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

[Waiman Long <longman@redhat.com>]

On 11/4/26 7:58 AM, Ridong Chen wrote:
>
>
> 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))

The del_cpus should normally be (cs->effective_xcpus & ~xcpus), but the 
top_cpuset_sd_nocpu() helper will skip CPUs that are in xcpus. So it 
will work in this case. It should be noted in the functional comment of 
top_cpuset_sd_nocpu() that @del_cpus can include common CPUs that will 
stay in the partition which will be skipped in the helper so that future 
change won't accidentally change that behavior.

Other than that, the code looks good to me.

Cheers,
Longman

> + 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))
>