linux-kernel - Re: [PATCH v11 6/8] sched/isolation: Force housekeeping if isolcpus and nohz

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Message-ID: <a4395bac-9c54-4a23-b5e0-2a84db9b4ec7@redhat.com>
Date: Tue, 12 Aug 2025 11:29:41 -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 v11 6/8] sched/isolation: Force housekeeping if isolcpus
 and nohz_full don't leave any

On 8/8/25 12:01 PM, 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 (domain) and nohz_full covers all CPUs.
>
> Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
> ---
>   kernel/sched/isolation.c | 23 +++++++++++++++++++++++
>   1 file changed, 23 insertions(+)
>
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index a4cf17b1fab0..3ad0d6df6a0a 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -167,6 +167,29 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
>   			}
>   		}
>   
> +		/*
> +		 * Check the combination of nohz_full and isolcpus=domain,
> +		 * necessary to avoid problems with the timer migration
> +		 * hierarchy. managed_irq is ignored by this check since it
> +		 * isn't considered in the timer migration logic.
> +		 */
> +		iter_flags = housekeeping.flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
> +		type = find_first_bit(&iter_flags, HK_TYPE_MAX);
> +		/*
> +		 * Pass the check if none of these flags were previously set or
> +		 * are not in the current selection.
> +		 */
> +		iter_flags = flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
> +		first_cpu = (type == HK_TYPE_MAX || !iter_flags) ? 0 :
> +			    cpumask_first_and_and(cpu_present_mask,
> +				    housekeeping_staging, housekeeping.cpumasks[type]);
> +		if (first_cpu >= min(nr_cpu_ids, setup_max_cpus)) {
> +			pr_warn("Housekeeping: must include one present CPU "
> +				"neither in nohz_full= nor in isolcpus=domain, "
> +				"ignoring setting %s\n", str);
> +			goto free_housekeeping_staging;
> +		}
> +
>   		iter_flags = flags & ~housekeeping.flags;
>   
>   		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
Reviewed-by: Waiman Long <longman@...hat.com>