Re: [PATCH v4] erofs: lazily initialize per-CPU workers and CPU hotplug hooks

[Chao Yu <chao@kernel.org>]

On 4/23/25 14:10, Sandeep Dhavale wrote:
> Currently, when EROFS is built with per-CPU workers, the workers are
> started and CPU hotplug hooks are registered during module initialization.
> This leads to unnecessary worker start/stop cycles during CPU hotplug
> events, particularly on Android devices that frequently suspend and resume.
> 
> This change defers the initialization of per-CPU workers and the
> registration of CPU hotplug hooks until the first EROFS mount. This
> ensures that these resources are only allocated and managed when EROFS is
> actually in use.
> 
> The tear down of per-CPU workers and unregistration of CPU hotplug hooks
> still occurs during z_erofs_exit_subsystem(), but only if they were
> initialized.
> 
> Signed-off-by: Sandeep Dhavale <dhavale@google.com>
> ---
> v3: https://lore.kernel.org/linux-erofs/20250422234546.2932092-1-dhavale@google.com/
> Changes since v3:
> - fold z_erofs_init_pcpu_workers() in the caller and rename the caller
> 
>  fs/erofs/zdata.c | 61 +++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 45 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
> index 0671184d9cf1..647a8340c9a1 100644
> --- a/fs/erofs/zdata.c
> +++ b/fs/erofs/zdata.c
> @@ -291,6 +291,9 @@ static struct workqueue_struct *z_erofs_workqueue __read_mostly;
>  
>  #ifdef CONFIG_EROFS_FS_PCPU_KTHREAD
>  static struct kthread_worker __rcu **z_erofs_pcpu_workers;
> +static atomic_t erofs_percpu_workers_initialized = ATOMIC_INIT(0);
> +static int erofs_cpu_hotplug_init(void);
> +static void erofs_cpu_hotplug_destroy(void);
>  
>  static void erofs_destroy_percpu_workers(void)
>  {
> @@ -336,9 +339,40 @@ static int erofs_init_percpu_workers(void)
>  	}
>  	return 0;
>  }
> +
> +static int z_erofs_init_pcpu_workers(void)
> +{
> +	int err;
> +
> +	if (atomic_xchg(&erofs_percpu_workers_initialized, 1))
> +		return 0;
> +
> +	err = erofs_init_percpu_workers();
> +	if (err)
> +		goto err_init_percpu_workers;
> +
> +	err = erofs_cpu_hotplug_init();
> +	if (err < 0)
> +		goto err_cpuhp_init;
> +	return err;
> +
> +err_cpuhp_init:
> +	erofs_destroy_percpu_workers();
> +err_init_percpu_workers:
> +	atomic_set(&erofs_percpu_workers_initialized, 0);
> +	return err;
> +}

- mount #1				- mount #2
 - z_erofs_init_pcpu_workers
  - atomic_xchg(, 1)
					 - z_erofs_init_pcpu_workers
					  - atomic_xchg(, 1)
					  : return 0 since atomic variable is 1
					  it will run w/o percpu workers and hotplug
  : update atomic variable to 1
  - erofs_init_percpu_workers
  : fail
  - atomic_set(, 0)
  : update atomic variable to 0 & fail the mount

Can we add some logs to show we succeed/fail to initialize workers or
hotplugs? As for mount #2, it expects it will run w/ them, but finally
it may not. So we'd better have a simple way to know?

Thanks,

> +
> +static void z_erofs_destroy_pcpu_workers(void)
> +{
> +	if (!atomic_xchg(&erofs_percpu_workers_initialized, 0))
> +		return;
> +	erofs_cpu_hotplug_destroy();
> +	erofs_destroy_percpu_workers();
> +}
>  #else
> -static inline void erofs_destroy_percpu_workers(void) {}
> -static inline int erofs_init_percpu_workers(void) { return 0; }
> +static inline int z_erofs_init_pcpu_workers(void) { return 0; }
> +static inline void z_erofs_destroy_pcpu_workers(void) {}
>  #endif
>  
>  #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_EROFS_FS_PCPU_KTHREAD)
> @@ -405,8 +439,7 @@ static inline void erofs_cpu_hotplug_destroy(void) {}
>  
>  void z_erofs_exit_subsystem(void)
>  {
> -	erofs_cpu_hotplug_destroy();
> -	erofs_destroy_percpu_workers();
> +	z_erofs_destroy_pcpu_workers();
>  	destroy_workqueue(z_erofs_workqueue);
>  	z_erofs_destroy_pcluster_pool();
>  	z_erofs_exit_decompressor();
> @@ -430,19 +463,8 @@ int __init z_erofs_init_subsystem(void)
>  		goto err_workqueue_init;
>  	}
>  
> -	err = erofs_init_percpu_workers();
> -	if (err)
> -		goto err_pcpu_worker;
> -
> -	err = erofs_cpu_hotplug_init();
> -	if (err < 0)
> -		goto err_cpuhp_init;
>  	return err;
>  
> -err_cpuhp_init:
> -	erofs_destroy_percpu_workers();
> -err_pcpu_worker:
> -	destroy_workqueue(z_erofs_workqueue);
>  err_workqueue_init:
>  	z_erofs_destroy_pcluster_pool();
>  err_pcluster_pool:
> @@ -644,10 +666,17 @@ static const struct address_space_operations z_erofs_cache_aops = {
>  
>  int z_erofs_init_super(struct super_block *sb)
>  {
> -	struct inode *const inode = new_inode(sb);
> +	struct inode *inode;
> +	int err;
>  
> +	err = z_erofs_init_pcpu_workers();
> +	if (err)
> +		return err;
> +
> +	inode = new_inode(sb);
>  	if (!inode)
>  		return -ENOMEM;
> +
>  	set_nlink(inode, 1);
>  	inode->i_size = OFFSET_MAX;
>  	inode->i_mapping->a_ops = &z_erofs_cache_aops;