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Message-ID: <20251105043848.382703-3-longman@redhat.com>
Date: Tue, 4 Nov 2025 23:38:45 -0500
From: Waiman Long <longman@...hat.com>
To: Tejun Heo <tj@...nel.org>,
Johannes Weiner <hannes@...xchg.org>,
Michal Koutný <mkoutny@...e.com>
Cc: cgroups@...r.kernel.org,
linux-kernel@...r.kernel.org,
Chen Ridong <chenridong@...wei.com>,
Gabriele Monaco <gmonaco@...hat.com>,
Frederic Weisbecker <frederic@...nel.org>,
Waiman Long <longman@...hat.com>
Subject: [cgroup/for-6.19 PATCH v3 2/5] cgroup/cpuset: Fail if isolated and nohz_full don't leave any housekeeping
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.
[longman: Remove isolated_cpus_should_update() and rewrite the checking
in update_prstate() and update_parent_effective_cpumask()]
Originally-by: Gabriele Monaco <gmonaco@...hat.com>
Signed-off-by: Waiman Long <longman@...hat.com>
---
kernel/cgroup/cpuset.c | 74 +++++++++++++++++++++++++++++++++++++++++-
1 file changed, 73 insertions(+), 1 deletion(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index da770dac955e..99622e90991a 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1393,6 +1393,45 @@ 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.
+ *
+ * TBD: Should consider merging this function into
+ * prstate_housekeeping_conflict().
+ */
+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_isolation_cpumasks(bool isolcpus_updated)
{
int ret;
@@ -1551,6 +1590,9 @@ 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 ((new_prs == PRS_ISOLATED) &&
+ !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);
@@ -1650,6 +1692,9 @@ 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 ((prs == PRS_ISOLATED) &&
+ !isolated_cpus_can_update(tmp->addmask, tmp->delmask))
+ cs->prs_err = PERR_HKEEPING;
if (cs->prs_err)
goto invalidate;
}
@@ -1750,6 +1795,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
int part_error = PERR_NONE; /* Partition error? */
int isolcpus_updated = 0;
struct cpumask *xcpus = user_xcpus(cs);
+ int parent_prs = parent->partition_root_state;
bool nocpu;
lockdep_assert_held(&cpuset_mutex);
@@ -1813,6 +1859,10 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
if (prstate_housekeeping_conflict(new_prs, xcpus))
return PERR_HKEEPING;
+ if ((new_prs == PRS_ISOLATED) && (new_prs != parent_prs) &&
+ !isolated_cpus_can_update(xcpus, NULL))
+ return PERR_HKEEPING;
+
if (tasks_nocpu_error(parent, cs, xcpus))
return PERR_NOCPUS;
@@ -1866,6 +1916,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
*
* For invalid partition:
* delmask = newmask & parent->effective_xcpus
+ * The partition may become valid soon.
*/
if (is_partition_invalid(cs)) {
adding = false;
@@ -1880,6 +1931,23 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
deleting = cpumask_and(tmp->delmask, tmp->delmask,
parent->effective_xcpus);
}
+
+ /*
+ * TBD: Invalidate a currently valid child root partition may
+ * still break isolated_cpus_can_update() rule if parent is an
+ * isolated partition.
+ */
+ if (is_partition_valid(cs) && (old_prs != parent_prs)) {
+ if ((parent_prs == PRS_ROOT) &&
+ /* Adding to parent means removing isolated CPUs */
+ !isolated_cpus_can_update(tmp->delmask, tmp->addmask))
+ part_error = PERR_HKEEPING;
+ if ((parent_prs == PRS_ISOLATED) &&
+ /* Adding to parent means adding isolated CPUs */
+ !isolated_cpus_can_update(tmp->addmask, tmp->delmask))
+ part_error = PERR_HKEEPING;
+ }
+
/*
* The new CPUs to be removed from parent's effective CPUs
* must be present.
@@ -2994,7 +3062,11 @@ static int update_prstate(struct cpuset *cs, int new_prs)
* A change in load balance state only, no change in cpumasks.
* Need to update isolated_cpus.
*/
- isolcpus_updated = true;
+ if ((new_prs == PRS_ISOLATED) &&
+ !isolated_cpus_can_update(cs->effective_xcpus, NULL))
+ err = PERR_HKEEPING;
+ else
+ isolcpus_updated = true;
} else {
/*
* Switching back to member is always allowed even if it
--
2.51.1
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