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Message-ID: <bdbc1a11-1546-47e6-bb14-d3ad940907ed@redhat.com>
Date: Thu, 31 Jul 2025 11:39:47 -0400
From: Waiman Long <llong@...hat.com>
To: Gabriele Monaco <gmonaco@...hat.com>, linux-kernel@...r.kernel.org,
Anna-Maria Behnsen <anna-maria@...utronix.de>,
Frederic Weisbecker <frederic@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v9 7/8] cgroup/cpuset: Fail if isolated and nohz_full
don't leave any housekeeping
On 7/30/25 9:11 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@...nel.org>
> Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
> ---
> kernel/cgroup/cpuset.c | 56 ++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 56 insertions(+)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 6e3f44ffaa21..a946d85ce954 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 +1352,35 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
> return isolcpus_updated;
> }
>
> +/*
> + * isolcpus_nohz_conflict - check for isolated & nohz_full conflicts
> + * @new_cpus: cpu mask for cpus that are going to be isolated
> + * Return: true if there is conflict, false otherwise
> + *
> + * If nohz_full is enabled and we have isolated CPUs, their combination must
> + * still leave housekeeping CPUs.
> + */
> +static bool isolcpus_nohz_conflict(struct cpumask *new_cpus)
> +{
> + cpumask_var_t full_hk_cpus;
> + int res = false;
> +
> + if (!housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
> + return false;
> +
> + if (!alloc_cpumask_var(&full_hk_cpus, GFP_KERNEL))
> + return true;
> +
> + cpumask_and(full_hk_cpus, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE),
> + housekeeping_cpumask(HK_TYPE_DOMAIN));
> + cpumask_and(full_hk_cpus, full_hk_cpus, cpu_online_mask);
> + if (!cpumask_weight_andnot(full_hk_cpus, new_cpus))
> + res = true;
> +
> + free_cpumask_var(full_hk_cpus);
> + return res;
> +}
First of all, isolated_cpus currently include those CPUs excluded from
boot time isolcpus=domain setting, but it also include new isolated cpus
created by used by cpuset isolated partitions. Your current
isolcpus_nohz_conflicts() does not check isolated_cpus which I think is
incomplete.
Cheers,
Longman
> +
> static void update_exclusion_cpumasks(bool isolcpus_updated)
> {
> int ret;
> @@ -1464,6 +1506,9 @@ 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) &&
> + isolcpus_nohz_conflict(tmp->new_cpus))
> + return PERR_HKEEPING;
>
> spin_lock_irq(&callback_lock);
> isolcpus_updated = partition_xcpus_add(new_prs, NULL, tmp->new_cpus);
> @@ -1563,6 +1608,9 @@ 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) &&
> + isolcpus_nohz_conflict(tmp->addmask))
> + cs->prs_err = PERR_HKEEPING;
> if (cs->prs_err)
> goto invalidate;
> }
> @@ -1914,6 +1962,12 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
> return err;
> }
>
> + if (deleting && isolated_cpus_should_update(new_prs, parent) &&
> + isolcpus_nohz_conflict(tmp->delmask)) {
> + cs->prs_err = PERR_HKEEPING;
> + return PERR_HKEEPING;
> + }
> +
> /*
> * Change the parent's effective_cpus & effective_xcpus (top cpuset
> * only).
> @@ -2934,6 +2988,8 @@ static int update_prstate(struct cpuset *cs, int new_prs)
> * Need to update isolated_cpus.
> */
> isolcpus_updated = true;
> + if (isolcpus_nohz_conflict(cs->effective_xcpus))
> + err = PERR_HKEEPING;
> } else {
> /*
> * Switching back to member is always allowed even if it
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