqueues settings
when there's a change in isolated CPUs, but it can be used for other
subsystems requiring updated when isolated CPUs change.

Generalise the name to update_isolation_cpumasks() to prepare for other
functions unrelated to workqueues to be called in that spot.

[longman: Change the function name to update_isolation_cpumasks()]

Acked-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Chen Ridong <chenridong@huaweicloud.com>
---
 kernel/cgroup/cpuset.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 27adb04df675..cf34623fe66f 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1339,7 +1339,7 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
 	return isolcpus_updated;
 }
 
-static void update_unbound_workqueue_cpumask(bool isolcpus_updated)
+static void update_isolation_cpumasks(bool isolcpus_updated)
 {
 	int ret;
 
@@ -1470,7 +1470,7 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
 	list_add(&cs->remote_sibling, &remote_children);
 	cpumask_copy(cs->effective_xcpus, tmp->new_cpus);
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 	cpuset_force_rebuild();
 	cs->prs_err = 0;
 
@@ -1511,7 +1511,7 @@ static void remote_partition_disable(struct cpuset *cs, struct tmpmasks *tmp)
 	compute_effective_exclusive_cpumask(cs, NULL, NULL);
 	reset_partition_data(cs);
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 	cpuset_force_rebuild();
 
 	/*
@@ -1580,7 +1580,7 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
 	if (xcpus)
 		cpumask_copy(cs->exclusive_cpus, xcpus);
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 	if (adding || deleting)
 		cpuset_force_rebuild();
 
@@ -1943,7 +1943,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 		WARN_ON_ONCE(parent->nr_subparts < 0);
 	}
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 
 	if ((old_prs != new_prs) && (cmd == partcmd_update))
 		update_partition_exclusive_flag(cs, new_prs);
@@ -2968,7 +2968,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 	else if (isolcpus_updated)
 		isolated_cpus_update(old_prs, new_prs, cs->effective_xcpus);
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 
 	/* Force update if switching back to member & update effective_xcpus */
 	update_cpumasks_hier(cs, &tmpmask, !new_prs);
-- 
2.51.1

[PATCH v16 5/7] sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any

[Gabriele Monaco]

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

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

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

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index a4cf17b1fab0..3ad0d6df6a0a 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -167,6 +167,29 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 			}
 		}
 
+		/*
+		 * Check the combination of nohz_full and isolcpus=domain,
+		 * necessary to avoid problems with the timer migration
+		 * hierarchy. managed_irq is ignored by this check since it
+		 * isn't considered in the timer migration logic.
+		 */
+		iter_flags = housekeeping.flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
+		type = find_first_bit(&iter_flags, HK_TYPE_MAX);
+		/*
+		 * Pass the check if none of these flags were previously set or
+		 * are not in the current selection.
+		 */
+		iter_flags = flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
+		first_cpu = (type == HK_TYPE_MAX || !iter_flags) ? 0 :
+			    cpumask_first_and_and(cpu_present_mask,
+				    housekeeping_staging, housekeeping.cpumasks[type]);
+		if (first_cpu >= min(nr_cpu_ids, setup_max_cpus)) {
+			pr_warn("Housekeeping: must include one present CPU "
+				"neither in nohz_full= nor in isolcpus=domain, "
+				"ignoring setting %s\n", str);
+			goto free_housekeeping_staging;
+		}
+
 		iter_flags = flags & ~housekeeping.flags;
 
 		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
-- 
2.51.1

[PATCH v16 6/7] cpumask: Add initialiser to use cleanup helpers

[Gabriele Monaco]

From: Yury Norov <yury.norov@gmail.com>

Now we can simplify a code that allocates cpumasks for local needs.

Automatic variables have to be initialized at declaration, or at least
before any possibility for the logic to return, so that compiler
wouldn't try to call an associate destructor function on a random stack
number.

Because cpumask_var_t, depending on the CPUMASK_OFFSTACK config, is
either a pointer or an array, we have to have a macro for initialization.

So define a CPUMASK_VAR_NULL macro, which allows to init struct cpumask
pointer with NULL when CPUMASK_OFFSTACK is enabled, and effectively a
no-op when CPUMASK_OFFSTACK is disabled (initialisation optimised out
with -O2).

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
---
 include/linux/cpumask.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index ff8f41ab7ce6..68be522449ec 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -1005,6 +1005,7 @@ static __always_inline unsigned int cpumask_size(void)
 
 #define this_cpu_cpumask_var_ptr(x)	this_cpu_read(x)
 #define __cpumask_var_read_mostly	__read_mostly
+#define CPUMASK_VAR_NULL		NULL
 
 bool alloc_cpumask_var_node(cpumask_var_t *mask, gfp_t flags, int node);
 
@@ -1051,6 +1052,7 @@ static __always_inline bool cpumask_available(cpumask_var_t mask)
 
 #define this_cpu_cpumask_var_ptr(x) this_cpu_ptr(x)
 #define __cpumask_var_read_mostly
+#define CPUMASK_VAR_NULL {}
 
 static __always_inline bool alloc_cpumask_var(cpumask_var_t *mask, gfp_t flags)
 {
-- 
2.51.1

[PATCH v16 7/7] timers/migration: Exclude isolated cpus from hierarchy

[Gabriele Monaco]

The timer migration mechanism allows active CPUs to pull timers from
idle ones to improve the overall idle time. This is however undesired
when CPU intensive workloads run on isolated cores, as the algorithm
would move the timers from housekeeping to isolated cores, negatively
affecting the isolation.

Exclude isolated cores from the timer migration algorithm, extend the
concept of unavailable cores, currently used for offline ones, to
isolated ones:
* A core is unavailable if isolated or offline;
* A core is available if non isolated and online;

A core is considered unavailable as isolated if it belongs to:
* the isolcpus (domain) list
* an isolated cpuset
Except if it is:
* in the nohz_full list (already idle for the hierarchy)
* the nohz timekeeper core (must be available to handle global timers)

CPUs are added to the hierarchy during late boot, excluding isolated
ones, the hierarchy is also adapted when the cpuset isolation changes.

Due to how the timer migration algorithm works, any CPU part of the
hierarchy can have their global timers pulled by remote CPUs and have to
pull remote timers, only skipping pulling remote timers would break the
logic.
For this reason, prevent isolated CPUs from pulling remote global
timers, but also the other way around: any global timer started on an
isolated CPU will run there. This does not break the concept of
isolation (global timers don't come from outside the CPU) and, if
considered inappropriate, can usually be mitigated with other isolation
techniques (e.g. IRQ pinning).

This effect was noticed on a 128 cores machine running oslat on the
isolated cores (1-31,33-63,65-95,97-127). The tool monopolises CPUs,
and the CPU with lowest count in a timer migration hierarchy (here 1
and 65) appears as always active and continuously pulls global timers,
from the housekeeping CPUs. This ends up moving driver work (e.g.
delayed work) to isolated CPUs and causes latency spikes:

before the change:

 # oslat -c 1-31,33-63,65-95,97-127 -D 62s
 ...
  Maximum:     1203 10 3 4 ... 5 (us)

after the change:

 # oslat -c 1-31,33-63,65-95,97-127 -D 62s
 ...
  Maximum:      10 4 3 4 3 ... 5 (us)

The same behaviour was observed on a machine with as few as 20 cores /
40 threads with isocpus set to: 1-9,11-39 with rtla-osnoise-top.

Tested-by: John B. Wyatt IV <jwyatt@redhat.com>
Tested-by: John B. Wyatt IV <sageofredondo@gmail.com>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 include/linux/timer.h         |   9 +++
 kernel/cgroup/cpuset.c        |   3 +
 kernel/time/timer_migration.c | 142 ++++++++++++++++++++++++++++++++++
 3 files changed, 154 insertions(+)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 0414d9e6b4fc..62e1cea71125 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -188,4 +188,13 @@ int timers_dead_cpu(unsigned int cpu);
 #define timers_dead_cpu		NULL
 #endif
 
+#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
+extern int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask);
+#else
+static inline int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
+{
+	return 0;
+}
+#endif
+
 #endif
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index cf34623fe66f..bfc3b319e1c0 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1350,6 +1350,9 @@ static void update_isolation_cpumasks(bool isolcpus_updated)
 
 	ret = workqueue_unbound_exclude_cpumask(isolated_cpus);
 	WARN_ON_ONCE(ret < 0);
+
+	ret = tmigr_isolated_exclude_cpumask(isolated_cpus);
+	WARN_ON_ONCE(ret < 0);
 }
 
 /**
diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index a01c7f8bdf52..4c5f2a576e13 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -10,6 +10,7 @@
 #include <linux/spinlock.h>
 #include <linux/timerqueue.h>
 #include <trace/events/ipi.h>
+#include <linux/sched/isolation.h>
 
 #include "timer_migration.h"
 #include "tick-internal.h"
@@ -427,8 +428,13 @@ static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
 /*
  * CPUs available for timer migration.
  * Protected by cpuset_mutex (with cpus_read_lock held) or cpus_write_lock.
+ * Additionally tmigr_available_mutex serialises set/clear operations with each other.
  */
 static cpumask_var_t tmigr_available_cpumask;
+static DEFINE_MUTEX(tmigr_available_mutex);
+
+/* Enabled during late initcall */
+static DEFINE_STATIC_KEY_FALSE(tmigr_exclude_isolated);
 
 #define TMIGR_NONE	0xFF
 #define BIT_CNT		8
@@ -438,6 +444,33 @@ static inline bool tmigr_is_not_available(struct tmigr_cpu *tmc)
 	return !(tmc->tmgroup && tmc->available);
 }
 
+/*
+ * Returns true if @cpu should be excluded from the hierarchy as isolated.
+ * Domain isolated CPUs don't participate in timer migration, nohz_full CPUs
+ * are still part of the hierarchy but become idle (from a tick and timer
+ * migration perspective) when they stop their tick. This lets the timekeeping
+ * CPU handle their global timers. Marking also isolated CPUs as idle would be
+ * too costly, hence they are completely excluded from the hierarchy.
+ * This check is necessary, for instance, to prevent offline isolated CPUs from
+ * being incorrectly marked as available once getting back online.
+ *
+ * This function returns false during early boot and the isolation logic is
+ * enabled only after isolated CPUs are marked as unavailable at late boot.
+ * The tick CPU can be isolated at boot, however we cannot mark it as
+ * unavailable to avoid having no global migrator for the nohz_full CPUs. This
+ * should be ensured by the callers of this function: implicitly from hotplug
+ * callbacs and explicitly in tmigr_init_isolation() and
+ * tmigr_isolated_exclude_cpumask().
+ */
+static inline bool tmigr_is_isolated(int cpu)
+{
+	if (!static_branch_unlikely(&tmigr_exclude_isolated))
+		return false;
+	return (!housekeeping_cpu(cpu, HK_TYPE_DOMAIN) ||
+		cpuset_cpu_is_isolated(cpu)) &&
+	       housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE);
+}
+
 /*
  * Returns true, when @childmask corresponds to the group migrator or when the
  * group is not active - so no migrator is set.
@@ -1439,8 +1472,12 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	int migrator;
 	u64 firstexp;
 
+	guard(mutex)(&tmigr_available_mutex);
+
 	cpumask_clear_cpu(cpu, tmigr_available_cpumask);
 	scoped_guard(raw_spinlock_irq, &tmc->lock) {
+		if (!tmc->available)
+			return 0;
 		tmc->available = false;
 		WRITE_ONCE(tmc->wakeup, KTIME_MAX);
 
@@ -1468,8 +1505,15 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	if (WARN_ON_ONCE(!tmc->tmgroup))
 		return -EINVAL;
 
+	if (tmigr_is_isolated(cpu))
+		return 0;
+
+	guard(mutex)(&tmigr_available_mutex);
+
 	cpumask_set_cpu(cpu, tmigr_available_cpumask);
 	scoped_guard(raw_spinlock_irq, &tmc->lock) {
+		if (tmc->available)
+			return 0;
 		trace_tmigr_cpu_available(tmc);
 		tmc->idle = timer_base_is_idle();
 		if (!tmc->idle)
@@ -1479,6 +1523,103 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	return 0;
 }
 
+static void tmigr_cpu_isolate(struct work_struct *ignored)
+{
+	tmigr_clear_cpu_available(smp_processor_id());
+}
+
+static void tmigr_cpu_unisolate(struct work_struct *ignored)
+{
+	tmigr_set_cpu_available(smp_processor_id());
+}
+
+/**
+ * tmigr_isolated_exclude_cpumask - Exclude given CPUs from hierarchy
+ * @exclude_cpumask: the cpumask to be excluded from timer migration hierarchy
+ *
+ * This function can be called from cpuset code to provide the new set of
+ * isolated CPUs that should be excluded from the hierarchy.
+ * Online CPUs not present in exclude_cpumask but already excluded are brought
+ * back to the hierarchy.
+ * Functions to isolate/unisolate need to be called locally and can sleep.
+ */
+int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
+{
+	struct work_struct __percpu *works __free(free_percpu) =
+		alloc_percpu(struct work_struct);
+	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
+	int cpu;
+
+	lockdep_assert_cpus_held();
+
+	if (!works)
+		return -ENOMEM;
+	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
+		return -ENOMEM;
+
+	/*
+	 * First set previously isolated CPUs as available (unisolate).
+	 * This cpumask contains only CPUs that switched to available now.
+	 */
+	cpumask_andnot(cpumask, cpu_online_mask, exclude_cpumask);
+	cpumask_andnot(cpumask, cpumask, tmigr_available_cpumask);
+
+	for_each_cpu(cpu, cpumask) {
+		struct work_struct *work = per_cpu_ptr(works, cpu);
+
+		INIT_WORK(work, tmigr_cpu_unisolate);
+		schedule_work_on(cpu, work);
+	}
+	for_each_cpu(cpu, cpumask)
+		flush_work(per_cpu_ptr(works, cpu));
+
+	/*
+	 * Then clear previously available CPUs (isolate).
+	 * This cpumask contains only CPUs that switched to not available now.
+	 * There cannot be overlap with the newly available ones.
+	 */
+	cpumask_and(cpumask, exclude_cpumask, tmigr_available_cpumask);
+	cpumask_and(cpumask, cpumask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
+	/*
+	 * Handle this here and not in the cpuset code because exclude_cpumask
+	 * might include also the tick CPU if included in isolcpus.
+	 */
+	for_each_cpu(cpu, cpumask) {
+		if (!tick_nohz_cpu_hotpluggable(cpu)) {
+			cpumask_clear_cpu(cpu, cpumask);
+			break;
+		}
+	}
+
+	for_each_cpu(cpu, cpumask) {
+		struct work_struct *work = per_cpu_ptr(works, cpu);
+
+		INIT_WORK(work, tmigr_cpu_isolate);
+		schedule_work_on(cpu, work);
+	}
+	for_each_cpu(cpu, cpumask)
+		flush_work(per_cpu_ptr(works, cpu));
+
+	return 0;
+}
+
+static int __init tmigr_init_isolation(void)
+{
+	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
+
+	static_branch_enable(&tmigr_exclude_isolated);
+
+	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
+		return 0;
+	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
+		return -ENOMEM;
+
+	cpumask_andnot(cpumask, cpu_possible_mask, housekeeping_cpumask(HK_TYPE_DOMAIN));
+
+	guard(cpus_read_lock)();
+	return tmigr_isolated_exclude_cpumask(cpumask);
+}
+
 static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,
 			     int node)
 {
@@ -1878,3 +2019,4 @@ static int __init tmigr_init(void)
 	return ret;
 }
 early_initcall(tmigr_init);
+late_initcall(tmigr_init_isolation);
-- 
2.51.1

Re: [PATCH v16 7/7] timers/migration: Exclude isolated cpus from hierarchy

[Thomas Gleixner]

On Thu, Nov 20 2025 at 15:56, Gabriele Monaco wrote:
> + * This function returns false during early boot and the isolation logic is
> + * enabled only after isolated CPUs are marked as unavailable at late boot.
> + * The tick CPU can be isolated at boot, however we cannot mark it as
> + * unavailable to avoid having no global migrator for the nohz_full CPUs. This
> + * should be ensured by the callers of this function: implicitly from hotplug
> + * callbacs and explicitly in tmigr_init_isolation() and

'callbacs' is still not correct :)

Other than that:

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Re: [PATCH v16 7/7] timers/migration: Exclude isolated cpus from hierarchy

[Gabriele Monaco]

On Thu, 2025-11-20 at 16:21 +0100, Thomas Gleixner wrote:
> On Thu, Nov 20 2025 at 15:56, Gabriele Monaco wrote:
> > + * This function returns false during early boot and the isolation logic is
> > + * enabled only after isolated CPUs are marked as unavailable at late boot.
> > + * The tick CPU can be isolated at boot, however we cannot mark it as
> > + * unavailable to avoid having no global migrator for the nohz_full CPUs.
> > This
> > + * should be ensured by the callers of this function: implicitly from
> > hotplug
> > + * callbacs and explicitly in tmigr_init_isolation() and
> 
> 'callbacs' is still not correct :)
> 
> Other than that:
> 
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Damn, I knew I was missing something..
Should I send a v17 for this or just re-send this patch?

Thanks,
Gabriele

Re: [PATCH v16 7/7] timers/migration: Exclude isolated cpus from hierarchy

[Thomas Gleixner]

On Thu, Nov 20 2025 at 16:23, Gabriele Monaco wrote:
> On Thu, 2025-11-20 at 16:21 +0100, Thomas Gleixner wrote:
>> On Thu, Nov 20 2025 at 15:56, Gabriele Monaco wrote:
>> > + * This function returns false during early boot and the isolation logic is
>> > + * enabled only after isolated CPUs are marked as unavailable at late boot.
>> > + * The tick CPU can be isolated at boot, however we cannot mark it as
>> > + * unavailable to avoid having no global migrator for the nohz_full CPUs.
>> > This
>> > + * should be ensured by the callers of this function: implicitly from
>> > hotplug
>> > + * callbacs and explicitly in tmigr_init_isolation() and
>> 
>> 'callbacs' is still not correct :)
>> 
>> Other than that:
>> 
>> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
>
> Damn, I knew I was missing something..
> Should I send a v17 for this or just re-send this patch?

That'll be fixed up when applying. I wait for Frederic to review it
first.

Re: [PATCH v16 7/7] timers/migration: Exclude isolated cpus from hierarchy

[Frederic Weisbecker]

Le Thu, Nov 20, 2025 at 03:56:53PM +0100, Gabriele Monaco a écrit :
> The timer migration mechanism allows active CPUs to pull timers from
> idle ones to improve the overall idle time. This is however undesired
> when CPU intensive workloads run on isolated cores, as the algorithm
> would move the timers from housekeeping to isolated cores, negatively
> affecting the isolation.
> 
> Exclude isolated cores from the timer migration algorithm, extend the
> concept of unavailable cores, currently used for offline ones, to
> isolated ones:
> * A core is unavailable if isolated or offline;
> * A core is available if non isolated and online;
> 
> A core is considered unavailable as isolated if it belongs to:
> * the isolcpus (domain) list
> * an isolated cpuset
> Except if it is:
> * in the nohz_full list (already idle for the hierarchy)
> * the nohz timekeeper core (must be available to handle global timers)
> 
> CPUs are added to the hierarchy during late boot, excluding isolated
> ones, the hierarchy is also adapted when the cpuset isolation changes.
> 
> Due to how the timer migration algorithm works, any CPU part of the
> hierarchy can have their global timers pulled by remote CPUs and have to
> pull remote timers, only skipping pulling remote timers would break the
> logic.
> For this reason, prevent isolated CPUs from pulling remote global
> timers, but also the other way around: any global timer started on an
> isolated CPU will run there. This does not break the concept of
> isolation (global timers don't come from outside the CPU) and, if
> considered inappropriate, can usually be mitigated with other isolation
> techniques (e.g. IRQ pinning).
> 
> This effect was noticed on a 128 cores machine running oslat on the
> isolated cores (1-31,33-63,65-95,97-127). The tool monopolises CPUs,
> and the CPU with lowest count in a timer migration hierarchy (here 1
> and 65) appears as always active and continuously pulls global timers,
> from the housekeeping CPUs. This ends up moving driver work (e.g.
> delayed work) to isolated CPUs and causes latency spikes:
> 
> before the change:
> 
>  # oslat -c 1-31,33-63,65-95,97-127 -D 62s
>  ...
>   Maximum:     1203 10 3 4 ... 5 (us)
> 
> after the change:
> 
>  # oslat -c 1-31,33-63,65-95,97-127 -D 62s
>  ...
>   Maximum:      10 4 3 4 3 ... 5 (us)
> 
> The same behaviour was observed on a machine with as few as 20 cores /
> 40 threads with isocpus set to: 1-9,11-39 with rtla-osnoise-top.
> 
> Tested-by: John B. Wyatt IV <jwyatt@redhat.com>
> Tested-by: John B. Wyatt IV <sageofredondo@gmail.com>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

Thanks a lot! Looks pretty good now!

-- 
Frederic Weisbecker
SUSE Labs

Re: [PATCH v16 7/7] timers/migration: Exclude isolated cpus from hierarchy

[Thomas Gleixner]

On Thu, Nov 20 2025 at 15:56, Gabriele Monaco wrote:
>
> Tested-by: John B. Wyatt IV <jwyatt@redhat.com>
> Tested-by: John B. Wyatt IV <sageofredondo@gmail.com>

I took the liberty to remove one of John's alter egos here :)

[tip: timers/core] timers/migration: Exclude isolated cpus from hierarchy

[tip-bot2 for Gabriele Monaco]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     7dec062cfcf27808dbb70a0b231d1a698792743d
Gitweb:        https://git.kernel.org/tip/7dec062cfcf27808dbb70a0b231d1a698792743d
Author:        Gabriele Monaco <gmonaco@redhat.com>
AuthorDate:    Thu, 20 Nov 2025 15:56:53 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 20 Nov 2025 20:17:32 +01:00

timers/migration: Exclude isolated cpus from hierarchy

The timer migration mechanism allows active CPUs to pull timers from
idle ones to improve the overall idle time. This is however undesired
when CPU intensive workloads run on isolated cores, as the algorithm
would move the timers from housekeeping to isolated cores, negatively
affecting the isolation.

Exclude isolated cores from the timer migration algorithm, extend the
concept of unavailable cores, currently used for offline ones, to
isolated ones:
* A core is unavailable if isolated or offline;
* A core is available if non isolated and online;

A core is considered unavailable as isolated if it belongs to:
* the isolcpus (domain) list
* an isolated cpuset
Except if it is:
* in the nohz_full list (already idle for the hierarchy)
* the nohz timekeeper core (must be available to handle global timers)

CPUs are added to the hierarchy during late boot, excluding isolated
ones, the hierarchy is also adapted when the cpuset isolation changes.

Due to how the timer migration algorithm works, any CPU part of the
hierarchy can have their global timers pulled by remote CPUs and have to
pull remote timers, only skipping pulling remote timers would break the
logic.
For this reason, prevent isolated CPUs from pulling remote global
timers, but also the other way around: any global timer started on an
isolated CPU will run there. This does not break the concept of
isolation (global timers don't come from outside the CPU) and, if
considered inappropriate, can usually be mitigated with other isolation
techniques (e.g. IRQ pinning).

This effect was noticed on a 128 cores machine running oslat on the
isolated cores (1-31,33-63,65-95,97-127). The tool monopolises CPUs,
and the CPU with lowest count in a timer migration hierarchy (here 1
and 65) appears as always active and continuously pulls global timers,
from the housekeeping CPUs. This ends up moving driver work (e.g.
delayed work) to isolated CPUs and causes latency spikes:

before the change:

 # oslat -c 1-31,33-63,65-95,97-127 -D 62s
 ...
  Maximum:     1203 10 3 4 ... 5 (us)

after the change:

 # oslat -c 1-31,33-63,65-95,97-127 -D 62s
 ...
  Maximum:      10 4 3 4 3 ... 5 (us)

The same behaviour was observed on a machine with as few as 20 cores /
40 threads with isocpus set to: 1-9,11-39 with rtla-osnoise-top.

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: John B. Wyatt IV <jwyatt@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://patch.msgid.link/20251120145653.296659-8-gmonaco@redhat.com
---
 include/linux/timer.h         |   9 ++-
 kernel/cgroup/cpuset.c        |   3 +-
 kernel/time/timer_migration.c | 143 +++++++++++++++++++++++++++++++++-
 3 files changed, 155 insertions(+)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 0414d9e..62e1cea 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -188,4 +188,13 @@ int timers_dead_cpu(unsigned int cpu);
 #define timers_dead_cpu		NULL
 #endif
 
+#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
+extern int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask);
+#else
+static inline int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
+{
+	return 0;
+}
+#endif
+
 #endif
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index cf34623..bfc3b31 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1350,6 +1350,9 @@ static void update_isolation_cpumasks(bool isolcpus_updated)
 
 	ret = workqueue_unbound_exclude_cpumask(isolated_cpus);
 	WARN_ON_ONCE(ret < 0);
+
+	ret = tmigr_isolated_exclude_cpumask(isolated_cpus);
+	WARN_ON_ONCE(ret < 0);
 }
 
 /**
diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index a01c7f8..18dda1a 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -10,6 +10,7 @@
 #include <linux/spinlock.h>
 #include <linux/timerqueue.h>
 #include <trace/events/ipi.h>
+#include <linux/sched/isolation.h>
 
 #include "timer_migration.h"
 #include "tick-internal.h"
@@ -427,8 +428,13 @@ static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
 /*
  * CPUs available for timer migration.
  * Protected by cpuset_mutex (with cpus_read_lock held) or cpus_write_lock.
+ * Additionally tmigr_available_mutex serializes set/clear operations with each other.
  */
 static cpumask_var_t tmigr_available_cpumask;
+static DEFINE_MUTEX(tmigr_available_mutex);
+
+/* Enabled during late initcall */
+static DEFINE_STATIC_KEY_FALSE(tmigr_exclude_isolated);
 
 #define TMIGR_NONE	0xFF
 #define BIT_CNT		8
@@ -439,6 +445,33 @@ static inline bool tmigr_is_not_available(struct tmigr_cpu *tmc)
 }
 
 /*
+ * Returns true if @cpu should be excluded from the hierarchy as isolated.
+ * Domain isolated CPUs don't participate in timer migration, nohz_full CPUs
+ * are still part of the hierarchy but become idle (from a tick and timer
+ * migration perspective) when they stop their tick. This lets the timekeeping
+ * CPU handle their global timers. Marking also isolated CPUs as idle would be
+ * too costly, hence they are completely excluded from the hierarchy.
+ * This check is necessary, for instance, to prevent offline isolated CPUs from
+ * being incorrectly marked as available once getting back online.
+ *
+ * This function returns false during early boot and the isolation logic is
+ * enabled only after isolated CPUs are marked as unavailable at late boot.
+ * The tick CPU can be isolated at boot, however we cannot mark it as
+ * unavailable to avoid having no global migrator for the nohz_full CPUs. This
+ * should be ensured by the callers of this function: implicitly from hotplug
+ * callbacks and explicitly in tmigr_init_isolation() and
+ * tmigr_isolated_exclude_cpumask().
+ */
+static inline bool tmigr_is_isolated(int cpu)
+{
+	if (!static_branch_unlikely(&tmigr_exclude_isolated))
+		return false;
+	return (!housekeeping_cpu(cpu, HK_TYPE_DOMAIN) ||
+		cpuset_cpu_is_isolated(cpu)) &&
+	       housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE);
+}
+
+/*
  * Returns true, when @childmask corresponds to the group migrator or when the
  * group is not active - so no migrator is set.
  */
@@ -1439,8 +1472,12 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	int migrator;
 	u64 firstexp;
 
+	guard(mutex)(&tmigr_available_mutex);
+
 	cpumask_clear_cpu(cpu, tmigr_available_cpumask);
 	scoped_guard(raw_spinlock_irq, &tmc->lock) {
+		if (!tmc->available)
+			return 0;
 		tmc->available = false;
 		WRITE_ONCE(tmc->wakeup, KTIME_MAX);
 
@@ -1468,8 +1505,15 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	if (WARN_ON_ONCE(!tmc->tmgroup))
 		return -EINVAL;
 
+	if (tmigr_is_isolated(cpu))
+		return 0;
+
+	guard(mutex)(&tmigr_available_mutex);
+
 	cpumask_set_cpu(cpu, tmigr_available_cpumask);
 	scoped_guard(raw_spinlock_irq, &tmc->lock) {
+		if (tmc->available)
+			return 0;
 		trace_tmigr_cpu_available(tmc);
 		tmc->idle = timer_base_is_idle();
 		if (!tmc->idle)
@@ -1479,6 +1523,105 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	return 0;
 }
 
+static void tmigr_cpu_isolate(struct work_struct *ignored)
+{
+	tmigr_clear_cpu_available(smp_processor_id());
+}
+
+static void tmigr_cpu_unisolate(struct work_struct *ignored)
+{
+	tmigr_set_cpu_available(smp_processor_id());
+}
+
+/**
+ * tmigr_isolated_exclude_cpumask - Exclude given CPUs from hierarchy
+ * @exclude_cpumask: the cpumask to be excluded from timer migration hierarchy
+ *
+ * This function can be called from cpuset code to provide the new set of
+ * isolated CPUs that should be excluded from the hierarchy.
+ * Online CPUs not present in exclude_cpumask but already excluded are brought
+ * back to the hierarchy.
+ * Functions to isolate/unisolate need to be called locally and can sleep.
+ */
+int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
+{
+	struct work_struct __percpu *works __free(free_percpu) =
+		alloc_percpu(struct work_struct);
+	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
+	int cpu;
+
+	lockdep_assert_cpus_held();
+
+	if (!works)
+		return -ENOMEM;
+	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
+		return -ENOMEM;
+
+	/*
+	 * First set previously isolated CPUs as available (unisolate).
+	 * This cpumask contains only CPUs that switched to available now.
+	 */
+	cpumask_andnot(cpumask, cpu_online_mask, exclude_cpumask);
+	cpumask_andnot(cpumask, cpumask, tmigr_available_cpumask);
+
+	for_each_cpu(cpu, cpumask) {
+		struct work_struct *work = per_cpu_ptr(works, cpu);
+
+		INIT_WORK(work, tmigr_cpu_unisolate);
+		schedule_work_on(cpu, work);
+	}
+	for_each_cpu(cpu, cpumask)
+		flush_work(per_cpu_ptr(works, cpu));
+
+	/*
+	 * Then clear previously available CPUs (isolate).
+	 * This cpumask contains only CPUs that switched to not available now.
+	 * There cannot be overlap with the newly available ones.
+	 */
+	cpumask_and(cpumask, exclude_cpumask, tmigr_available_cpumask);
+	cpumask_and(cpumask, cpumask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
+	/*
+	 * Handle this here and not in the cpuset code because exclude_cpumask
+	 * might include also the tick CPU if included in isolcpus.
+	 */
+	for_each_cpu(cpu, cpumask) {
+		if (!tick_nohz_cpu_hotpluggable(cpu)) {
+			cpumask_clear_cpu(cpu, cpumask);
+			break;
+		}
+	}
+
+	for_each_cpu(cpu, cpumask) {
+		struct work_struct *work = per_cpu_ptr(works, cpu);
+
+		INIT_WORK(work, tmigr_cpu_isolate);
+		schedule_work_on(cpu, work);
+	}
+	for_each_cpu(cpu, cpumask)
+		flush_work(per_cpu_ptr(works, cpu));
+
+	return 0;
+}
+
+static int __init tmigr_init_isolation(void)
+{
+	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
+
+	static_branch_enable(&tmigr_exclude_isolated);
+
+	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
+		return 0;
+	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
+		return -ENOMEM;
+
+	cpumask_andnot(cpumask, cpu_possible_mask, housekeeping_cpumask(HK_TYPE_DOMAIN));
+
+	/* Protect against RCU torture hotplug testing */
+	guard(cpus_read_lock)();
+	return tmigr_isolated_exclude_cpumask(cpumask);
+}
+late_initcall(tmigr_init_isolation);
+
 static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,
 			     int node)
 {

[tip: timers/core] cpumask: Add initialiser to use cleanup helpers

[tip-bot2 for Yury Norov]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     b56651007fc018effe695a68d48caa6970b23094
Gitweb:        https://git.kernel.org/tip/b56651007fc018effe695a68d48caa6970b23094
Author:        Yury Norov <yury.norov@gmail.com>
AuthorDate:    Thu, 20 Nov 2025 15:56:52 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 20 Nov 2025 20:17:32 +01:00

cpumask: Add initialiser to use cleanup helpers

Now we can simplify a code that allocates cpumasks for local needs.

Automatic variables have to be initialized at declaration, or at least
before any possibility for the logic to return, so that compiler
wouldn't try to call an associate destructor function on a random stack
number.

Because cpumask_var_t, depending on the CPUMASK_OFFSTACK config, is
either a pointer or an array, we have to have a macro for initialization.

So define a CPUMASK_VAR_NULL macro, which allows to init struct cpumask
pointer with NULL when CPUMASK_OFFSTACK is enabled, and effectively a
no-op when CPUMASK_OFFSTACK is disabled (initialisation optimised out
with -O2).

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://patch.msgid.link/20251120145653.296659-7-gmonaco@redhat.com
---
 include/linux/cpumask.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index ff8f41a..68be522 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -1005,6 +1005,7 @@ static __always_inline unsigned int cpumask_size(void)
 
 #define this_cpu_cpumask_var_ptr(x)	this_cpu_read(x)
 #define __cpumask_var_read_mostly	__read_mostly
+#define CPUMASK_VAR_NULL		NULL
 
 bool alloc_cpumask_var_node(cpumask_var_t *mask, gfp_t flags, int node);
 
@@ -1051,6 +1052,7 @@ static __always_inline bool cpumask_available(cpumask_var_t mask)
 
 #define this_cpu_cpumask_var_ptr(x) this_cpu_ptr(x)
 #define __cpumask_var_read_mostly
+#define CPUMASK_VAR_NULL {}
 
 static __always_inline bool alloc_cpumask_var(cpumask_var_t *mask, gfp_t flags)
 {

[tip: timers/core] sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any

[tip-bot2 for Gabriele Monaco]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     185bccc79797d71477e672a1b2a2b7d0325044e7
Gitweb:        https://git.kernel.org/tip/185bccc79797d71477e672a1b2a2b7d0325044e7
Author:        Gabriele Monaco <gmonaco@redhat.com>
AuthorDate:    Thu, 20 Nov 2025 15:56:51 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 20 Nov 2025 20:17:31 +01:00

sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any

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

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

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Waiman Long <longman@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://patch.msgid.link/20251120145653.296659-6-gmonaco@redhat.com
---
 kernel/sched/isolation.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index a4cf17b..3ad0d6d 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -167,6 +167,29 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 			}
 		}
 
+		/*
+		 * Check the combination of nohz_full and isolcpus=domain,
+		 * necessary to avoid problems with the timer migration
+		 * hierarchy. managed_irq is ignored by this check since it
+		 * isn't considered in the timer migration logic.
+		 */
+		iter_flags = housekeeping.flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
+		type = find_first_bit(&iter_flags, HK_TYPE_MAX);
+		/*
+		 * Pass the check if none of these flags were previously set or
+		 * are not in the current selection.
+		 */
+		iter_flags = flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
+		first_cpu = (type == HK_TYPE_MAX || !iter_flags) ? 0 :
+			    cpumask_first_and_and(cpu_present_mask,
+				    housekeeping_staging, housekeeping.cpumasks[type]);
+		if (first_cpu >= min(nr_cpu_ids, setup_max_cpus)) {
+			pr_warn("Housekeeping: must include one present CPU "
+				"neither in nohz_full= nor in isolcpus=domain, "
+				"ignoring setting %s\n", str);
+			goto free_housekeeping_staging;
+		}
+
 		iter_flags = flags & ~housekeeping.flags;
 
 		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)

[tip: timers/core] cgroup/cpuset: Rename update_unbound_workqueue_cpumask() to update_isolation_cpumasks()

[tip-bot2 for Gabriele Monaco]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     22f8e41680efec63ead03d4693676587814f7a24
Gitweb:        https://git.kernel.org/tip/22f8e41680efec63ead03d4693676587814f7a24
Author:        Gabriele Monaco <gmonaco@redhat.com>
AuthorDate:    Thu, 20 Nov 2025 15:56:50 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 20 Nov 2025 20:17:31 +01:00

cgroup/cpuset: Rename update_unbound_workqueue_cpumask() to update_isolation_cpumasks()

update_unbound_workqueue_cpumask() updates unbound workqueues settings
when there's a change in isolated CPUs, but it can be used for other
subsystems requiring updated when isolated CPUs change.

Generalise the name to update_isolation_cpumasks() to prepare for other
functions unrelated to workqueues to be called in that spot.

[longman: Change the function name to update_isolation_cpumasks()]

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Chen Ridong <chenridong@huaweicloud.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
Link: https://patch.msgid.link/20251120145653.296659-5-gmonaco@redhat.com
---
 kernel/cgroup/cpuset.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 27adb04..cf34623 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1339,7 +1339,7 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
 	return isolcpus_updated;
 }
 
-static void update_unbound_workqueue_cpumask(bool isolcpus_updated)
+static void update_isolation_cpumasks(bool isolcpus_updated)
 {
 	int ret;
 
@@ -1470,7 +1470,7 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
 	list_add(&cs->remote_sibling, &remote_children);
 	cpumask_copy(cs->effective_xcpus, tmp->new_cpus);
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 	cpuset_force_rebuild();
 	cs->prs_err = 0;
 
@@ -1511,7 +1511,7 @@ static void remote_partition_disable(struct cpuset *cs, struct tmpmasks *tmp)
 	compute_effective_exclusive_cpumask(cs, NULL, NULL);
 	reset_partition_data(cs);
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 	cpuset_force_rebuild();
 
 	/*
@@ -1580,7 +1580,7 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
 	if (xcpus)
 		cpumask_copy(cs->exclusive_cpus, xcpus);
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 	if (adding || deleting)
 		cpuset_force_rebuild();
 
@@ -1943,7 +1943,7 @@ write_error:
 		WARN_ON_ONCE(parent->nr_subparts < 0);
 	}
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 
 	if ((old_prs != new_prs) && (cmd == partcmd_update))
 		update_partition_exclusive_flag(cs, new_prs);
@@ -2968,7 +2968,7 @@ out:
 	else if (isolcpus_updated)
 		isolated_cpus_update(old_prs, new_prs, cs->effective_xcpus);
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 
 	/* Force update if switching back to member & update effective_xcpus */
 	update_cpumasks_hier(cs, &tmpmask, !new_prs);

[tip: timers/core] timers/migration: Use scoped_guard on available flag set/clear

[tip-bot2 for Gabriele Monaco]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     4c2374ed86847c71dab5602c7882d21a0d56a4c7
Gitweb:        https://git.kernel.org/tip/4c2374ed86847c71dab5602c7882d21a0d56a4c7
Author:        Gabriele Monaco <gmonaco@redhat.com>
AuthorDate:    Thu, 20 Nov 2025 15:56:49 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 20 Nov 2025 20:17:31 +01:00

timers/migration: Use scoped_guard on available flag set/clear

Cleanup tmigr_clear_cpu_available() and tmigr_set_cpu_available() to
prepare for easier checks on the available flag.

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251120145653.296659-4-gmonaco@redhat.com
---
 kernel/time/timer_migration.c | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 3325ca7..a01c7f8 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1440,17 +1440,17 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	u64 firstexp;
 
 	cpumask_clear_cpu(cpu, tmigr_available_cpumask);
-	raw_spin_lock_irq(&tmc->lock);
-	tmc->available = false;
-	WRITE_ONCE(tmc->wakeup, KTIME_MAX);
+	scoped_guard(raw_spinlock_irq, &tmc->lock) {
+		tmc->available = false;
+		WRITE_ONCE(tmc->wakeup, KTIME_MAX);
 
-	/*
-	 * CPU has to handle the local events on his own, when on the way to
-	 * offline; Therefore nextevt value is set to KTIME_MAX
-	 */
-	firstexp = __tmigr_cpu_deactivate(tmc, KTIME_MAX);
-	trace_tmigr_cpu_unavailable(tmc);
-	raw_spin_unlock_irq(&tmc->lock);
+		/*
+		 * CPU has to handle the local events on his own, when on the way to
+		 * offline; Therefore nextevt value is set to KTIME_MAX
+		 */
+		firstexp = __tmigr_cpu_deactivate(tmc, KTIME_MAX);
+		trace_tmigr_cpu_unavailable(tmc);
+	}
 
 	if (firstexp != KTIME_MAX) {
 		migrator = cpumask_any(tmigr_available_cpumask);
@@ -1469,13 +1469,13 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 		return -EINVAL;
 
 	cpumask_set_cpu(cpu, tmigr_available_cpumask);
-	raw_spin_lock_irq(&tmc->lock);
-	trace_tmigr_cpu_available(tmc);
-	tmc->idle = timer_base_is_idle();
-	if (!tmc->idle)
-		__tmigr_cpu_activate(tmc);
-	tmc->available = true;
-	raw_spin_unlock_irq(&tmc->lock);
+	scoped_guard(raw_spinlock_irq, &tmc->lock) {
+		trace_tmigr_cpu_available(tmc);
+		tmc->idle = timer_base_is_idle();
+		if (!tmc->idle)
+			__tmigr_cpu_activate(tmc);
+		tmc->available = true;
+	}
 	return 0;
 }
 

[tip: timers/core] timers/migration: Add mask for CPUs available in the hierarchy

[tip-bot2 for Gabriele Monaco]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     a048ca5f00ebd5a44f8551d546a3cd81fed7a204
Gitweb:        https://git.kernel.org/tip/a048ca5f00ebd5a44f8551d546a3cd81fed7a204
Author:        Gabriele Monaco <gmonaco@redhat.com>
AuthorDate:    Thu, 20 Nov 2025 15:56:48 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 20 Nov 2025 20:17:31 +01:00

timers/migration: Add mask for CPUs available in the hierarchy

Keep track of the CPUs available for timer migration in a cpumask. This
prepares the ground to generalise the concept of unavailable CPUs.

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251120145653.296659-3-gmonaco@redhat.com
---
 kernel/time/timer_migration.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 2cfebed..3325ca7 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -424,6 +424,12 @@ static struct tmigr_group *tmigr_root;
 
 static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
 
+/*
+ * CPUs available for timer migration.
+ * Protected by cpuset_mutex (with cpus_read_lock held) or cpus_write_lock.
+ */
+static cpumask_var_t tmigr_available_cpumask;
+
 #define TMIGR_NONE	0xFF
 #define BIT_CNT		8
 
@@ -1433,6 +1439,7 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	int migrator;
 	u64 firstexp;
 
+	cpumask_clear_cpu(cpu, tmigr_available_cpumask);
 	raw_spin_lock_irq(&tmc->lock);
 	tmc->available = false;
 	WRITE_ONCE(tmc->wakeup, KTIME_MAX);
@@ -1446,7 +1453,7 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	raw_spin_unlock_irq(&tmc->lock);
 
 	if (firstexp != KTIME_MAX) {
-		migrator = cpumask_any_but(cpu_online_mask, cpu);
+		migrator = cpumask_any(tmigr_available_cpumask);
 		work_on_cpu(migrator, tmigr_trigger_active, NULL);
 	}
 
@@ -1461,6 +1468,7 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	if (WARN_ON_ONCE(!tmc->tmgroup))
 		return -EINVAL;
 
+	cpumask_set_cpu(cpu, tmigr_available_cpumask);
 	raw_spin_lock_irq(&tmc->lock);
 	trace_tmigr_cpu_available(tmc);
 	tmc->idle = timer_base_is_idle();
@@ -1805,6 +1813,11 @@ static int __init tmigr_init(void)
 	if (ncpus == 1)
 		return 0;
 
+	if (!zalloc_cpumask_var(&tmigr_available_cpumask, GFP_KERNEL)) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
 	/*
 	 * Calculate the required hierarchy levels. Unfortunately there is no
 	 * reliable information available, unless all possible CPUs have been

[tip: timers/core] timers/migration: Rename 'online' bit to 'available'

[tip-bot2 for Gabriele Monaco]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     8312cab5ff4702389a86129051eba6ea046a71a1
Gitweb:        https://git.kernel.org/tip/8312cab5ff4702389a86129051eba6ea046a71a1
Author:        Gabriele Monaco <gmonaco@redhat.com>
AuthorDate:    Thu, 20 Nov 2025 15:56:47 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 20 Nov 2025 20:17:31 +01:00

timers/migration: Rename 'online' bit to 'available'

The timer migration hierarchy excludes offline CPUs via the
tmigr_is_not_available function, which is essentially checking the
online bit for the CPU.

Rename the online bit to available and all references in function names
and tracepoint to generalise the concept of available CPUs.

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251120145653.296659-2-gmonaco@redhat.com
---
 include/trace/events/timer_migration.h |  4 ++--
 kernel/time/timer_migration.c          | 24 ++++++++++++------------
 kernel/time/timer_migration.h          |  2 +-
 3 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/include/trace/events/timer_migration.h b/include/trace/events/timer_migration.h
index 47db5ea..61171b1 100644
--- a/include/trace/events/timer_migration.h
+++ b/include/trace/events/timer_migration.h
@@ -173,14 +173,14 @@ DEFINE_EVENT(tmigr_cpugroup, tmigr_cpu_active,
 	TP_ARGS(tmc)
 );
 
-DEFINE_EVENT(tmigr_cpugroup, tmigr_cpu_online,
+DEFINE_EVENT(tmigr_cpugroup, tmigr_cpu_available,
 
 	TP_PROTO(struct tmigr_cpu *tmc),
 
 	TP_ARGS(tmc)
 );
 
-DEFINE_EVENT(tmigr_cpugroup, tmigr_cpu_offline,
+DEFINE_EVENT(tmigr_cpugroup, tmigr_cpu_unavailable,
 
 	TP_PROTO(struct tmigr_cpu *tmc),
 
diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 57e3867..2cfebed 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -429,7 +429,7 @@ static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
 
 static inline bool tmigr_is_not_available(struct tmigr_cpu *tmc)
 {
-	return !(tmc->tmgroup && tmc->online);
+	return !(tmc->tmgroup && tmc->available);
 }
 
 /*
@@ -926,7 +926,7 @@ static void tmigr_handle_remote_cpu(unsigned int cpu, u64 now,
 	 * updated the event takes care when hierarchy is completely
 	 * idle. Otherwise the migrator does it as the event is enqueued.
 	 */
-	if (!tmc->online || tmc->remote || tmc->cpuevt.ignore ||
+	if (!tmc->available || tmc->remote || tmc->cpuevt.ignore ||
 	    now < tmc->cpuevt.nextevt.expires) {
 		raw_spin_unlock_irq(&tmc->lock);
 		return;
@@ -973,7 +973,7 @@ static void tmigr_handle_remote_cpu(unsigned int cpu, u64 now,
 	 * (See also section "Required event and timerqueue update after a
 	 * remote expiry" in the documentation at the top)
 	 */
-	if (!tmc->online || !tmc->idle) {
+	if (!tmc->available || !tmc->idle) {
 		timer_unlock_remote_bases(cpu);
 		goto unlock;
 	}
@@ -1422,19 +1422,19 @@ static long tmigr_trigger_active(void *unused)
 {
 	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
 
-	WARN_ON_ONCE(!tmc->online || tmc->idle);
+	WARN_ON_ONCE(!tmc->available || tmc->idle);
 
 	return 0;
 }
 
-static int tmigr_cpu_offline(unsigned int cpu)
+static int tmigr_clear_cpu_available(unsigned int cpu)
 {
 	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
 	int migrator;
 	u64 firstexp;
 
 	raw_spin_lock_irq(&tmc->lock);
-	tmc->online = false;
+	tmc->available = false;
 	WRITE_ONCE(tmc->wakeup, KTIME_MAX);
 
 	/*
@@ -1442,7 +1442,7 @@ static int tmigr_cpu_offline(unsigned int cpu)
 	 * offline; Therefore nextevt value is set to KTIME_MAX
 	 */
 	firstexp = __tmigr_cpu_deactivate(tmc, KTIME_MAX);
-	trace_tmigr_cpu_offline(tmc);
+	trace_tmigr_cpu_unavailable(tmc);
 	raw_spin_unlock_irq(&tmc->lock);
 
 	if (firstexp != KTIME_MAX) {
@@ -1453,7 +1453,7 @@ static int tmigr_cpu_offline(unsigned int cpu)
 	return 0;
 }
 
-static int tmigr_cpu_online(unsigned int cpu)
+static int tmigr_set_cpu_available(unsigned int cpu)
 {
 	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
 
@@ -1462,11 +1462,11 @@ static int tmigr_cpu_online(unsigned int cpu)
 		return -EINVAL;
 
 	raw_spin_lock_irq(&tmc->lock);
-	trace_tmigr_cpu_online(tmc);
+	trace_tmigr_cpu_available(tmc);
 	tmc->idle = timer_base_is_idle();
 	if (!tmc->idle)
 		__tmigr_cpu_activate(tmc);
-	tmc->online = true;
+	tmc->available = true;
 	raw_spin_unlock_irq(&tmc->lock);
 	return 0;
 }
@@ -1758,7 +1758,7 @@ static int tmigr_add_cpu(unsigned int cpu)
 		 * The (likely) current CPU is expected to be online in the hierarchy,
 		 * otherwise the old root may not be active as expected.
 		 */
-		WARN_ON_ONCE(!per_cpu_ptr(&tmigr_cpu, raw_smp_processor_id())->online);
+		WARN_ON_ONCE(!per_cpu_ptr(&tmigr_cpu, raw_smp_processor_id())->available);
 		ret = tmigr_setup_groups(-1, old_root->numa_node, old_root, true);
 	}
 
@@ -1854,7 +1854,7 @@ static int __init tmigr_init(void)
 		goto err;
 
 	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
-				tmigr_cpu_online, tmigr_cpu_offline);
+				tmigr_set_cpu_available, tmigr_clear_cpu_available);
 	if (ret)
 		goto err;
 
diff --git a/kernel/time/timer_migration.h b/kernel/time/timer_migration.h
index ae19f70..70879cd 100644
--- a/kernel/time/timer_migration.h
+++ b/kernel/time/timer_migration.h
@@ -97,7 +97,7 @@ struct tmigr_group {
  */
 struct tmigr_cpu {
 	raw_spinlock_t		lock;
-	bool			online;
+	bool			available;
 	bool			idle;
 	bool			remote;
 	struct tmigr_group	*tmgroup;