[PATCH v14 0/7] timers: Exclude isolated cpus from timer migration

[PATCH v14 0/7] timers: Exclude isolated cpus from timer migration

[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 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: Rename tmigr 'online' bit to 'available'
  timers: Add the available mask in timer migration
  timers: Use scoped_guard when setting/clearing the tmigr available
    flag
  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: Exclude isolated cpus from timer migration

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          | 227 ++++++++++++++++++++++---
 kernel/time/timer_migration.h          |   2 +-
 7 files changed, 247 insertions(+), 35 deletions(-)


base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
-- 
2.43.0

[PATCH v14 1/7] timers: Rename tmigr '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>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 include/trace/events/timer_migration.h |  4 ++--
 kernel/time/timer_migration.c          | 22 +++++++++++-----------
 kernel/time/timer_migration.h          |  2 +-
 3 files changed, 14 insertions(+), 14 deletions(-)

diff --git a/include/trace/events/timer_migration.h b/include/trace/events/timer_migration.h
index 47db5eaf2f9ab..61171b13c687c 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 c0c54dc5314c3..78700f90944f0 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -427,7 +427,7 @@ 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;
 	}
@@ -1432,19 +1432,19 @@ 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);
 
 	/*
@@ -1452,7 +1452,7 @@ 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) {
@@ -1463,7 +1463,7 @@ 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);
 
@@ -1472,11 +1472,11 @@ 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;
 }
@@ -1847,7 +1847,7 @@ 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 ae19f70f8170f..70879cde6fdd0 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;
-- 
2.43.0

[PATCH v14 2/7] timers: Add the available mask in timer migration

[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>
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 78700f90944f0..39f734dbbb0fc 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -422,6 +422,12 @@ static unsigned int tmigr_crossnode_level __read_mostly;
 
 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
 
@@ -1443,6 +1449,7 @@ 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);
@@ -1456,7 +1463,7 @@ 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);
 	}
 
@@ -1471,6 +1478,7 @@ 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();
@@ -1798,6 +1806,11 @@ 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
-- 
2.43.0

[PATCH v14 3/7] timers: Use scoped_guard when setting/clearing the tmigr available flag

[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>
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 39f734dbbb0fc..99503d34f151d 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1450,17 +1450,17 @@ 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);
@@ -1479,13 +1479,13 @@ 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;
 }
 
-- 
2.43.0

[PATCH v14 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 52468d2c178a3..7ad5a92af5c73 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1391,7 +1391,7 @@ 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;
 
@@ -1555,7 +1555,7 @@ 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;
 
@@ -1596,7 +1596,7 @@ static void remote_partition_disable(struct cpuset *cs, struct tmpmasks *tmp)
 	compute_excpus(cs, cs->effective_xcpus);
 	reset_partition_data(cs);
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
+	update_isolation_cpumasks(isolcpus_updated);
 	cpuset_force_rebuild();
 
 	/*
@@ -1665,7 +1665,7 @@ 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();
 
@@ -2023,7 +2023,7 @@ 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);
@@ -3043,7 +3043,7 @@ 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);
-- 
2.43.0

[PATCH v14 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 a4cf17b1fab06..3ad0d6df6a0a2 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)
-- 
2.43.0

[PATCH v14 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 ff8f41ab7ce63..68be522449ec5 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)
 {
-- 
2.43.0

[PATCH v14 7/7] timers: Exclude isolated cpus from timer migration

[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 | 172 +++++++++++++++++++++++++++++++++-
 3 files changed, 179 insertions(+), 5 deletions(-)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 0414d9e6b4fcd..62e1cea711257 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 7ad5a92af5c73..719f119a563cf 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1402,6 +1402,9 @@ 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 99503d34f151d..f93c5fba24fd8 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"
@@ -428,6 +429,9 @@ static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
  */
 static cpumask_var_t tmigr_available_cpumask;
 
+/* Enabled during late initcall */
+DEFINE_STATIC_KEY_FALSE(tmigr_exclude_isolated);
+
 #define TMIGR_NONE	0xFF
 #define BIT_CNT		8
 
@@ -436,6 +440,33 @@ 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 (!housekeeping_cpu(cpu, HK_TYPE_DOMAIN) ||
+			cpuset_cpu_is_isolated(cpu)) &&
+		       housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE);
+	return false;
+}
+
 /*
  * Returns true, when @childmask corresponds to the group migrator or when the
  * group is not active - so no migrator is set.
@@ -1449,8 +1480,9 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	int migrator;
 	u64 firstexp;
 
-	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);
 
@@ -1463,11 +1495,11 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	}
 
 	if (firstexp != KTIME_MAX) {
-		migrator = cpumask_any(tmigr_available_cpumask);
+		migrator = cpumask_any_but(tmigr_available_cpumask, cpu);
 		work_on_cpu(migrator, tmigr_trigger_active, NULL);
 	}
 
-	return 0;
+	return 1;
 }
 
 static int tmigr_set_cpu_available(unsigned int cpu)
@@ -1478,14 +1510,143 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	if (WARN_ON_ONCE(!tmc->tmgroup))
 		return -EINVAL;
 
-	cpumask_set_cpu(cpu, tmigr_available_cpumask);
+	if (tmigr_is_isolated(cpu))
+		return 0;
+
 	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)
 			__tmigr_cpu_activate(tmc);
 		tmc->available = true;
 	}
+	return 1;
+}
+
+static int tmigr_online_cpu(unsigned int cpu)
+{
+	if (tmigr_set_cpu_available(cpu) > 0)
+		cpumask_set_cpu(cpu, tmigr_available_cpumask);
+	return 0;
+}
+
+static int tmigr_offline_cpu(unsigned int cpu)
+{
+	if (tmigr_clear_cpu_available(cpu) > 0)
+		cpumask_clear_cpu(cpu, tmigr_available_cpumask);
+	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_unisol __free(free_cpumask_var) = CPUMASK_VAR_NULL;
+	cpumask_var_t cpumask_isol __free(free_cpumask_var) = CPUMASK_VAR_NULL;
+	int cpu;
+
+	lockdep_assert_cpus_held();
+
+	if (!alloc_cpumask_var(&cpumask_isol, GFP_KERNEL))
+		return -ENOMEM;
+	if (!alloc_cpumask_var(&cpumask_unisol, GFP_KERNEL))
+		return -ENOMEM;
+	if (!works)
+		return -ENOMEM;
+
+	cpumask_andnot(cpumask_unisol, cpu_online_mask, exclude_cpumask);
+	cpumask_andnot(cpumask_unisol, cpumask_unisol, tmigr_available_cpumask);
+	/* Set up the mask earlier to avoid races with the migrator CPU */
+	cpumask_or(tmigr_available_cpumask, tmigr_available_cpumask, cpumask_unisol);
+	for_each_cpu(cpu, cpumask_unisol) {
+		struct work_struct *work = per_cpu_ptr(works, cpu);
+
+		INIT_WORK(work, tmigr_cpu_unisolate);
+		schedule_work_on(cpu, work);
+	}
+
+	cpumask_and(cpumask_isol, exclude_cpumask, tmigr_available_cpumask);
+	cpumask_and(cpumask_isol, cpumask_isol, 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_isol) {
+		if (!tick_nohz_cpu_hotpluggable(cpu)) {
+			cpumask_clear_cpu(cpu, cpumask_isol);
+			break;
+		}
+	}
+	/* Set up the mask earlier to avoid races with the migrator CPU */
+	cpumask_andnot(tmigr_available_cpumask, tmigr_available_cpumask, cpumask_isol);
+	for_each_cpu(cpu, cpumask_isol) {
+		struct work_struct *work = per_cpu_ptr(works, cpu);
+
+		INIT_WORK(work, tmigr_cpu_isolate);
+		schedule_work_on(cpu, work);
+	}
+
+	for_each_cpu_or(cpu, cpumask_isol, cpumask_unisol)
+		flush_work(per_cpu_ptr(works, cpu));
+
+	return 0;
+}
+
+static int __init tmigr_init_isolation(void)
+{
+	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;
+
+	static_branch_enable(&tmigr_exclude_isolated);
+	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
+		return 0;
+	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
+		return -ENOMEM;
+	if (!works)
+		return -ENOMEM;
+	cpumask_andnot(cpumask, tmigr_available_cpumask,
+		       housekeeping_cpumask(HK_TYPE_DOMAIN));
+	cpumask_and(cpumask, cpumask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
+	/* Never disable the tick CPU, see tmigr_is_isolated for details */
+	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;
 }
 
@@ -1860,7 +2021,7 @@ static int __init tmigr_init(void)
 		goto err;
 
 	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
-				tmigr_set_cpu_available, tmigr_clear_cpu_available);
+				tmigr_online_cpu, tmigr_offline_cpu);
 	if (ret)
 		goto err;
 
@@ -1871,3 +2032,4 @@ static int __init tmigr_init(void)
 	return ret;
 }
 early_initcall(tmigr_init);
+late_initcall(tmigr_init_isolation);
-- 
2.43.0

Re: [PATCH v14 7/7] timers: Exclude isolated cpus from timer migration

[kernel test robot]

Hi Gabriele,

kernel test robot noticed the following build warnings:

[auto build test WARNING on 6146a0f1dfae5d37442a9ddcba012add260bceb0]

url:    https://github.com/intel-lab-lkp/linux/commits/Gabriele-Monaco/timers-Rename-tmigr-online-bit-to-available/20251104-185437
base:   6146a0f1dfae5d37442a9ddcba012add260bceb0
patch link:    https://lore.kernel.org/r/20251104104740.70512-8-gmonaco%40redhat.com
patch subject: [PATCH v14 7/7] timers: Exclude isolated cpus from timer migration
config: s390-randconfig-r132-20251106 (https://download.01.org/0day-ci/archive/20251106/202511061957.rIK5HZq6-lkp@intel.com/config)
compiler: clang version 22.0.0git (https://github.com/llvm/llvm-project d2625a438020ad35330cda29c3def102c1687b1b)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251106/202511061957.rIK5HZq6-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202511061957.rIK5HZq6-lkp@intel.com/

sparse warnings: (new ones prefixed by >>)
>> kernel/time/timer_migration.c:433:1: sparse: sparse: symbol 'tmigr_exclude_isolated' was not declared. Should it be static?
   kernel/time/timer_migration.c:903:9: sparse: sparse: context imbalance in 'tmigr_update_events' - different lock contexts for basic block

vim +/tmigr_exclude_isolated +433 kernel/time/timer_migration.c

   431	
   432	/* Enabled during late initcall */
 > 433	DEFINE_STATIC_KEY_FALSE(tmigr_exclude_isolated);
   434	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH v14 7/7] timers: Exclude isolated cpus from timer migration

[Gabriele Monaco]

On Tue, 2025-11-04 at 11:47 +0100, Gabriele Monaco wrote:
> 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;
> 
> ...
> 
> +/* Enabled during late initcall */
> +DEFINE_STATIC_KEY_FALSE(tmigr_exclude_isolated);
> +

I forgot to mark this as static:

+static DEFINE_STATIC_KEY_FALSE(tmigr_exclude_isolated);

will send a new version after getting a review/comments.

Frederic, Thomas, besides that, does this series look ready to you?

Thanks,
Gabriele

Re: [PATCH v14 1/7] timers: Rename tmigr 'online' bit to 'available'

[Frederic Weisbecker]

Le Tue, Nov 04, 2025 at 11:47:33AM +0100, Gabriele Monaco a écrit :
> 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>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> ---
>  include/trace/events/timer_migration.h |  4 ++--
>  kernel/time/timer_migration.c          | 22 +++++++++++-----------
>  kernel/time/timer_migration.h          |  2 +-
>  3 files changed, 14 insertions(+), 14 deletions(-)
> 
> diff --git a/include/trace/events/timer_migration.h b/include/trace/events/timer_migration.h
> index 47db5eaf2f9ab..61171b13c687c 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 c0c54dc5314c3..78700f90944f0 100644
> --- a/kernel/time/timer_migration.c
> +++ b/kernel/time/timer_migration.c
> @@ -427,7 +427,7 @@ 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);

Please rebase and test your patchset on top of:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
	timers/core

There is one more "->online" field to rename there.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs

Re: [PATCH v14 7/7] timers: Exclude isolated cpus from timer migration

[Frederic Weisbecker]

Le Tue, Nov 04, 2025 at 11:47:39AM +0100, Gabriele Monaco a écrit :
> +static int __init tmigr_init_isolation(void)
> +{
> +	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;
> +
> +	static_branch_enable(&tmigr_exclude_isolated);
> +	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
> +		return 0;
> +	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
> +		return -ENOMEM;
> +	if (!works)
> +		return -ENOMEM;
> +	cpumask_andnot(cpumask, tmigr_available_cpumask,
> +		       housekeeping_cpumask(HK_TYPE_DOMAIN));
> +	cpumask_and(cpumask, cpumask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
> +	/* Never disable the tick CPU, see tmigr_is_isolated for details */
> +	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;

This duplicates a lot tmigr_isolated_exclude_cpumask().
Would this work?

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));

	return tmigr_isolated_exclude_cpumask(cpumask);
}

If so please add my Reviewed-by to the next version.

Thanks!

Re: [PATCH v14 7/7] timers: Exclude isolated cpus from timer migration

[Gabriele Monaco]

On Wed, 2025-11-12 at 17:04 +0100, Frederic Weisbecker wrote:
> Le Tue, Nov 04, 2025 at 11:47:39AM +0100, Gabriele Monaco a écrit :
> 
> 
> This duplicates a lot tmigr_isolated_exclude_cpumask().
> Would this work?
> 
> 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));
> 
> 	return tmigr_isolated_exclude_cpumask(cpumask);
> }
> 
> If so please add my Reviewed-by to the next version.

Good point, it should work, I'm going to test it tomorrow.

What comes to my mind is that tmigr_isolated_exclude_cpumask expects the hotplug
lock to be held, so I should probably just hold it before calling it:

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);
}

Thanks,
Gabriele

Re: [PATCH v14 7/7] timers: Exclude isolated cpus from timer migration

[Frederic Weisbecker]

Le Wed, Nov 12, 2025 at 05:18:26PM +0100, Gabriele Monaco a écrit :
> 
> 
> On Wed, 2025-11-12 at 17:04 +0100, Frederic Weisbecker wrote:
> > Le Tue, Nov 04, 2025 at 11:47:39AM +0100, Gabriele Monaco a écrit :
> > 
> > 
> > This duplicates a lot tmigr_isolated_exclude_cpumask().
> > Would this work?
> > 
> > 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));
> > 
> > 	return tmigr_isolated_exclude_cpumask(cpumask);
> > }
> > 
> > If so please add my Reviewed-by to the next version.
> 
> Good point, it should work, I'm going to test it tomorrow.
> 
> What comes to my mind is that tmigr_isolated_exclude_cpumask expects the hotplug
> lock to be held, so I should probably just hold it before calling it:
> 
> 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);
> }

Well it doesn't make much sense to lock hotplug at this stage.

How about:

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index e2f9a4637d7b..6ca870f1b951 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1539,17 +1539,7 @@ 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)
+static int __tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
 {
 	struct work_struct __percpu *works __free(free_percpu) =
 		alloc_percpu(struct work_struct);
@@ -1557,8 +1547,6 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
 	cpumask_var_t cpumask_isol __free(free_cpumask_var) = CPUMASK_VAR_NULL;
 	int cpu;
 
-	lockdep_assert_cpus_held();
-
 	if (!alloc_cpumask_var(&cpumask_isol, GFP_KERNEL))
 		return -ENOMEM;
 	if (!alloc_cpumask_var(&cpumask_unisol, GFP_KERNEL))
@@ -1604,40 +1592,36 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
 	return 0;
 }
 
+/**
+ * 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)
+{
+	lockdep_assert_cpus_held();
+	return __tmigr_isolated_exclude_cpumask(exclude_cpumask);
+}
+
 static int __init tmigr_init_isolation(void)
 {
-	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;
 
 	static_branch_enable(&tmigr_exclude_isolated);
+
 	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
 		return 0;
 	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
 		return -ENOMEM;
-	if (!works)
-		return -ENOMEM;
-	cpumask_andnot(cpumask, tmigr_available_cpumask,
-		       housekeeping_cpumask(HK_TYPE_DOMAIN));
-	cpumask_and(cpumask, cpumask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
-	/* Never disable the tick CPU, see tmigr_is_isolated for details */
-	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));
+	cpumask_andnot(cpumask, cpu_possible_mask, housekeeping_cpumask(HK_TYPE_DOMAIN));
 
-	return 0;
+	return __tmigr_isolated_exclude_cpumask(cpumask);
 }
 
 static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,