Re: [PATCH] sched/isolation: fix boot crash when the boot CPU is nohz_full

[Phil Auld <pauld@redhat.com>]

On Thu, Apr 11, 2024 at 04:39:05PM +0200 Oleg Nesterov wrote:
> Documentation/timers/no_hz.rst states that the "nohz_full=" mask must not
> include the boot CPU, this is no longer true after the commit 08ae95f4fd3b
> ("nohz_full: Allow the boot CPU to be nohz_full").
> 
> However after another commit aae17ebb53cd ("workqueue: Avoid using isolated
> cpus' timers on queue_delayed_work") the kernel will crash at boot time in
> this case; housekeeping_any_cpu() returns an invalid cpu nr until smp_init()
> paths bring the 1st housekeeping CPU up.
> 
> Change housekeeping_any_cpu() to check the result of cpumask_any_and() and
> return smp_processor_id() in this case. Yes, this is just the simple and
> backportable workaround which fixes the symptom, but smp_processor_id() at
> boot time should be safe at least for type == HK_TYPE_TIMER, this more or
> less matches the tick_do_timer_boot_cpu logic.
> 
> We should not worry about cpu_down(); tick_nohz_cpu_down() will not allow
> to offline tick_do_timer_cpu (the 1st online housekeeping CPU).
> 
> Fixes: aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue_delayed_work")
> Reported-by: Chris von Recklinghausen <crecklin@redhat.com>
> Closes: https://lore.kernel.org/all/20240402105847.GA24832@redhat.com/
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Checking the returned value instead of assuming seems safer in any case.

Reviewed-by: Phil Auld <pauld@redhat.com>

> ---
>  Documentation/timers/no_hz.rst |  7 ++-----
>  kernel/sched/isolation.c       | 11 ++++++++++-
>  2 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/timers/no_hz.rst b/Documentation/timers/no_hz.rst
> index f8786be15183..7fe8ef9718d8 100644
> --- a/Documentation/timers/no_hz.rst
> +++ b/Documentation/timers/no_hz.rst
> @@ -129,11 +129,8 @@ adaptive-tick CPUs:  At least one non-adaptive-tick CPU must remain
>  online to handle timekeeping tasks in order to ensure that system
>  calls like gettimeofday() returns accurate values on adaptive-tick CPUs.
>  (This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running
> -user processes to observe slight drifts in clock rate.)  Therefore, the
> -boot CPU is prohibited from entering adaptive-ticks mode.  Specifying a
> -"nohz_full=" mask that includes the boot CPU will result in a boot-time
> -error message, and the boot CPU will be removed from the mask.  Note that
> -this means that your system must have at least two CPUs in order for
> +user processes to observe slight drifts in clock rate.) Note that this
> +means that your system must have at least two CPUs in order for
>  CONFIG_NO_HZ_FULL=y to do anything for you.
>  
>  Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 373d42c707bc..2a262d3ecb3d 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -46,7 +46,16 @@ int housekeeping_any_cpu(enum hk_type type)
>  			if (cpu < nr_cpu_ids)
>  				return cpu;
>  
> -			return cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
> +			cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
> +			if (likely(cpu < nr_cpu_ids))
> +				return cpu;
> +			/*
> +			 * Unless we have another problem this can only happen
> +			 * at boot time before start_secondary() brings the 1st
> +			 * housekeeping CPU up.
> +			 */
> +			WARN_ON_ONCE(system_state == SYSTEM_RUNNING ||
> +				     type != HK_TYPE_TIMER);
>  		}
>  	}
>  	return smp_processor_id();
> -- 
> 2.25.1.362.g51ebf55
> 
> 
> 

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