[PATCH v12 0/9] timers: Exclude isolated cpus from timer migration

[PATCH v12 0/9] 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 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 [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 (8):
  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
  cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers
  timers: Exclude isolated cpus from timer migration

 include/linux/cpumask.h                |   2 +
 include/linux/timer.h                  |   9 ++
 include/trace/events/timer_migration.h |   4 +-
 kernel/cgroup/cpuset.c                 |  78 +++++++++-
 kernel/sched/isolation.c               |  23 +++
 kernel/time/timer_migration.c          | 191 ++++++++++++++++++++-----
 kernel/time/timer_migration.h          |   2 +-
 7 files changed, 265 insertions(+), 44 deletions(-)


base-commit: 320475fbd590dc94a0a3d9173f81e0797ee1a232
-- 
2.51.0

[PATCH v12 1/9] 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);
-- 
2.51.0

[PATCH v12 2/9] 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;
-- 
2.51.0

[PATCH v12 3/9] 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
-- 
2.51.0

[PATCH v12 4/9] 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;
 }
 
-- 
2.51.0

[PATCH v12 5/9] 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 27adb04df675..81a9239053a7 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);
-- 
2.51.0

[PATCH v12 6/9] sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any

[Gabriele Monaco]

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

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

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

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index a4cf17b1fab0..3ad0d6df6a0a 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -167,6 +167,29 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 			}
 		}
 
+		/*
+		 * Check the combination of nohz_full and isolcpus=domain,
+		 * necessary to avoid problems with the timer migration
+		 * hierarchy. managed_irq is ignored by this check since it
+		 * isn't considered in the timer migration logic.
+		 */
+		iter_flags = housekeeping.flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
+		type = find_first_bit(&iter_flags, HK_TYPE_MAX);
+		/*
+		 * Pass the check if none of these flags were previously set or
+		 * are not in the current selection.
+		 */
+		iter_flags = flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
+		first_cpu = (type == HK_TYPE_MAX || !iter_flags) ? 0 :
+			    cpumask_first_and_and(cpu_present_mask,
+				    housekeeping_staging, housekeeping.cpumasks[type]);
+		if (first_cpu >= min(nr_cpu_ids, setup_max_cpus)) {
+			pr_warn("Housekeeping: must include one present CPU "
+				"neither in nohz_full= nor in isolcpus=domain, "
+				"ignoring setting %s\n", str);
+			goto free_housekeeping_staging;
+		}
+
 		iter_flags = flags & ~housekeeping.flags;
 
 		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
-- 
2.51.0

[PATCH v12 7/9] 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 | 63 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 81a9239053a7..3cedc3580373 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 +1352,42 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
 	return isolcpus_updated;
 }
 
+/*
+ * isolated_cpus_can_update - check for isolated & nohz_full conflicts
+ * @add_cpus: cpu mask for cpus that are going to be isolated
+ * @del_cpus: cpu mask for cpus that are no longer isolated, can be NULL
+ * Return: false if there is conflict, true otherwise
+ *
+ * If nohz_full is enabled and we have isolated CPUs, their combination must
+ * still leave housekeeping CPUs.
+ */
+static bool isolated_cpus_can_update(struct cpumask *add_cpus,
+				     struct cpumask *del_cpus)
+{
+	cpumask_var_t full_hk_cpus;
+	int res = true;
+
+	if (!housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
+		return true;
+
+	if (del_cpus && cpumask_weight_and(del_cpus,
+			housekeeping_cpumask(HK_TYPE_KERNEL_NOISE)))
+		return true;
+
+	if (!alloc_cpumask_var(&full_hk_cpus, GFP_KERNEL))
+		return false;
+
+	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_active_mask);
+	if (!cpumask_weight_andnot(full_hk_cpus, add_cpus))
+		res = false;
+
+	free_cpumask_var(full_hk_cpus);
+	return res;
+}
+
 static void update_exclusion_cpumasks(bool isolcpus_updated)
 {
 	int ret;
@@ -1464,6 +1513,9 @@ 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) &&
+	    !isolated_cpus_can_update(tmp->new_cpus, NULL))
+		return PERR_HKEEPING;
 
 	spin_lock_irq(&callback_lock);
 	isolcpus_updated = partition_xcpus_add(new_prs, NULL, tmp->new_cpus);
@@ -1563,6 +1615,9 @@ 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) &&
+			 !isolated_cpus_can_update(tmp->addmask, tmp->delmask))
+			cs->prs_err = PERR_HKEEPING;
 		if (cs->prs_err)
 			goto invalidate;
 	}
@@ -1914,6 +1969,12 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 			return err;
 	}
 
+	if (deleting && isolated_cpus_should_update(new_prs, parent) &&
+	    !isolated_cpus_can_update(tmp->delmask, tmp->addmask)) {
+		cs->prs_err = PERR_HKEEPING;
+		return PERR_HKEEPING;
+	}
+
 	/*
 	 * Change the parent's effective_cpus & effective_xcpus (top cpuset
 	 * only).
@@ -2934,6 +2995,8 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 		 * Need to update isolated_cpus.
 		 */
 		isolcpus_updated = true;
+		if (!isolated_cpus_can_update(cs->effective_xcpus, NULL))
+			err = PERR_HKEEPING;
 	} else {
 		/*
 		 * Switching back to member is always allowed even if it
-- 
2.51.0

[PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Gabriele Monaco]

The cleanup helper __free(free_cpumask_var) works only if the function
returns after alloc_cpumask_var, this can complicate the code in case
there are multiple cpumasks using the cleanup helper.

Define a cpumask initialiser that is NULL if the cpumask is a pointer
and {} if it's on stack. This allows users of the cleanup helper to use
it freely on initialised cpumasks as the actual free will be called only
if the mask is not NULL (and of course if it's a pointer).

This solution was first used in [1], dropped as eventually a single mask
was sufficient.

[1] - https://lore.kernel.org/lkml/20240120025053.684838-8-yury.norov@gmail.com

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 include/linux/cpumask.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index ff8f41ab7ce6..5fb9c3fe4256 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_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_NULL {}
 
 static __always_inline bool alloc_cpumask_var(cpumask_var_t *mask, gfp_t flags)
 {
-- 
2.51.0

[PATCH v12 9/9] 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)

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 | 108 +++++++++++++++++++++++++++++++++-
 3 files changed, 119 insertions(+), 1 deletion(-)

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 3cedc3580373..6e9d86fab27e 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1399,6 +1399,9 @@ 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..08e29fc01479 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 +437,29 @@ 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.
+ *
+ * Additionally, 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 check is only necessary at boot time.
+ */
+static inline bool tmigr_is_isolated(int cpu)
+{
+	if (!tick_nohz_cpu_hotpluggable(cpu))
+		return false;
+	return (!housekeeping_cpu(cpu, HK_TYPE_DOMAIN) ||
+		cpuset_cpu_is_isolated(cpu)) &&
+	       housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE);
+}
+
 /*
  * Returns true, when @childmask corresponds to the group migrator or when the
  * group is not active - so no migrator is set.
@@ -1451,6 +1475,8 @@ 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);
 
@@ -1478,8 +1504,12 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	if (WARN_ON_ONCE(!tmc->tmgroup))
 		return -EINVAL;
 
+	if (tmigr_is_isolated(cpu))
+		return 0;
 	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)
@@ -1489,6 +1519,81 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	return 0;
 }
 
+static void tmigr_cpu_isolate(struct work_struct *ignored)
+{
+	tmigr_clear_cpu_available(smp_processor_id());
+}
+
+static void tmigr_cpu_unisolate(struct work_struct *ignored)
+{
+	tmigr_set_cpu_available(smp_processor_id());
+}
+
+/**
+ * tmigr_isolated_exclude_cpumask - Exclude given CPUs from hierarchy
+ * @exclude_cpumask: the cpumask to be excluded from timer migration hierarchy
+ *
+ * This function can be called from cpuset code to provide the new set of
+ * isolated CPUs that should be excluded from the hierarchy.
+ * Online CPUs not present in exclude_cpumask but already excluded are brought
+ * back to the hierarchy.
+ * Functions to isolate/unisolate need to be called locally and can sleep.
+ */
+int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
+{
+	struct work_struct __percpu *works __free(free_percpu) =
+		alloc_percpu(struct work_struct);
+	cpumask_var_t cpumask_unisol __free(free_cpumask_var) = CPUMASK_NULL;
+	cpumask_var_t cpumask_isol __free(free_cpumask_var) = CPUMASK_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;
+}
+
 /*
  * NOHZ can only be enabled after clocksource_done_booting(). Don't
  * bother trashing the cache in the tree before.
@@ -1496,7 +1601,8 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 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);
+				 tmigr_set_cpu_available,
+				 tmigr_clear_cpu_available);
 }
 
 static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,
-- 
2.51.0

Re: [PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Yury Norov]

On Mon, Sep 15, 2025 at 04:59:29PM +0200, Gabriele Monaco wrote:
> The cleanup helper __free(free_cpumask_var) works only if the function
> returns after alloc_cpumask_var, this can complicate the code in case
> there are multiple cpumasks using the cleanup helper.
> 
> Define a cpumask initialiser that is NULL if the cpumask is a pointer
> and {} if it's on stack. This allows users of the cleanup helper to use
> it freely on initialised cpumasks as the actual free will be called only
> if the mask is not NULL (and of course if it's a pointer).
> 
> This solution was first used in [1], dropped as eventually a single mask
> was sufficient.
> 
> [1] - https://lore.kernel.org/lkml/20240120025053.684838-8-yury.norov@gmail.com

So why don't you pick the original patch instead?
 
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> ---
>  include/linux/cpumask.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index ff8f41ab7ce6..5fb9c3fe4256 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_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_NULL {}
>  
>  static __always_inline bool alloc_cpumask_var(cpumask_var_t *mask, gfp_t flags)
>  {
> -- 
> 2.51.0

Re: [PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Gabriele Monaco]

2025-09-15T16:04:53Z Yury Norov <yury.norov@gmail.com>:
> So why don't you pick the original patch instead?

In my opinion, the /juice/ of that patch was included with [1], here I'm just adding part of it.
If you prefer I can pick that patch and adapt the commit message to reflect only the part included here.

[1] - https://lore.kernel.org/lkml/1706509267-17754-3-git-send-email-schakrabarti@linux.microsoft.com/

Re: [PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Yury Norov]

On Mon, Sep 15, 2025 at 05:02:45PM +0000, Gabriele Monaco wrote:
> 2025-09-15T16:04:53Z Yury Norov <yury.norov@gmail.com>:
> > So why don't you pick the original patch instead?
> 
> In my opinion, the /juice/ of that patch was included with [1], here I'm just adding part of it.
> If you prefer I can pick that patch and adapt the commit message to reflect only the part included here.
> 
> [1] - https://lore.kernel.org/lkml/1706509267-17754-3-git-send-email-schakrabarti@linux.microsoft.com/

Yes please.

Re: [PATCH v12 7/9] cgroup/cpuset: Fail if isolated and nohz_full don't leave any housekeeping

[Waiman Long]

On 9/15/25 10:59 AM, 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 | 63 ++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 63 insertions(+)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 81a9239053a7..3cedc3580373 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 +1352,42 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
>   	return isolcpus_updated;
>   }
>   
> +/*
> + * isolated_cpus_can_update - check for isolated & nohz_full conflicts
> + * @add_cpus: cpu mask for cpus that are going to be isolated
> + * @del_cpus: cpu mask for cpus that are no longer isolated, can be NULL
> + * Return: false if there is conflict, true otherwise
> + *
> + * If nohz_full is enabled and we have isolated CPUs, their combination must
> + * still leave housekeeping CPUs.
> + */
> +static bool isolated_cpus_can_update(struct cpumask *add_cpus,
> +				     struct cpumask *del_cpus)
> +{
> +	cpumask_var_t full_hk_cpus;
> +	int res = true;
> +
> +	if (!housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
> +		return true;
> +
> +	if (del_cpus && cpumask_weight_and(del_cpus,
> +			housekeeping_cpumask(HK_TYPE_KERNEL_NOISE)))
> +		return true;
> +
> +	if (!alloc_cpumask_var(&full_hk_cpus, GFP_KERNEL))
> +		return false;
> +
> +	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_active_mask);
> +	if (!cpumask_weight_andnot(full_hk_cpus, add_cpus))
> +		res = false;
> +
> +	free_cpumask_var(full_hk_cpus);
> +	return res;
> +}
> +
>   static void update_exclusion_cpumasks(bool isolcpus_updated)
>   {
>   	int ret;
> @@ -1464,6 +1513,9 @@ 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) &&
> +	    !isolated_cpus_can_update(tmp->new_cpus, NULL))
> +		return PERR_HKEEPING;
>   
>   	spin_lock_irq(&callback_lock);
>   	isolcpus_updated = partition_xcpus_add(new_prs, NULL, tmp->new_cpus);
> @@ -1563,6 +1615,9 @@ 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) &&
> +			 !isolated_cpus_can_update(tmp->addmask, tmp->delmask))
> +			cs->prs_err = PERR_HKEEPING;
>   		if (cs->prs_err)
>   			goto invalidate;
>   	}
> @@ -1914,6 +1969,12 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>   			return err;
>   	}
>   
> +	if (deleting && isolated_cpus_should_update(new_prs, parent) &&
> +	    !isolated_cpus_can_update(tmp->delmask, tmp->addmask)) {
> +		cs->prs_err = PERR_HKEEPING;
> +		return PERR_HKEEPING;
> +	}
> +
>   	/*
>   	 * Change the parent's effective_cpus & effective_xcpus (top cpuset
>   	 * only).
> @@ -2934,6 +2995,8 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>   		 * Need to update isolated_cpus.
>   		 */
>   		isolcpus_updated = true;
> +		if (!isolated_cpus_can_update(cs->effective_xcpus, NULL))
> +			err = PERR_HKEEPING;
>   	} else {
>   		/*
>   		 * Switching back to member is always allowed even if it
Reviewed-by: Waiman Long <longman@redhat.com>

Re: [PATCH v12 9/9] timers: Exclude isolated cpus from timer migration

[John B. Wyatt IV]

On Mon, Sep 15, 2025 at 04:59:30PM +0200, Gabriele Monaco wrote:

Your patchset continues to pass when applied against v6.17-rc4-rt3 on a
preview of RHEL 10.2.

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

duration:   0 00:30:00 | time is in us
CPU Period       Runtime        Noise  % CPU Aval   Max Noise   Max Single          HW          NMI          IRQ      Softirq       Thread
  1 #1799     1799000001      3351316    99.81371        2336            9         400            0      1799011            0        23795

> 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:
> 

If you do another version; you may want to amend the cover letter to include
this affect can be noticed with a machine with as few as 20cores/40threads
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>

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

Re: [PATCH v12 9/9] timers: Exclude isolated cpus from timer migration

[Gabriele Monaco]

2025-09-15T20:51:21Z John B. Wyatt IV <jwyatt@redhat.com>:

> On Mon, Sep 15, 2025 at 04:59:30PM +0200, Gabriele Monaco wrote:
>
> Your patchset continues to pass when applied against v6.17-rc4-rt3 on a
> preview of RHEL 10.2.
>
> rtla osnoise top -c 1 -e sched:sched_switch -s 20 -T 1 -t -d 30m -q
>
> duration:   0 00:30:00 | time is in us
> CPU Period       Runtime        Noise  % CPU Aval   Max Noise   Max Single          HW          NMI          IRQ      Softirq       Thread
>   1 #1799     1799000001      3351316    99.81371        2336            9         400            0      1799011            0        23795
>
>> 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:
>>
>
> If you do another version; you may want to amend the cover letter to include
> this affect can be noticed with a machine with as few as 20cores/40threads
> 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>
>

Thanks John for testing again, I'll mention your results with the next version.

Cheers,
Gabriele

Re: [PATCH v12 7/9] cgroup/cpuset: Fail if isolated and nohz_full don't leave any housekeeping

[Chen Ridong]

On 2025/9/15 22:59, 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 | 63 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 63 insertions(+)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 81a9239053a7..3cedc3580373 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;
> +}
> +

Hi all,

I'm a bit confused about the logic for updating isolated CPUs.

As I understand it, the isolated_cpus set should be updated in two scenarios:
1. When changing to an isolated partition.
2. When a valid isolated partition becomes invalid or changes its membership.

However, I find the current approach of comparing the parent's partition_root_state with prs to
determine whether to update the isolated CPUs somewhat difficult to follow.

Wouldn't a more straightforward approach be something like this?

static bool isolated_cpus_should_update(int old_prs, int new_prs)
{
    if (old_prs == new_prs)
        return false;

    /* Changing to an isolated partition */
    if (new_prs == PRS_ISOLATED)
        return true;

    /* Isolated partition changing to another state */
    if (old_prs == PRS_ISOLATED)
        return true;

    return false;
}

I'd greatly appreciate it if someone could help clarify this. Thank you.

>  /*
>   * partition_xcpus_add - Add new exclusive CPUs to partition
>   * @new_prs: new partition_root_state
> @@ -1339,6 +1352,42 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
>  	return isolcpus_updated;
>  }
>  
> +/*
> + * isolated_cpus_can_update - check for isolated & nohz_full conflicts
> + * @add_cpus: cpu mask for cpus that are going to be isolated
> + * @del_cpus: cpu mask for cpus that are no longer isolated, can be NULL
> + * Return: false if there is conflict, true otherwise
> + *
> + * If nohz_full is enabled and we have isolated CPUs, their combination must
> + * still leave housekeeping CPUs.
> + */
> +static bool isolated_cpus_can_update(struct cpumask *add_cpus,
> +				     struct cpumask *del_cpus)
> +{
> +	cpumask_var_t full_hk_cpus;
> +	int res = true;
> +
> +	if (!housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
> +		return true;
> +
> +	if (del_cpus && cpumask_weight_and(del_cpus,
> +			housekeeping_cpumask(HK_TYPE_KERNEL_NOISE)))
> +		return true;
> +
> +	if (!alloc_cpumask_var(&full_hk_cpus, GFP_KERNEL))
> +		return false;
> +
> +	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_active_mask);
> +	if (!cpumask_weight_andnot(full_hk_cpus, add_cpus))
> +		res = false;
> +
> +	free_cpumask_var(full_hk_cpus);
> +	return res;
> +}
> +
>  static void update_exclusion_cpumasks(bool isolcpus_updated)
>  {
>  	int ret;
> @@ -1464,6 +1513,9 @@ 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) &&
> +	    !isolated_cpus_can_update(tmp->new_cpus, NULL))
> +		return PERR_HKEEPING;
>  
>  	spin_lock_irq(&callback_lock);
>  	isolcpus_updated = partition_xcpus_add(new_prs, NULL, tmp->new_cpus);
> @@ -1563,6 +1615,9 @@ 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) &&
> +			 !isolated_cpus_can_update(tmp->addmask, tmp->delmask))
> +			cs->prs_err = PERR_HKEEPING;
>  		if (cs->prs_err)
>  			goto invalidate;
>  	}
> @@ -1914,6 +1969,12 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>  			return err;
>  	}
>  
> +	if (deleting && isolated_cpus_should_update(new_prs, parent) &&
> +	    !isolated_cpus_can_update(tmp->delmask, tmp->addmask)) {
> +		cs->prs_err = PERR_HKEEPING;
> +		return PERR_HKEEPING;
> +	}
> +
>  	/*
>  	 * Change the parent's effective_cpus & effective_xcpus (top cpuset
>  	 * only).
> @@ -2934,6 +2995,8 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>  		 * Need to update isolated_cpus.
>  		 */
>  		isolcpus_updated = true;
> +		if (!isolated_cpus_can_update(cs->effective_xcpus, NULL))
> +			err = PERR_HKEEPING;
>  	} else {
>  		/*
>  		 * Switching back to member is always allowed even if it

-- 
Best regards,
Ridong

Re: [PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Frederic Weisbecker]

Le Mon, Sep 15, 2025 at 02:35:12PM -0400, Yury Norov a écrit :
> On Mon, Sep 15, 2025 at 05:02:45PM +0000, Gabriele Monaco wrote:
> > 2025-09-15T16:04:53Z Yury Norov <yury.norov@gmail.com>:
> > > So why don't you pick the original patch instead?
> > 
> > In my opinion, the /juice/ of that patch was included with [1], here I'm just adding part of it.
> > If you prefer I can pick that patch and adapt the commit message to reflect only the part included here.
> > 
> > [1] - https://lore.kernel.org/lkml/1706509267-17754-3-git-send-email-schakrabarti@linux.microsoft.com/
> 
> Yes please.

And can we rename CPUMASK_NULL to CPUMASK_VAR_NULL to avoid accidents/confusion with real
cpumasks initializations?

Thanks.

-- 
Frederic Weisbecker
SUSE Labs

Re: [PATCH v12 9/9] timers: Exclude isolated cpus from timer migration

[Frederic Weisbecker]

Le Mon, Sep 15, 2025 at 04:59:30PM +0200, Gabriele Monaco a écrit :
> Tested-by: John B. Wyatt IV <jwyatt@redhat.com>
> Tested-by: John B. Wyatt IV <sageofredondo@gmail.com>

Two people, one tester? :-)

> +/**
> + * 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_NULL;
> +	cpumask_var_t cpumask_isol __free(free_cpumask_var) = CPUMASK_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);
> +	}

This is racy at various levels:

* The tmigr_available_cpumask clear can race with the cpumask_set_cpu() in
  tmigr_cpu_unisolate(), risking overwrites when CPUs are on the same bitmap
  chunk (bitmap operations aren't atomic).

* tmigr_cpu_isolate() and tmigr_cpu_unisolate() can now run concurrently and
  then cpumask_set_cpu() can race with cpumask_clear_cpu() on
  tmigr_available_cpumask, risking overwrites, though the real problem is
  on the precedent point.
  
* tmigr_cpu_isolate() can race with tmigr_cpu_isolate() on other CPUs so
  the calls to cpumask_clear_cpu() can race. That's fine because
  tmigr_available_cpumask is already set to those CPUs but that still
  leaves un uncomfortable taste. That would leave an excuse for KSCAN to warn
  for example.

* Similar with tmigr_cpu_unisolate() racing together.

So, something like that should be added?

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 08e29fc01479..6615e56c8b0d 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1473,7 +1473,6 @@ 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;
@@ -1489,11 +1488,11 @@ 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)
@@ -1506,7 +1505,7 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 
 	if (tmigr_is_isolated(cpu))
 		return 0;
-	cpumask_set_cpu(cpu, tmigr_available_cpumask);
+
 	scoped_guard(raw_spinlock_irq, &tmc->lock) {
 		if (tmc->available)
 			return 0;
@@ -1516,7 +1515,19 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 			__tmigr_cpu_activate(tmc);
 		tmc->available = true;
 	}
-	return 0;
+	return 1;
+}
+
+static int tmigr_online_cpu(unsigned int cpu)
+{
+	if (tmigr_set_cpu_available(cpu) > 0)
+		cpumask_set_cpu(cpu, tmigr_available_cpumask);
+}
+
+static int tmigr_offline_cpu(unsigned int cpu)
+{
+	if (tmigr_clear_cpu_available(cpu) > 0)
+		cpumask_clear_cpu(cpu, tmigr_available_cpumask);
 }
 
 static void tmigr_cpu_isolate(struct work_struct *ignored)
@@ -1601,8 +1612,7 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
 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);
+				 tmigr_online_cpu, tmigr_offline_cpu);
 }
 
 static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,

-- 
Frederic Weisbecker
SUSE Labs

Re: [PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Gabriele Monaco]

On Tue, 2025-09-16 at 13:27 +0200, Frederic Weisbecker wrote:
> Le Mon, Sep 15, 2025 at 02:35:12PM -0400, Yury Norov a écrit :
> > On Mon, Sep 15, 2025 at 05:02:45PM +0000, Gabriele Monaco wrote:
> > > 2025-09-15T16:04:53Z Yury Norov <yury.norov@gmail.com>:
> > > > So why don't you pick the original patch instead?
> > > 
> > > In my opinion, the /juice/ of that patch was included with [1],
> > > here I'm just adding part of it.
> > > If you prefer I can pick that patch and adapt the commit message
> > > to reflect only the part included here.
> > > 
> > > [1] -
> > > https://lore.kernel.org/lkml/1706509267-17754-3-git-send-email-schakrabarti@linux.microsoft.com/
> > 
> > Yes please.

Alright, will use your commit in v13 while changing the macro name to
CPUMASK_VAR_NULL as suggested.

> And can we rename CPUMASK_NULL to CPUMASK_VAR_NULL to avoid
> accidents/confusion with real
> cpumasks initializations?

Note that in the linked commit message, you have what I believe is an
incorrect assumption:

> So define a CPUMASK_VAR_NULL macro, ... and effectively a no-op
> when CPUMASK_OFFSTACK is disabled.

According to what I can understand from the standard, the C list
initialisation sets to the default value (e.g. 0) all elements not
present in the initialiser. Since in {} no element is present, {} is
not a no-op but it initialises the entire cpumask to 0.

Am I missing your original intent here?
It doesn't look like a big price to pay, but I'd still reword the
sentence to something like:
"and a valid struct initializer when CPUMASK_OFFSTACK is disabled."

Thanks,
Gabriele

Re: [PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Yury Norov]

On Wed, Sep 17, 2025 at 09:51:47AM +0200, Gabriele Monaco wrote:
> 
> 
> On Tue, 2025-09-16 at 13:27 +0200, Frederic Weisbecker wrote:
> > Le Mon, Sep 15, 2025 at 02:35:12PM -0400, Yury Norov a écrit :
> > > On Mon, Sep 15, 2025 at 05:02:45PM +0000, Gabriele Monaco wrote:
> > > > 2025-09-15T16:04:53Z Yury Norov <yury.norov@gmail.com>:
> > > > > So why don't you pick the original patch instead?
> > > > 
> > > > In my opinion, the /juice/ of that patch was included with [1],
> > > > here I'm just adding part of it.
> > > > If you prefer I can pick that patch and adapt the commit message
> > > > to reflect only the part included here.
> > > > 
> > > > [1] -
> > > > https://lore.kernel.org/lkml/1706509267-17754-3-git-send-email-schakrabarti@linux.microsoft.com/
> > > 
> > > Yes please.
> 
> Alright, will use your commit in v13 while changing the macro name to
> CPUMASK_VAR_NULL as suggested.
> 
> > And can we rename CPUMASK_NULL to CPUMASK_VAR_NULL to avoid
> > accidents/confusion with real
> > cpumasks initializations?
> 
> Note that in the linked commit message, you have what I believe is an
> incorrect assumption:
> 
> > So define a CPUMASK_VAR_NULL macro, ... and effectively a no-op
> > when CPUMASK_OFFSTACK is disabled.
> 
> According to what I can understand from the standard, the C list
> initialisation sets to the default value (e.g. 0) all elements not
> present in the initialiser. Since in {} no element is present, {} is
> not a no-op but it initialises the entire cpumask to 0.
> 
> Am I missing your original intent here?
> It doesn't look like a big price to pay, but I'd still reword the
> sentence to something like:
> "and a valid struct initializer when CPUMASK_OFFSTACK is disabled."

The full quote is:

  So define a CPUMASK_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.

If you read the 'which allows' part, it makes more sense, isn't?

Re: [PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Gabriele Monaco]

On Wed, 2025-09-17 at 07:38 -0400, Yury Norov wrote:
> On Wed, Sep 17, 2025 at 09:51:47AM +0200, Gabriele Monaco wrote:
> > According to what I can understand from the standard, the C list
> > initialisation sets to the default value (e.g. 0) all elements not
> > present in the initialiser. Since in {} no element is present, {}
> > is not a no-op but it initialises the entire cpumask to 0.
> > 
> > Am I missing your original intent here?
> > It doesn't look like a big price to pay, but I'd still reword the
> > sentence to something like:
> > "and a valid struct initializer when CPUMASK_OFFSTACK is disabled."
> 
> The full quote is:
> 
>   So define a CPUMASK_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.
> 
> If you read the 'which allows' part, it makes more sense, isn't?

Alright, my bad for trimming the sentence, what I wanted to highlight
is that with !CPUMASK_OFFSTACK this CPUMASK_NULL becomes something like

  struct cpumask mask[1] = {};

which, to me, doesn't look like a no-op as the description suggests,
but an initialisation of the entire array.

Now I'm not sure if the compiler would be smart enough to optimise this
assignment out, but it doesn't look obvious to me.

Unless you were meaning the __free() becomes a no-op (which is true but
out of scope in this version of the patch), I would avoid mentioning
the no-op altogether.

Am I missing something and that initialisation is proven to be compiled
out?

Re: [PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Yury Norov]

On Wed, Sep 17, 2025 at 02:08:06PM +0200, Gabriele Monaco wrote:
> On Wed, 2025-09-17 at 07:38 -0400, Yury Norov wrote:
> > On Wed, Sep 17, 2025 at 09:51:47AM +0200, Gabriele Monaco wrote:
> > > According to what I can understand from the standard, the C list
> > > initialisation sets to the default value (e.g. 0) all elements not
> > > present in the initialiser. Since in {} no element is present, {}
> > > is not a no-op but it initialises the entire cpumask to 0.
> > > 
> > > Am I missing your original intent here?
> > > It doesn't look like a big price to pay, but I'd still reword the
> > > sentence to something like:
> > > "and a valid struct initializer when CPUMASK_OFFSTACK is disabled."
> > 
> > The full quote is:
> > 
> >   So define a CPUMASK_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.
> > 
> > If you read the 'which allows' part, it makes more sense, isn't?
> 
> Alright, my bad for trimming the sentence, what I wanted to highlight
> is that with !CPUMASK_OFFSTACK this CPUMASK_NULL becomes something like
> 
>   struct cpumask mask[1] = {};
> 
> which, to me, doesn't look like a no-op as the description suggests,
> but an initialisation of the entire array.
> 
> Now I'm not sure if the compiler would be smart enough to optimise this
> assignment out, but it doesn't look obvious to me.
> 
> Unless you were meaning the __free() becomes a no-op (which is true but
> out of scope in this version of the patch), I would avoid mentioning
> the no-op altogether.
> 
> Am I missing something and that initialisation is proven to be compiled
> out?

When you create a non-initialized variable on stack, compiler does
nothing about it, except for adjusting an argument to brk() emitted in
the function prologue.

In this case, non-initialized struct cpumask is already on stack, and
switching from

        struct cpumask mask[1];

to

        struct cpumask mask[1] = {};

is really a no-op.

Thanks,
Yury

Re: [PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Yury Norov]

On Wed, Sep 17, 2025 at 08:34:54AM -0400, Yury Norov wrote:
> On Wed, Sep 17, 2025 at 02:08:06PM +0200, Gabriele Monaco wrote:
> > On Wed, 2025-09-17 at 07:38 -0400, Yury Norov wrote:
> > > On Wed, Sep 17, 2025 at 09:51:47AM +0200, Gabriele Monaco wrote:
> > > > According to what I can understand from the standard, the C list
> > > > initialisation sets to the default value (e.g. 0) all elements not
> > > > present in the initialiser. Since in {} no element is present, {}
> > > > is not a no-op but it initialises the entire cpumask to 0.
> > > > 
> > > > Am I missing your original intent here?
> > > > It doesn't look like a big price to pay, but I'd still reword the
> > > > sentence to something like:
> > > > "and a valid struct initializer when CPUMASK_OFFSTACK is disabled."
> > > 
> > > The full quote is:
> > > 
> > >   So define a CPUMASK_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.
> > > 
> > > If you read the 'which allows' part, it makes more sense, isn't?
> > 
> > Alright, my bad for trimming the sentence, what I wanted to highlight
> > is that with !CPUMASK_OFFSTACK this CPUMASK_NULL becomes something like
> > 
> >   struct cpumask mask[1] = {};
> > 
> > which, to me, doesn't look like a no-op as the description suggests,
> > but an initialisation of the entire array.
> > 
> > Now I'm not sure if the compiler would be smart enough to optimise this
> > assignment out, but it doesn't look obvious to me.
> > 
> > Unless you were meaning the __free() becomes a no-op (which is true but
> > out of scope in this version of the patch), I would avoid mentioning
> > the no-op altogether.
> > 
> > Am I missing something and that initialisation is proven to be compiled
> > out?
> 
> When you create a non-initialized variable on stack, compiler does
> nothing about it, except for adjusting an argument to brk() emitted in
> the function prologue.
> 
> In this case, non-initialized struct cpumask is already on stack, and
> switching from
> 
>         struct cpumask mask[1];
> 
> to
> 
>         struct cpumask mask[1] = {};
> 
> is really a no-op.

Alright... The above is correct for optimization levels > 0.
With -O0, 2nd version really makes GCC to initialize the array.

https://godbolt.org/z/e1zG4K7M8

This is not relevant for the kernel because -O2 is our default
optimization level (I'm not even sure that -O0 is buildable).
But you may want to mention that in commit message.

Thanks,
Yury

Re: [PATCH v12 8/9] cpumask: Add initialiser CPUMASK_NULL to use cleanup helpers

[Gabriele Monaco]

2025-09-19T14:58:18Z Yury Norov <yury.norov@gmail.com>:

>> When you create a non-initialized variable on stack, compiler does
>> nothing about it, except for adjusting an argument to brk() emitted in
>> the function prologue.
>>
>> In this case, non-initialized struct cpumask is already on stack, and
>> switching from
>>
>>         struct cpumask mask[1];
>>
>> to
>>
>>         struct cpumask mask[1] = {};
>>
>> is really a no-op.
>
> Alright... The above is correct for optimization levels > 0.
> With -O0, 2nd version really makes GCC to initialize the array.
>
> https://godbolt.org/z/e1zG4K7M8
>
> This is not relevant for the kernel because -O2 is our default
> optimization level (I'm not even sure that -O0 is buildable).
> But you may want to mention that in commit message.

Thanks for testing that!

Makes totally sense. Although if it's something really unreachable in the kernel it may not make too much sense to mention in the commit message.
Also I'm not planning on sending another version after V13 [1], so we may let it slide.

Thanks,
Gabriele

[1] - https://lore.kernel.org/lkml/20250917161958.178925-1-gmonaco@redhat.com