[PATCH 02/13] kernel/cpu: Add support for declaring CPU hotplug not supported

[Kuppuswamy Sathyanarayanan]

On 10/5/2023 6:13 AM, Kirill A. Shutemov wrote:
> The function cpu_hotplug_not_supported() can be called to indicate that
> CPU hotplug should be disabled. It does not prevent the initial bring up
> of the CPU, but it stops subsequent offlining.
> 
> This function is intended to replace CC_ATTR_HOTPLUG_DISABLED.
> 

Looks good to me.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy at linux.intel.com>

> Signed-off-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
> ---
>  include/linux/cpu.h |  2 ++
>  kernel/cpu.c        | 17 ++++++++++++++++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index f19f56501809..aab3887cadbc 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -132,6 +132,7 @@ extern void cpus_read_lock(void);
>  extern void cpus_read_unlock(void);
>  extern int  cpus_read_trylock(void);
>  extern void lockdep_assert_cpus_held(void);
> +extern void cpu_hotplug_not_supported(void);
>  extern void cpu_hotplug_disable(void);
>  extern void cpu_hotplug_enable(void);
>  void clear_tasks_mm_cpumask(int cpu);
> @@ -147,6 +148,7 @@ static inline void cpus_read_lock(void) { }
>  static inline void cpus_read_unlock(void) { }
>  static inline int  cpus_read_trylock(void) { return true; }
>  static inline void lockdep_assert_cpus_held(void) { }
> +static inline void cpu_hotplug_not_supported(void) { }
>  static inline void cpu_hotplug_disable(void) { }
>  static inline void cpu_hotplug_enable(void) { }
>  static inline int remove_cpu(unsigned int cpu) { return -EPERM; }
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 6de7c6bb74ee..cf536fe1a88a 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -484,6 +484,9 @@ static int cpu_hotplug_disabled;
>  
>  DEFINE_STATIC_PERCPU_RWSEM(cpu_hotplug_lock);
>  
> +/* Cleared if platform declares CPU hotplug not supported */
> +static bool cpu_hotplug_supported = true;
> +
>  void cpus_read_lock(void)
>  {
>  	percpu_down_read(&cpu_hotplug_lock);
> @@ -543,6 +546,18 @@ static void lockdep_release_cpus_lock(void)
>  	rwsem_release(&cpu_hotplug_lock.dep_map, _THIS_IP_);
>  }
>  
> +/*
> + * Declare CPU hotplug not supported.
> + *
> + * It doesn't prevent initial bring up of the CPU, but stops offlining.
> + */
> +void cpu_hotplug_not_supported(void)
> +{
> +	cpu_maps_update_begin();
> +	cpu_hotplug_supported = false;
> +	cpu_maps_update_done();
> +}

Since this function is not used in this patch, do you need to add __maybe_unused to
avoid warnings?

> +
>  /*
>   * Wait for currently running CPU hotplug operations to complete (if any) and
>   * disable future CPU hotplug (from sysfs). The 'cpu_add_remove_lock' protects
> @@ -1507,7 +1522,7 @@ static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target)
>  	 * If the platform does not support hotplug, report it explicitly to
>  	 * differentiate it from a transient offlining failure.
>  	 */
> -	if (cc_platform_has(CC_ATTR_HOTPLUG_DISABLED))
> +	if (cc_platform_has(CC_ATTR_HOTPLUG_DISABLED) || !cpu_hotplug_supported)
>  		return -EOPNOTSUPP;
>  	if (cpu_hotplug_disabled)
>  		return -EBUSY;

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer