[PATCH 26/33] cgroup/cpuset: Fail if isolated and nohz_full don't leave any housekeeping

Frederic Weisbecker
2025-10-13T20:35:22Z

From: Gabriele Monaco <gmonaco@redhat.com>

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>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/cgroup/cpuset.c | 63 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 63 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index e19d3375a4ec..d1a799e361c3 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1327,6 +1327,19 @@ 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
@@ -1391,6 +1404,42 @@ 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_housekeeping_cpumask(bool isolcpus_updated)
{
int ret;
@@ -1538,6 +1587,9 @@ 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);
@@ -1637,6 +1689,9 @@ 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;
}
@@ -1984,6 +2039,12 @@ 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).
@@ -2999,6 +3060,8 @@ 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