Re: [PATCH] cgroup/cpuset: Fail if isolated CPUs don't leave any boot domain housekeeping CPU

[Ridong Chen <ridong.chen@linux.dev>]

On 11/4/2026 3:16 PM, Guopeng Zhang wrote:
> From: Guopeng Zhang<zhangguopeng@kylinos.cn>
> 
> With the "domain" flag of the isolcpus= boot option, the CPUs isolated
> at boot are excluded from the sched domains and the remaining CPUs
> become the boot domain housekeeping CPUs. An isolated partition is
> allowed to use the boot isolated CPUs, but it must not take over all
> the boot domain housekeeping CPUs.
> 
> prstate_housekeeping_conflict() checks that rule for root partitions
> only. The 2nd check covers the nohz_full case as it is gated by
> housekeeping_enabled(HK_TYPE_KERNEL_NOISE) which is not enabled by
> isolcpus=domain. So on an isolcpus=domain,<cpus> boot, an isolated
> partition can claim all the remaining boot domain housekeeping CPUs.
> The state is committed, but the sched domain rebuild at the end of
> the write finds no housekeeping CPU left for the boot domain and
> hits WARNs and an OOPS in partition_sched_domains().
> housekeeping_update() cannot accept the configuration either as it
> cannot empty the boot domain housekeeping mask.
> With an "isolcpus=domain,10-31" boot where CPUs 0-9 are the boot
> domain housekeeping CPUs, for example:
> 
>    # echo 0-9 > A/cpuset.cpus
>    # echo isolated > A/cpuset.cpus.partition
>    WARNING: kernel/sched/topology.c:3089 at build_sched_domains+...
>    WARNING: kernel/cgroup/cpuset.c:1478 at cpuset_update_sd_hk_unlock+...
>    RIP: 0010:partition_sched_domains+0x3b8
> 
> Fix that by extending prstate_housekeeping_conflict() to reject an
> isolated partition update that would leave no active boot domain
> housekeeping CPU outside the isolated CPUs. The CPUs released back
> to the boot domain by the same update are taken into account, so
> replacing one of the last housekeeping CPUs with another one in a
> single update remains allowed. The new check is only performed when
> housekeeping_enabled(HK_TYPE_KERNEL_NOISE) is disabled so that the
> existing nohz_full behavior is unchanged.
> 
> Add test cases in test_cpuset_prs.sh to verify that taking over all
> the boot domain housekeeping CPUs is rejected while swapping one of
> them with another one is allowed.

Hi Guopeng,

Thanks for the report and the reproducer. I can reproduce the WARN and
the OOPS in partition_sched_domains() here.

However, I don't think prstate_housekeeping_conflict() is the right
place to fix it, and the fix doesn't seem complete. The same problem
can be triggered with a root partition, which this patch doesn't cover.
With an "isolcpus=domain,4-7" boot on an 8-CPU VM:

   # echo 0-3 > A/cpuset.cpus
   # echo root > A/cpuset.cpus.partition
   WARNING: kernel/sched/topology.c:3111 at build_sched_domains+0x2b96/0x5500
   Oops: general protection fault, ... [#1] SMP KASAN NOPTI
   RIP: 0010:partition_sched_domains+0x10a3/0x1a10

The faulting address is in build_perf_domains(), where cpumask_first()
on the empty cpu_map returns nr_cpu_ids and cpu_rq() goes out of
bounds.

The root cause looks like it isn't a housekeeping conflict. Boot time
isolated CPUs stay in top_cpuset.effective_cpus by design, and they are
only filtered out of the top_cpuset sched domain in
generate_sched_domains(). But the checks that make sure the parent
still has CPUs left, i.e. tasks_nocpu_error() for local partitions and
the cpumask_subset(top_cpuset.effective_cpus, ...) checks in
remote_partition_enable()/remote_cpus_update(), only look at
top_cpuset.effective_cpus. So any partition, root or isolated, local
or remote, can take away all the boot domain housekeeping CPUs as long
as some boot isolated CPUs are still left in top_cpuset, which leaves
the top_cpuset sched domain empty.

So I think the check should be done at the "parent has no CPU left"
level instead. Something like the diff below. It is not tested yet;
could you give it a try with your reproducer and the test cases?
Note that the root partition and remote partition cases should be
added to the selftest as well.

Without isolcpus=domain, HK_TYPE_DOMAIN_BOOT is cpu_possible_mask and
the new check is equivalent to the existing one, so no extra
housekeeping_enabled() gating is needed.

---
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 3219f927c35e..fb3e81405f5b 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1180,12 +1180,51 @@ static void update_partition_sd_lb(struct cpuset *cs, 
int old_prs)
                 cpuset_force_rebuild();
  }

+/*
+ * top_cpuset_sd_nocpu - Check if top_cpuset will have no sched domain CPU
+ * @add_cpus: CPUs to be taken away from top_cpuset
+ * @del_cpus: CPUs to be given back to top_cpuset, can be NULL
+ * Return: true if no active CPU will be left in the top_cpuset sched domain
+ *
+ * Boot time isolated CPUs stay in top_cpuset.effective_cpus, but they are
+ * excluded from the sched domain of top_cpuset in generate_sched_domains().
+ * So checking top_cpuset.effective_cpus alone is not enough to ensure that
+ * the top_cpuset sched domain won't become empty after @add_cpus are taken
+ * away from it. The sched domain of top_cpuset must keep at least one
+ * active boot domain housekeeping CPU.
+ */
+static bool top_cpuset_sd_nocpu(const struct cpumask *add_cpus,
+                               const struct cpumask *del_cpus)
+{
+       const struct cpumask *hk_mask = housekeeping_cpumask(HK_TYPE_DOMAIN_BOOT);
+       int cpu;
+
+       for_each_cpu_and(cpu, hk_mask, cpu_active_mask) {
+               if (cpumask_test_cpu(cpu, add_cpus))
+                       continue;
+               if (cpumask_test_cpu(cpu, top_cpuset.effective_cpus) ||
+                   (del_cpus && cpumask_test_cpu(cpu, del_cpus)))
+                       return false;
+       }
+       return true;
+}
+
  /*
   * tasks_nocpu_error - Return true if tasks will have no effective_cpus
   */
  static bool tasks_nocpu_error(struct cpuset *parent, struct cpuset *cs,
                               struct cpumask *xcpus)
  {
+       /*
+        * The top_cpuset always has tasks (kthreads at least), so its sched
+        * domain can't be empty. The current effective_xcpus of a valid
+        * partition will be given back to top_cpuset if not in @xcpus.
+        */
+       if ((parent == &top_cpuset) &&
+           top_cpuset_sd_nocpu(xcpus, is_partition_valid(cs)
+                                      ? cs->effective_xcpus : NULL))
+               return true;
+
         /*
          * A populated partition (cs or parent) can't have empty effective_cpus
          */
@@ -1724,7 +1763,7 @@ static int remote_partition_enable(struct cpuset *cs, int 
new_prs,
          */
         compute_excpus(cs, tmp->new_cpus);
         if (!cpumask_intersects(tmp->new_cpus, cpu_active_mask) ||
-           cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus))
+           top_cpuset_sd_nocpu(tmp->new_cpus, NULL))
                 return PERR_INVCPUS;
         if (cpumask_intersects(tmp->new_cpus, subpartitions_cpus))
                 return PERR_NOCPUS;
@@ -1828,7 +1867,7 @@ static void remote_cpus_update(struct cpuset *cs, struct 
cpumask *xcpus,
                 if (!capable(CAP_SYS_ADMIN))
                         WRITE_ONCE(cs->prs_err, PERR_ACCESS);
                 else if (cpumask_intersects(tmp->addmask, subpartitions_cpus) ||
-                        cpumask_subset(top_cpuset.effective_cpus, tmp->addmask))
+                        top_cpuset_sd_nocpu(tmp->addmask, tmp->delmask))
                         WRITE_ONCE(cs->prs_err, PERR_NOCPUS);
                 else if (prstate_housekeeping_conflict(prs, PRS_ROOT,
                                                        excpus, tmp->delmask))

-- 
Best regards
Ridong