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Message-ID: <a2ef7773-bec6-466f-81b3-e1d8f6cbe7b6@redhat.com>
Date: Thu, 31 Jul 2025 11:09:27 -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 6/8] sched/isolation: Force housekeeping if isolcpus
and nohz_full don't leave any
On 7/30/25 9:11 AM, Gabriele Monaco wrote:
> Currently the user can set up isolcpus 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 invalidating the last setting in case the
> union of isolcpus and nohz_full covers all CPUs.
>
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
> Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
> ---
> kernel/sched/isolation.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 93b038d48900..0019d941de68 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -165,6 +165,18 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
> }
> }
>
> + /* Check in combination with the previously set cpumask */
> + type = find_first_bit(&housekeeping.flags, HK_TYPE_MAX);
> + first_cpu = cpumask_first_and_and(cpu_present_mask,
> + housekeeping_staging,
> + housekeeping.cpumasks[type]);
> + if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
> + pr_warn("Housekeeping: must include one present CPU neither "
> + "in nohz_full= nor in isolcpus=, ignoring setting %s\n",
> + str);
> + goto free_housekeeping_staging;
> + }
> +
> iter_flags = flags & ~housekeeping.flags;
>
> for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
I do have a question about this check. Currently isolcpus=domain is bit
0, managed_irq is bit 1 and nohz_full is bit 2. If manage_irq come first
followed by nohz_full and then isolcpus=domain. By the time,
isolcpus=domain is being set, you are comparing its cpumask with that of
manage_irq, not nohz_full.
Perhaps you can reuse the non_housekeeping_mask for doing the check, e.g.
cpumask_and(non_housekeeping_mask, cpu_present_mask,
housekeeping_staging);
iter_flags = housekeeping.flags & ~flags;
for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
cpumask_and(non_housekeeping_mask,
non_housekeeping_mask, housekeeping.cpumasks[type]);
if (cpumask_empty(non_housekeeping_mask)) {
pr_warn(...
Regards,
Longman
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