linux-kernel - Re: [PATCH v5 4/6] sched/isolation: Force housekeeping if isolcpus and nohz

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Message-ID: <aCxWwVy2UUmvinyZ@localhost.localdomain>
Date: Tue, 20 May 2025 12:17:37 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v5 4/6] sched/isolation: Force housekeeping if isolcpus
 and nohz_full don't leave any

Le Thu, May 08, 2025 at 04:53:24PM +0200, Gabriele Monaco a écrit :
> 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 setting the boot CPU as housekeeping if
> the union of isolcpus and nohz_full covers all CPUs. In a similar
> fashion as it already happens if either of them covers all CPUs.
> 
> Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
> ---
>  include/linux/tick.h     |  2 ++
>  kernel/sched/isolation.c | 20 ++++++++++++++++++++
>  kernel/time/tick-sched.c |  7 +++++++
>  3 files changed, 29 insertions(+)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index b8ddc8e631a3..0b32c0bd3512 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -278,6 +278,7 @@ static inline void tick_dep_clear_signal(struct signal_struct *signal,
>  extern void tick_nohz_full_kick_cpu(int cpu);
>  extern void __tick_nohz_task_switch(void);
>  extern void __init tick_nohz_full_setup(cpumask_var_t cpumask);
> +extern void __init tick_nohz_full_clear_cpu(unsigned int cpu);
>  #else
>  static inline bool tick_nohz_full_enabled(void) { return false; }
>  static inline bool tick_nohz_full_cpu(int cpu) { return false; }
> @@ -304,6 +305,7 @@ static inline void tick_dep_clear_signal(struct signal_struct *signal,
>  static inline void tick_nohz_full_kick_cpu(int cpu) { }
>  static inline void __tick_nohz_task_switch(void) { }
>  static inline void tick_nohz_full_setup(cpumask_var_t cpumask) { }
> +static inline void tick_nohz_full_clear_cpu(unsigned int cpu) { }
>  #endif
>  
>  static inline void tick_nohz_task_switch(void)
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 81bc8b329ef1..27b65b401534 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -165,6 +165,26 @@ 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=, using boot CPU:%d\n",
> +				smp_processor_id());
> +			for_each_set_bit(type, &housekeeping.flags, HK_TYPE_MAX)
> +				__cpumask_set_cpu(smp_processor_id(),
> +						  housekeeping.cpumasks[type]);
> +			__cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
> +			__cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask);
> +			tick_nohz_full_clear_cpu(smp_processor_id());
> +
> +			if (cpumask_empty(non_housekeeping_mask))
> +				goto free_housekeeping_staging;
> +		}
> +

Looking again at that, how is it possible to set a different CPU between
isolcpus= and nohz_full= ?

		enum hk_type type;
		unsigned long iter_flags = flags & housekeeping.flags;

		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX) {
			if (!cpumask_equal(housekeeping_staging,
					   housekeeping.cpumasks[type])) {
				pr_warn("Housekeeping: nohz_full= must match isolcpus=\n");
				goto free_housekeeping_staging;
			}
		}

-- 
Frederic Weisbecker
SUSE Labs