[PATCH v11 0/8] timers: Exclude isolated cpus from timer migration

[PATCH v11 0/8] 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 first 5 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 6 and 7 adapt isolation and cpuset 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.

Patch 8 extends the unavailable status to domain isolated CPUs, which
is the main contribution of the series.

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 [2]:
* 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 [3]:
* 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 [4]:
* 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/20250530142031.215594-1-gmonaco@redhat.com
[3] - https://lore.kernel.org/lkml/20250506091534.42117-7-gmonaco@redhat.com
[4] - https://lore.kernel.org/lkml/20250410065446.57304-2-gmonaco@redhat.com

Frederic Weisbecker (1):
  timers/migration: Postpone online/offline callbacks registration to
    late initcall

Gabriele Monaco (7):
  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_exclusion_cpumasks()
  sched/isolation: Force housekeeping if isolcpus and nohz_full don't
    leave any
  cgroup/cpuset: Fail if isolated and nohz_full don't leave any
    housekeeping
  timers: Exclude isolated cpus from timer migration

 include/linux/timer.h                  |   9 ++
 include/trace/events/timer_migration.h |   4 +-
 kernel/cgroup/cpuset.c                 |  72 +++++++++-
 kernel/sched/isolation.c               |  23 ++++
 kernel/time/timer_migration.c          | 182 ++++++++++++++++++++-----
 kernel/time/timer_migration.h          |   2 +-
 6 files changed, 248 insertions(+), 44 deletions(-)


base-commit: 260f6f4fda93c8485c8037865c941b42b9cba5d2
-- 
2.50.1

[PATCH v11 1/8] timers/migration: Postpone online/offline callbacks registration to late initcall

[Gabriele Monaco]

From: Frederic Weisbecker <frederic@kernel.org>

During the early boot process, the default clocksource used for
timekeeping is the jiffies. Better clocksources can only be selected
once clocksource_done_booting() is called as an fs initcall.

NOHZ can only be enabled after that stage, making global timer migration
irrelevant up to that point.

The tree remains inactive before NOHZ is enabled anyway. Therefore it
makes sense to enable each CPUs to the tree only once that is setup.

Make the CPUs available to the tree on late initcall, after the right
clocksource had a chance to be selected. This will also simplify the
handling of domain isolated CPUs on further patches.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 kernel/time/timer_migration.c | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index c0c54dc5314c..891891794b92 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1481,6 +1481,16 @@ static int tmigr_cpu_online(unsigned int cpu)
 	return 0;
 }
 
+/*
+ * NOHZ can only be enabled after clocksource_done_booting(). Don't
+ * bother trashing the cache in the tree before.
+ */
+static int __init tmigr_late_init(void)
+{
+	return cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
+				 tmigr_cpu_online, tmigr_cpu_offline);
+}
+
 static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,
 			     int node)
 {
@@ -1843,18 +1853,10 @@ static int __init tmigr_init(void)
 
 	ret = cpuhp_setup_state(CPUHP_TMIGR_PREPARE, "tmigr:prepare",
 				tmigr_cpu_prepare, NULL);
-	if (ret)
-		goto err;
-
-	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
-				tmigr_cpu_online, tmigr_cpu_offline);
-	if (ret)
-		goto err;
-
-	return 0;
-
 err:
-	pr_err("Timer migration setup failed\n");
+	if (ret)
+		pr_err("Timer migration setup failed\n");
 	return ret;
 }
 early_initcall(tmigr_init);
+late_initcall(tmigr_late_init);
-- 
2.50.1

[PATCH v11 2/8] 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 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 891891794b92..55b186fd146c 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;
 }
@@ -1488,7 +1488,7 @@ static int tmigr_cpu_online(unsigned int cpu)
 static int __init tmigr_late_init(void)
 {
 	return cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
-				 tmigr_cpu_online, tmigr_cpu_offline);
+				 tmigr_set_cpu_available, tmigr_clear_cpu_available);
 }
 
 static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,
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;
-- 
2.50.1

[PATCH v11 3/8] 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 55b186fd146c..865071ab5062 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();
@@ -1808,6 +1816,11 @@ 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
-- 
2.50.1

[PATCH v11 4/8] 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 865071ab5062..0a3a26e766d0 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;
 }
 
-- 
2.50.1

[PATCH v11 5/8] cgroup/cpuset: Rename update_unbound_workqueue_cpumask() to update_exclusion_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_exclusion_cpumasks() to prepare for other
functions unrelated to workqueues to be called in that spot.

Acked-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Gabriele Monaco <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 3bc4301466f3..6e3f44ffaa21 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_exclusion_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_exclusion_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_exclusion_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_exclusion_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_exclusion_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_exclusion_cpumasks(isolcpus_updated);
 
 	/* Force update if switching back to member & update effective_xcpus */
 	update_cpumasks_hier(cs, &tmpmask, !new_prs);
-- 
2.50.1

[PATCH v11 6/8] 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.

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)
-- 
2.50.1

[PATCH v11 7/8] cgroup/cpuset: Fail if isolated and nohz_full don't leave any housekeeping

[Gabriele Monaco]

Currently the user can set up isolated cpus via cpuset 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 blocking any assignation that would cause
the union of domain isolated cpus and nohz_full to covers all CPUs.

Acked-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 kernel/cgroup/cpuset.c | 57 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 6e3f44ffaa21..7b66ccedbc53 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1275,6 +1275,19 @@ static void isolated_cpus_update(int old_prs, int new_prs, struct cpumask *xcpus
 		cpumask_andnot(isolated_cpus, isolated_cpus, xcpus);
 }
 
+/*
+ * isolated_cpus_should_update - Returns if the isolated_cpus mask needs update
+ * @prs: new or old partition_root_state
+ * @parent: parent cpuset
+ * Return: true if isolated_cpus needs modification, false otherwise
+ */
+static bool isolated_cpus_should_update(int prs, struct cpuset *parent)
+{
+	if (!parent)
+		parent = &top_cpuset;
+	return prs != parent->partition_root_state;
+}
+
 /*
  * partition_xcpus_add - Add new exclusive CPUs to partition
  * @new_prs: new partition_root_state
@@ -1339,6 +1352,36 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
 	return isolcpus_updated;
 }
 
+/*
+ * isolcpus_nohz_conflict - check for isolated & nohz_full conflicts
+ * @new_cpus: cpu mask for cpus that are going to be isolated
+ * Return: true if there is conflict, false otherwise
+ *
+ * If nohz_full is enabled and we have isolated CPUs, their combination must
+ * still leave housekeeping CPUs.
+ */
+static bool isolcpus_nohz_conflict(struct cpumask *new_cpus)
+{
+	cpumask_var_t full_hk_cpus;
+	int res = false;
+
+	if (!housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
+		return false;
+
+	if (!alloc_cpumask_var(&full_hk_cpus, GFP_KERNEL))
+		return true;
+
+	cpumask_and(full_hk_cpus, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE),
+		    housekeeping_cpumask(HK_TYPE_DOMAIN));
+	cpumask_andnot(full_hk_cpus, full_hk_cpus, isolated_cpus);
+	cpumask_and(full_hk_cpus, full_hk_cpus, cpu_online_mask);
+	if (!cpumask_weight_andnot(full_hk_cpus, new_cpus))
+		res = true;
+
+	free_cpumask_var(full_hk_cpus);
+	return res;
+}
+
 static void update_exclusion_cpumasks(bool isolcpus_updated)
 {
 	int ret;
@@ -1464,6 +1507,9 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
 	if (!cpumask_intersects(tmp->new_cpus, cpu_active_mask) ||
 	    cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus))
 		return PERR_INVCPUS;
+	if (isolated_cpus_should_update(new_prs, NULL) &&
+	    isolcpus_nohz_conflict(tmp->new_cpus))
+		return PERR_HKEEPING;
 
 	spin_lock_irq(&callback_lock);
 	isolcpus_updated = partition_xcpus_add(new_prs, NULL, tmp->new_cpus);
@@ -1563,6 +1609,9 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
 		else if (cpumask_intersects(tmp->addmask, subpartitions_cpus) ||
 			 cpumask_subset(top_cpuset.effective_cpus, tmp->addmask))
 			cs->prs_err = PERR_NOCPUS;
+		else if (isolated_cpus_should_update(prs, NULL) &&
+			 isolcpus_nohz_conflict(tmp->addmask))
+			cs->prs_err = PERR_HKEEPING;
 		if (cs->prs_err)
 			goto invalidate;
 	}
@@ -1914,6 +1963,12 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 			return err;
 	}
 
+	if (deleting && isolated_cpus_should_update(new_prs, parent) &&
+	    isolcpus_nohz_conflict(tmp->delmask)) {
+		cs->prs_err = PERR_HKEEPING;
+		return PERR_HKEEPING;
+	}
+
 	/*
 	 * Change the parent's effective_cpus & effective_xcpus (top cpuset
 	 * only).
@@ -2934,6 +2989,8 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 		 * Need to update isolated_cpus.
 		 */
 		isolcpus_updated = true;
+		if (isolcpus_nohz_conflict(cs->effective_xcpus))
+			err = PERR_HKEEPING;
 	} else {
 		/*
 		 * Switching back to member is always allowed even if it
-- 
2.50.1

[PATCH v11 8/8] 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)

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 include/linux/timer.h         |   9 +++
 kernel/cgroup/cpuset.c        |   3 +
 kernel/time/timer_migration.c | 103 +++++++++++++++++++++++++++++++++-
 3 files changed, 112 insertions(+), 3 deletions(-)

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 7b66ccedbc53..2e73fc450a81 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1393,6 +1393,9 @@ static void update_exclusion_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 0a3a26e766d0..07b63be18f83 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"
@@ -436,6 +437,23 @@ 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.
+ */
+static inline bool tmigr_is_isolated(int cpu)
+{
+	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.
@@ -1451,6 +1469,8 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 
 	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);
 
@@ -1470,7 +1490,7 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	return 0;
 }
 
-static int tmigr_set_cpu_available(unsigned int cpu)
+static inline int _tmigr_set_cpu_available(unsigned int cpu)
 {
 	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
 
@@ -1480,6 +1500,8 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 
 	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)
@@ -1489,14 +1511,89 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	return 0;
 }
 
+static int tmigr_set_cpu_available(unsigned int cpu)
+{
+	if (tmigr_is_isolated(cpu))
+		return 0;
+	return _tmigr_set_cpu_available(cpu);
+}
+
+static bool tmigr_should_isolate_cpu(int cpu, void *ignored)
+{
+	/*
+	 * The tick CPU can be marked as isolated by the cpuset code, however
+	 * we cannot mark it as unavailable to avoid having no global migrator
+	 * for the nohz_full CPUs.
+	 */
+	return tick_nohz_cpu_hotpluggable(cpu);
+}
+
+static void tmigr_cpu_isolate(void *ignored)
+{
+	tmigr_clear_cpu_available(smp_processor_id());
+}
+
+static void tmigr_cpu_unisolate(void *ignored)
+{
+	tmigr_set_cpu_available(smp_processor_id());
+}
+
+static void tmigr_cpu_unisolate_force(void *ignored)
+{
+	/*
+	 * Required at boot to restore the tick CPU if nohz_full is available.
+	 * Hotplug handlers don't check for tick CPUs during runtime.
+	 */
+	_tmigr_set_cpu_available(smp_processor_id());
+}
+
+int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
+{
+	cpumask_var_t cpumask;
+
+	lockdep_assert_cpus_held();
+
+	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
+		return -ENOMEM;
+
+	cpumask_and(cpumask, exclude_cpumask, tmigr_available_cpumask);
+	cpumask_and(cpumask, cpumask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
+	on_each_cpu_cond_mask(tmigr_should_isolate_cpu, tmigr_cpu_isolate, NULL,
+			      1, cpumask);
+
+	cpumask_andnot(cpumask, cpu_online_mask, exclude_cpumask);
+	cpumask_andnot(cpumask, cpumask, tmigr_available_cpumask);
+	on_each_cpu_mask(cpumask, tmigr_cpu_unisolate, NULL, 1);
+
+	free_cpumask_var(cpumask);
+	return 0;
+}
+
 /*
  * NOHZ can only be enabled after clocksource_done_booting(). Don't
  * bother trashing the cache in the tree before.
  */
 static int __init tmigr_late_init(void)
 {
-	return cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
-				 tmigr_set_cpu_available, tmigr_clear_cpu_available);
+	int cpu, ret;
+
+	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
+				tmigr_set_cpu_available, tmigr_clear_cpu_available);
+	if (ret)
+		return ret;
+	/*
+	 * The tick CPU may not be marked as available in the above call, this
+	 * can occur only at boot as hotplug handlers are not called on the
+	 * tick CPU. Force it enabled here.
+	 */
+	for_each_possible_cpu(cpu) {
+		if (!tick_nohz_cpu_hotpluggable(cpu)) {
+			ret = smp_call_function_single(
+				cpu, tmigr_cpu_unisolate_force, NULL, 1);
+			break;
+		}
+	}
+	return ret;
 }
 
 static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,
-- 
2.50.1

Re: [PATCH v11 6/8] sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any

[Waiman Long]

On 8/8/25 12:01 PM, Gabriele Monaco wrote:
> 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>
> ---
>   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)
Reviewed-by: Waiman Long <longman@redhat.com>

Re: [PATCH v11 7/8] cgroup/cpuset: Fail if isolated and nohz_full don't leave any housekeeping

[Waiman Long]

On 8/8/25 12:01 PM, Gabriele Monaco wrote:
> Currently the user can set up isolated cpus via cpuset 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 blocking any assignation that would cause
> the union of domain isolated cpus and nohz_full to covers all CPUs.
>
> Acked-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> ---
>   kernel/cgroup/cpuset.c | 57 ++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 57 insertions(+)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 6e3f44ffaa21..7b66ccedbc53 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1275,6 +1275,19 @@ static void isolated_cpus_update(int old_prs, int new_prs, struct cpumask *xcpus
>   		cpumask_andnot(isolated_cpus, isolated_cpus, xcpus);
>   }
>   
> +/*
> + * isolated_cpus_should_update - Returns if the isolated_cpus mask needs update
> + * @prs: new or old partition_root_state
> + * @parent: parent cpuset
> + * Return: true if isolated_cpus needs modification, false otherwise
> + */
> +static bool isolated_cpus_should_update(int prs, struct cpuset *parent)
> +{
> +	if (!parent)
> +		parent = &top_cpuset;
> +	return prs != parent->partition_root_state;
> +}
> +
>   /*
>    * partition_xcpus_add - Add new exclusive CPUs to partition
>    * @new_prs: new partition_root_state
> @@ -1339,6 +1352,36 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
>   	return isolcpus_updated;
>   }
>   
> +/*
> + * isolcpus_nohz_conflict - check for isolated & nohz_full conflicts
> + * @new_cpus: cpu mask for cpus that are going to be isolated
> + * Return: true if there is conflict, false otherwise
> + *
> + * If nohz_full is enabled and we have isolated CPUs, their combination must
> + * still leave housekeeping CPUs.
> + */
> +static bool isolcpus_nohz_conflict(struct cpumask *new_cpus)
> +{
> +	cpumask_var_t full_hk_cpus;
> +	int res = false;
> +
> +	if (!housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
> +		return false;
> +
> +	if (!alloc_cpumask_var(&full_hk_cpus, GFP_KERNEL))
> +		return true;
> +
> +	cpumask_and(full_hk_cpus, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE),
> +		    housekeeping_cpumask(HK_TYPE_DOMAIN));
> +	cpumask_andnot(full_hk_cpus, full_hk_cpus, isolated_cpus);
> +	cpumask_and(full_hk_cpus, full_hk_cpus, cpu_online_mask);
> +	if (!cpumask_weight_andnot(full_hk_cpus, new_cpus))
> +		res = true;
> +
> +	free_cpumask_var(full_hk_cpus);
> +	return res;
> +}
> +
The check could probably be further optimized. This check will need to 
be updated when the relationship between isolated CPUs and nohz_full 
CPUs changes in the future. It does get the job done for now.
>   static void update_exclusion_cpumasks(bool isolcpus_updated)
>   {
>   	int ret;
> @@ -1464,6 +1507,9 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
>   	if (!cpumask_intersects(tmp->new_cpus, cpu_active_mask) ||
>   	    cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus))
>   		return PERR_INVCPUS;
> +	if (isolated_cpus_should_update(new_prs, NULL) &&
> +	    isolcpus_nohz_conflict(tmp->new_cpus))
> +		return PERR_HKEEPING;
>   
>   	spin_lock_irq(&callback_lock);
>   	isolcpus_updated = partition_xcpus_add(new_prs, NULL, tmp->new_cpus);
> @@ -1563,6 +1609,9 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
>   		else if (cpumask_intersects(tmp->addmask, subpartitions_cpus) ||
>   			 cpumask_subset(top_cpuset.effective_cpus, tmp->addmask))
>   			cs->prs_err = PERR_NOCPUS;
> +		else if (isolated_cpus_should_update(prs, NULL) &&
> +			 isolcpus_nohz_conflict(tmp->addmask))
> +			cs->prs_err = PERR_HKEEPING;
>   		if (cs->prs_err)
>   			goto invalidate;
>   	}
> @@ -1914,6 +1963,12 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>   			return err;
>   	}
>   
> +	if (deleting && isolated_cpus_should_update(new_prs, parent) &&
> +	    isolcpus_nohz_conflict(tmp->delmask)) {
> +		cs->prs_err = PERR_HKEEPING;
> +		return PERR_HKEEPING;
> +	}
> +
>   	/*
>   	 * Change the parent's effective_cpus & effective_xcpus (top cpuset
>   	 * only).
> @@ -2934,6 +2989,8 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>   		 * Need to update isolated_cpus.
>   		 */
>   		isolcpus_updated = true;
> +		if (isolcpus_nohz_conflict(cs->effective_xcpus))
> +			err = PERR_HKEEPING;
>   	} else {
>   		/*
>   		 * Switching back to member is always allowed even if it
Acked-by: Waiman Long <longman@redhat.com>

Re: [PATCH v11 7/8] cgroup/cpuset: Fail if isolated and nohz_full don't leave any housekeeping

[Waiman Long]

On 8/8/25 12:01 PM, Gabriele Monaco wrote:
> Currently the user can set up isolated cpus via cpuset 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 blocking any assignation that would cause
> the union of domain isolated cpus and nohz_full to covers all CPUs.
>
> Acked-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> ---
>   kernel/cgroup/cpuset.c | 57 ++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 57 insertions(+)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 6e3f44ffaa21..7b66ccedbc53 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1275,6 +1275,19 @@ static void isolated_cpus_update(int old_prs, int new_prs, struct cpumask *xcpus
>   		cpumask_andnot(isolated_cpus, isolated_cpus, xcpus);
>   }
>   
> +/*
> + * isolated_cpus_should_update - Returns if the isolated_cpus mask needs update
> + * @prs: new or old partition_root_state
> + * @parent: parent cpuset
> + * Return: true if isolated_cpus needs modification, false otherwise
> + */
> +static bool isolated_cpus_should_update(int prs, struct cpuset *parent)
> +{
> +	if (!parent)
> +		parent = &top_cpuset;
> +	return prs != parent->partition_root_state;
> +}
> +
>   /*
>    * partition_xcpus_add - Add new exclusive CPUs to partition
>    * @new_prs: new partition_root_state
> @@ -1339,6 +1352,36 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
>   	return isolcpus_updated;
>   }
>   
> +/*
> + * isolcpus_nohz_conflict - check for isolated & nohz_full conflicts
> + * @new_cpus: cpu mask for cpus that are going to be isolated
> + * Return: true if there is conflict, false otherwise
> + *
> + * If nohz_full is enabled and we have isolated CPUs, their combination must
> + * still leave housekeeping CPUs.
> + */
> +static bool isolcpus_nohz_conflict(struct cpumask *new_cpus)
> +{
> +	cpumask_var_t full_hk_cpus;
> +	int res = false;
> +
> +	if (!housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
> +		return false;
> +
> +	if (!alloc_cpumask_var(&full_hk_cpus, GFP_KERNEL))
> +		return true;
> +
> +	cpumask_and(full_hk_cpus, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE),
> +		    housekeeping_cpumask(HK_TYPE_DOMAIN));
> +	cpumask_andnot(full_hk_cpus, full_hk_cpus, isolated_cpus);
> +	cpumask_and(full_hk_cpus, full_hk_cpus, cpu_online_mask);

BTW, could you replace cpu_online_mask to cpu_active_mask in case you 
need to update this patch series again?

Thanks,
Longman

Re: [PATCH v11 8/8] timers: Exclude isolated cpus from timer migration

[Waiman Long]

On 8/8/25 12:01 PM, 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;
>
> 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)
>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> ---
>   include/linux/timer.h         |   9 +++
>   kernel/cgroup/cpuset.c        |   3 +
>   kernel/time/timer_migration.c | 103 +++++++++++++++++++++++++++++++++-
>   3 files changed, 112 insertions(+), 3 deletions(-)
>
> 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 7b66ccedbc53..2e73fc450a81 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1393,6 +1393,9 @@ static void update_exclusion_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 0a3a26e766d0..07b63be18f83 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"
> @@ -436,6 +437,23 @@ 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.
> + */
> +static inline bool tmigr_is_isolated(int cpu)
> +{
> +	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.
> @@ -1451,6 +1469,8 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
>   
>   	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);
>   
> @@ -1470,7 +1490,7 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
>   	return 0;
>   }
>   
> -static int tmigr_set_cpu_available(unsigned int cpu)
> +static inline int _tmigr_set_cpu_available(unsigned int cpu)
>   {
>   	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
>   
> @@ -1480,6 +1500,8 @@ static int tmigr_set_cpu_available(unsigned int cpu)
>   
>   	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)
> @@ -1489,14 +1511,89 @@ static int tmigr_set_cpu_available(unsigned int cpu)
>   	return 0;
>   }
>   
> +static int tmigr_set_cpu_available(unsigned int cpu)
> +{
> +	if (tmigr_is_isolated(cpu))
> +		return 0;
> +	return _tmigr_set_cpu_available(cpu);
> +}
> +
> +static bool tmigr_should_isolate_cpu(int cpu, void *ignored)
> +{
> +	/*
> +	 * The tick CPU can be marked as isolated by the cpuset code, however
> +	 * we cannot mark it as unavailable to avoid having no global migrator
> +	 * for the nohz_full CPUs.
> +	 */
> +	return tick_nohz_cpu_hotpluggable(cpu);
> +}
We may have to update the cpuset code to fail isolated partition 
formation if it includes the nohz_full tick CPU as that CPU cannot be 
fully isolated. That will also make this patch simpler.
> +
> +static void tmigr_cpu_isolate(void *ignored)
> +{
> +	tmigr_clear_cpu_available(smp_processor_id());
> +}
> +
> +static void tmigr_cpu_unisolate(void *ignored)
> +{
> +	tmigr_set_cpu_available(smp_processor_id());
> +}
> +
> +static void tmigr_cpu_unisolate_force(void *ignored)
> +{
> +	/*
> +	 * Required at boot to restore the tick CPU if nohz_full is available.
> +	 * Hotplug handlers don't check for tick CPUs during runtime.
> +	 */
> +	_tmigr_set_cpu_available(smp_processor_id());
> +}
> +
> +int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
> +{
> +	cpumask_var_t cpumask;
> +
> +	lockdep_assert_cpus_held();
> +
> +	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
> +		return -ENOMEM;
> +
> +	cpumask_and(cpumask, exclude_cpumask, tmigr_available_cpumask);
> +	cpumask_and(cpumask, cpumask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
> +	on_each_cpu_cond_mask(tmigr_should_isolate_cpu, tmigr_cpu_isolate, NULL,
> +			      1, cpumask);
> +
> +	cpumask_andnot(cpumask, cpu_online_mask, exclude_cpumask);
> +	cpumask_andnot(cpumask, cpumask, tmigr_available_cpumask);
> +	on_each_cpu_mask(cpumask, tmigr_cpu_unisolate, NULL, 1);
> +
> +	free_cpumask_var(cpumask);
> +	return 0;
> +}
> +
>   /*
>    * NOHZ can only be enabled after clocksource_done_booting(). Don't
>    * bother trashing the cache in the tree before.
>    */
>   static int __init tmigr_late_init(void)
>   {
> -	return cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
> -				 tmigr_set_cpu_available, tmigr_clear_cpu_available);
> +	int cpu, ret;
> +
> +	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
> +				tmigr_set_cpu_available, tmigr_clear_cpu_available);
> +	if (ret)
> +		return ret;
> +	/*
> +	 * The tick CPU may not be marked as available in the above call, this
> +	 * can occur only at boot as hotplug handlers are not called on the
> +	 * tick CPU. Force it enabled here.
> +	 */
> +	for_each_possible_cpu(cpu) {
> +		if (!tick_nohz_cpu_hotpluggable(cpu)) {
> +			ret = smp_call_function_single(
> +				cpu, tmigr_cpu_unisolate_force, NULL, 1);
> +			break;
> +		}
> +	}
> +	return ret;
>   }
Can you integrate the 
tick_nohz_cpu_hotpluggable/tmigr_should_isolate_cpu check into 
tmigr_set_cpu_available() instead of special-casing the tick CPU here?

Cheers,
Longman

Re: [PATCH v11 8/8] timers: Exclude isolated cpus from timer migration

[Gabriele Monaco]

On Tue, 2025-08-12 at 13:20 -0400, Waiman Long wrote:
> On 8/8/25 12:01 PM, Gabriele Monaco wrote:
> > +static bool tmigr_should_isolate_cpu(int cpu, void *ignored)
> > +{
> > +	/*
> > +	 * The tick CPU can be marked as isolated by the cpuset
> > code, however
> > +	 * we cannot mark it as unavailable to avoid having no
> > global migrator
> > +	 * for the nohz_full CPUs.
> > +	 */
> > +	return tick_nohz_cpu_hotpluggable(cpu);
> > +}
> We may have to update the cpuset code to fail isolated partition 
> formation if it includes the nohz_full tick CPU as that CPU cannot be
> fully isolated. That will also make this patch simpler.

Good idea, I can check that!

[...]
> >   /*
> >    * NOHZ can only be enabled after clocksource_done_booting().
> > Don't
> >    * bother trashing the cache in the tree before.
> >    */
> >   static int __init tmigr_late_init(void)
> >   {
> > -	return cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE,
> > "tmigr:online",
> > -				 tmigr_set_cpu_available,
> > tmigr_clear_cpu_available);
> > +	int cpu, ret;
> > +
> > +	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE,
> > "tmigr:online",
> > +				tmigr_set_cpu_available,
> > tmigr_clear_cpu_available);
> > +	if (ret)
> > +		return ret;
> > +	/*
> > +	 * The tick CPU may not be marked as available in the
> > above call, this
> > +	 * can occur only at boot as hotplug handlers are not
> > called on the
> > +	 * tick CPU. Force it enabled here.
> > +	 */
> > +	for_each_possible_cpu(cpu) {
> > +		if (!tick_nohz_cpu_hotpluggable(cpu)) {
> > +			ret = smp_call_function_single(
> > +				cpu, tmigr_cpu_unisolate_force,
> > NULL, 1);
> > +			break;
> > +		}
> > +	}
> > +	return ret;
> >   }
> Can you integrate the
> tick_nohz_cpu_hotpluggable/tmigr_should_isolate_cpu check into 
> tmigr_set_cpu_available() instead of special-casing the tick CPU
> here?


I could but I'm afraid Thomas won't like that [1]:
> This is really horribly confusing. This function is also invoked from
> the CPU offline hotplug callback and in context of CPU hotplug this
> check makes absolutely no sense at all.

The whole thing changed a bit since then, but I think his point still
stands, tick_nohz_cpu_hotpluggable is not meaningful from hotplug
callbacks besides that very first invocation.

Or am I missing your point?

Thanks,
Gabriele

[1] - https://lore.kernel.org/lkml/875xgqqrel.ffs@tglx/

Re: [PATCH v11 0/8] timers: Exclude isolated cpus from timer migration

[John B. Wyatt IV]

On Fri, Aug 08, 2025 at 06:01:34PM +0200, 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;

Hello Gabriele,

I tested your v11 patchset against 6.17-rc1-rt1 to see if it was the same issue with rtla-onoise getting latencies in the 20,000-40000 us on RHEL 10.2. It seems to have resolved the issue. Adding my tested-by tags and test results.

rtla osnoise top -c 1 -e sched:sched_switch -s 20 -T 1 -t -d 30m -q

CPU Period       Runtime        Noise  % CPU Aval   Max Noise   Max Single          HW          NMI          IRQ      Softirq       Thread
  1 #1799     1799000003      3382675    99.81196        2840            9        1578            0      1799012            0        24369

Tested-by: John B. Wyatt IV <jwyatt@redhat.com>
Tested-by: John B. Wyatt IV <sageofredondo@gmail.com>

-- 
Sincerely,
John Wyatt
Software Engineer, Core Kernel
Red Hat

Re: [PATCH v11 8/8] timers: Exclude isolated cpus from timer migration

[Frederic Weisbecker]

Le Fri, Aug 08, 2025 at 06:01:42PM +0200, Gabriele Monaco a écrit :
>  /*
>   * NOHZ can only be enabled after clocksource_done_booting(). Don't
>   * bother trashing the cache in the tree before.
>   */
>  static int __init tmigr_late_init(void)
>  {
> -	return cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
> -				 tmigr_set_cpu_available, tmigr_clear_cpu_available);
> +	int cpu, ret;
> +
> +	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
> +				tmigr_set_cpu_available, tmigr_clear_cpu_available);
> +	if (ret)
> +		return ret;
> +	/*
> +	 * The tick CPU may not be marked as available in the above call, this
> +	 * can occur only at boot as hotplug handlers are not called on the
> +	 * tick CPU. Force it enabled here.
> +	 */
> +	for_each_possible_cpu(cpu) {
> +		if (!tick_nohz_cpu_hotpluggable(cpu)) {
> +			ret = smp_call_function_single(
> +				cpu, tmigr_cpu_unisolate_force, NULL, 1);
> +			break;
> +		}
> +	}

Why not evaluate tick_nohz_cpu_hotpluggable() from tmigr_clear_cpu_available()
instead of this force IPI?

But if I understand correctly, this will be handled by cpuset, right?

Thanks.

-- 
Frederic Weisbecker
SUSE Labs

Re: [PATCH v11 8/8] timers: Exclude isolated cpus from timer migration

[Gabriele Monaco]

On Mon, 2025-09-01 at 14:41 +0200, Frederic Weisbecker wrote:
> Le Fri, Aug 08, 2025 at 06:01:42PM +0200, Gabriele Monaco a écrit :
> >  /*
> >   * NOHZ can only be enabled after clocksource_done_booting().
> > Don't
> >   * bother trashing the cache in the tree before.
> >   */
> >  static int __init tmigr_late_init(void)
> >  {
> > -	return cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE,
> > "tmigr:online",
> > -				 tmigr_set_cpu_available,
> > tmigr_clear_cpu_available);
> > +	int cpu, ret;
> > +
> > +	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE,
> > "tmigr:online",
> > +				tmigr_set_cpu_available,
> > tmigr_clear_cpu_available);
> > +	if (ret)
> > +		return ret;
> > +	/*
> > +	 * The tick CPU may not be marked as available in the
> > above call, this
> > +	 * can occur only at boot as hotplug handlers are not
> > called on the
> > +	 * tick CPU. Force it enabled here.
> > +	 */
> > +	for_each_possible_cpu(cpu) {
> > +		if (!tick_nohz_cpu_hotpluggable(cpu)) {
> > +			ret = smp_call_function_single(
> > +				cpu, tmigr_cpu_unisolate_force,
> > NULL, 1);
> > +			break;
> > +		}
> > +	}
> 
> Why not evaluate tick_nohz_cpu_hotpluggable() from
> tmigr_clear_cpu_available() instead of this force IPI?

The idea is that this IPI runs once during late boot only for the tick
CPU, while the call to tick_nohz_cpu_hotpluggable() would be running at
every hotplug event if I move it to tmigr_clear_cpu_available.
In that scenario, it's guaranteed to return true (besides the very
first call).

I don't have a strong opinion against running that check every time
although it's needed only at boot time and remove this IPI, but in my
understanding that's one of the thing Thomas was finding confusing [1].

Am I missing anything here?

> But if I understand correctly, this will be handled by cpuset, right?

Currently tick_nohz_cpu_hotpluggable() is called by
tmigr_should_isolate_cpu() and that is called by cpuset code, changing
cpuset would save that call but won't deal with the tick CPU not
enabled at boot time, unless I'm misunderstanding what Waiman implied.

Thanks,
Gabriele

[1] - https://lore.kernel.org/lkml/875xgqqrel.ffs@tglx

Re: [PATCH v11 8/8] timers: Exclude isolated cpus from timer migration

[Frederic Weisbecker]

Le Mon, Sep 01, 2025 at 03:48:15PM +0200, Gabriele Monaco a écrit :
> On Mon, 2025-09-01 at 14:41 +0200, Frederic Weisbecker wrote:
> > Le Fri, Aug 08, 2025 at 06:01:42PM +0200, Gabriele Monaco a écrit :
> > >  /*
> > >   * NOHZ can only be enabled after clocksource_done_booting().
> > > Don't
> > >   * bother trashing the cache in the tree before.
> > >   */
> > >  static int __init tmigr_late_init(void)
> > >  {
> > > -	return cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE,
> > > "tmigr:online",
> > > -				 tmigr_set_cpu_available,
> > > tmigr_clear_cpu_available);
> > > +	int cpu, ret;
> > > +
> > > +	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE,
> > > "tmigr:online",
> > > +				tmigr_set_cpu_available,
> > > tmigr_clear_cpu_available);
> > > +	if (ret)
> > > +		return ret;
> > > +	/*
> > > +	 * The tick CPU may not be marked as available in the
> > > above call, this
> > > +	 * can occur only at boot as hotplug handlers are not
> > > called on the
> > > +	 * tick CPU. Force it enabled here.
> > > +	 */
> > > +	for_each_possible_cpu(cpu) {
> > > +		if (!tick_nohz_cpu_hotpluggable(cpu)) {
> > > +			ret = smp_call_function_single(
> > > +				cpu, tmigr_cpu_unisolate_force,
> > > NULL, 1);
> > > +			break;
> > > +		}
> > > +	}
> > 
> > Why not evaluate tick_nohz_cpu_hotpluggable() from
> > tmigr_clear_cpu_available() instead of this force IPI?
> 
> The idea is that this IPI runs once during late boot only for the tick
> CPU, while the call to tick_nohz_cpu_hotpluggable() would be running at
> every hotplug event if I move it to tmigr_clear_cpu_available.
> In that scenario, it's guaranteed to return true (besides the very
> first call).
> 
> I don't have a strong opinion against running that check every time
> although it's needed only at boot time and remove this IPI, but in my
> understanding that's one of the thing Thomas was finding confusing [1].
> 
> Am I missing anything here?

Right, Thomas didn't like it, but the organization of the code has changed
a bit since then with the late initcall. If the best we can do to workaround
the situation is to make the CPU unavailable regardless and then undo that
right after with an IPI, then it's a good sign that we should just simplify
and eventually check tick_nohz_cpu_hotpluggable() from tmigr_is_isolated().

> > But if I understand correctly, this will be handled by cpuset, right?
> 
> Currently tick_nohz_cpu_hotpluggable() is called by
> tmigr_should_isolate_cpu() and that is called by cpuset code, changing
> cpuset would save that call but won't deal with the tick CPU not
> enabled at boot time, unless I'm misunderstanding what Waiman implied.

Good point!

Thanks.

> 
> Thanks,
> Gabriele
> 
> [1] - https://lore.kernel.org/lkml/875xgqqrel.ffs@tglx
> 

-- 
Frederic Weisbecker
SUSE Labs

Re: [PATCH v11 8/8] timers: Exclude isolated cpus from timer migration

[Gabriele Monaco]

On Mon, 2025-09-01 at 23:11 +0200, Frederic Weisbecker wrote:
> Le Mon, Sep 01, 2025 at 03:48:15PM +0200, Gabriele Monaco a écrit :
> > On Mon, 2025-09-01 at 14:41 +0200, Frederic Weisbecker wrote:
> > > Why not evaluate tick_nohz_cpu_hotpluggable() from
> > > tmigr_clear_cpu_available() instead of this force IPI?
> > 
> > The idea is that this IPI runs once during late boot only for the
> > tick CPU, while the call to tick_nohz_cpu_hotpluggable() would be
> > running at every hotplug event if I move it to
> > tmigr_clear_cpu_available. In that scenario, it's guaranteed to
> > return true (besides the very first call).
> > 
> > I don't have a strong opinion against running that check every time
> > although it's needed only at boot time and remove this IPI, but in
> > my understanding that's one of the thing Thomas was finding
> > confusing [1].
> > 
> > Am I missing anything here?
> 
> Right, Thomas didn't like it, but the organization of the code has
> changed a bit since then with the late initcall. If the best we can
> do to workaround the situation is to make the CPU unavailable
> regardless and then undo that right after with an IPI, then it's a
> good sign that we should just simplify and eventually check
> tick_nohz_cpu_hotpluggable() from tmigr_is_isolated().

Makes sense.
I'd be tempted using a static branch but since the call to
tick_nohz_cpu_hotpluggable() isn't really heavy, we can just be fine
including it in the tmigr_is_isolated() check.


> > > But if I understand correctly, this will be handled by cpuset,
> > > right?
> > 
> > Currently tick_nohz_cpu_hotpluggable() is called by
> > tmigr_should_isolate_cpu() and that is called by cpuset code,
> > changing cpuset would save that call but won't deal with the tick
> > CPU not enabled at boot time, unless I'm misunderstanding what
> > Waiman implied.
> 
> Good point!

Here I'm a bit unsure how to proceed though. We want to fail any single
isolated cpuset that includes the tick CPU under nohz_full. I can do it
directly in isolcpus_nohz_conflict and that looks easy.

But is that going to be clear for the user?
Can the user even know what the tick CPU is? Besides /assuming/ 0.

Thanks,
Gabriele

> Thanks.
> 
> > 
> > Thanks,
> > Gabriele
> > 
> > [1] - https://lore.kernel.org/lkml/875xgqqrel.ffs@tglx
> > 

Re: [PATCH v11 6/8] sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any

[Frederic Weisbecker]

Le Fri, Aug 08, 2025 at 06:01:40PM +0200, Gabriele Monaco a écrit :
> 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>

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

-- 
Frederic Weisbecker
SUSE Labs

Re: [PATCH v11 8/8] timers: Exclude isolated cpus from timer migration

[Frederic Weisbecker]

Le Tue, Sep 02, 2025 at 01:08:25PM +0200, Gabriele Monaco a écrit :
> On Mon, 2025-09-01 at 23:11 +0200, Frederic Weisbecker wrote:
> > Le Mon, Sep 01, 2025 at 03:48:15PM +0200, Gabriele Monaco a écrit :
> > > On Mon, 2025-09-01 at 14:41 +0200, Frederic Weisbecker wrote:
> > > > Why not evaluate tick_nohz_cpu_hotpluggable() from
> > > > tmigr_clear_cpu_available() instead of this force IPI?
> > > 
> > > The idea is that this IPI runs once during late boot only for the
> > > tick CPU, while the call to tick_nohz_cpu_hotpluggable() would be
> > > running at every hotplug event if I move it to
> > > tmigr_clear_cpu_available. In that scenario, it's guaranteed to
> > > return true (besides the very first call).
> > > 
> > > I don't have a strong opinion against running that check every time
> > > although it's needed only at boot time and remove this IPI, but in
> > > my understanding that's one of the thing Thomas was finding
> > > confusing [1].
> > > 
> > > Am I missing anything here?
> > 
> > Right, Thomas didn't like it, but the organization of the code has
> > changed a bit since then with the late initcall. If the best we can
> > do to workaround the situation is to make the CPU unavailable
> > regardless and then undo that right after with an IPI, then it's a
> > good sign that we should just simplify and eventually check
> > tick_nohz_cpu_hotpluggable() from tmigr_is_isolated().
> 
> Makes sense.
> I'd be tempted using a static branch but since the call to
> tick_nohz_cpu_hotpluggable() isn't really heavy, we can just be fine
> including it in the tmigr_is_isolated() check.

Right.

> 
> 
> > > > But if I understand correctly, this will be handled by cpuset,
> > > > right?
> > > 
> > > Currently tick_nohz_cpu_hotpluggable() is called by
> > > tmigr_should_isolate_cpu() and that is called by cpuset code,
> > > changing cpuset would save that call but won't deal with the tick
> > > CPU not enabled at boot time, unless I'm misunderstanding what
> > > Waiman implied.
> > 
> > Good point!
> 
> Here I'm a bit unsure how to proceed though. We want to fail any single
> isolated cpuset that includes the tick CPU under nohz_full. I can do it
> directly in isolcpus_nohz_conflict and that looks easy.
> 
> But is that going to be clear for the user?
> Can the user even know what the tick CPU is? Besides /assuming/ 0.

You're right the user can't know in advance which CPU is the tick.
I don't mind if we prevent or not cpuset from allowing to isolate
the timekeeper but either way, a pr_info() could be helpful to tell
that either:

* The isolated timekeeper will have limited isolation
or
* The timekeeper can't be isolated.

Thanks.

> 
> Thanks,
> Gabriele
> 
> > Thanks.
> > 
> > > 
> > > Thanks,
> > > Gabriele
> > > 
> > > [1] - https://lore.kernel.org/lkml/875xgqqrel.ffs@tglx
> > > 
> 

-- 
Frederic Weisbecker
SUSE Labs