linux-kernel - Re: [PATCH v5 1/2] kernfs: use hashed mutex and spinlock in place of global ones.

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Date:   Tue, 8 Feb 2022 12:27:25 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Imran Khan <imran.f.khan@...cle.com>
Cc:     tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/2] kernfs: use hashed mutex and spinlock in place of
 global ones.

On Sun, Feb 06, 2022 at 12:09:24PM +1100, Imran Khan wrote:
> diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
> index f9cc912c31e1b..cc49a6cd94154 100644
> --- a/fs/kernfs/kernfs-internal.h
> +++ b/fs/kernfs/kernfs-internal.h
> @@ -31,6 +31,7 @@ struct kernfs_iattrs {
>  	atomic_t		user_xattr_size;
>  };
>  
> +
>  /* +1 to avoid triggering overflow warning when negating it */
>  #define KN_DEACTIVATED_BIAS		(INT_MIN + 1)
>  

Nit, the above change isn't needed :)

> @@ -147,4 +148,54 @@ void kernfs_drain_open_files(struct kernfs_node *kn);
>   */
>  extern const struct inode_operations kernfs_symlink_iops;
>  
> +static inline spinlock_t *kernfs_open_node_lock_ptr(struct kernfs_node *kn)
> +{
> +	struct kernfs_root *root;
> +	int idx = hash_ptr(kn, NR_KERNFS_LOCK_BITS);
> +
> +	root = kernfs_root(kn);
> +
> +	return &root->open_node_locks[idx].lock;
> +}
> +
> +static inline spinlock_t *kernfs_open_node_lock(struct kernfs_node *kn)
> +{
> +	struct kernfs_root *root;
> +	spinlock_t *lock;
> +	int idx = hash_ptr(kn, NR_KERNFS_LOCK_BITS);
> +
> +	root = kernfs_root(kn);
> +
> +	lock = &root->open_node_locks[idx].lock;
> +
> +	spin_lock_irq(lock);
> +
> +	return lock;
> +}

Can't you use kernfs_open_node_lock_ptr() in kernfs_open_node_lock() to
make this use less duplicated code?


> +
> +static inline struct mutex *kernfs_open_file_mutex_ptr(struct kernfs_node *kn)
> +{
> +	struct kernfs_root *root;
> +	int idx = hash_ptr(kn, NR_KERNFS_LOCK_BITS);
> +
> +	root = kernfs_root(kn);
> +
> +	return &root->open_file_mutex[idx].lock;
> +}
> +
> +static inline struct mutex *kernfs_open_file_mutex_lock(struct kernfs_node *kn)
> +{
> +	struct kernfs_root *root;
> +	struct mutex *lock;
> +	int idx = hash_ptr(kn, NR_KERNFS_LOCK_BITS);
> +
> +	root = kernfs_root(kn);
> +
> +	lock = &root->open_file_mutex[idx].lock;
> +
> +	mutex_lock(lock);
> +
> +	return lock;
> +}

Same thing here.



> +
>  #endif	/* __KERNFS_INTERNAL_H */
> diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
> index 861c4f0f8a29f..5bf9f02ce9dce 100644
> --- a/include/linux/kernfs.h
> +++ b/include/linux/kernfs.h
> @@ -18,6 +18,8 @@
>  #include <linux/uidgid.h>
>  #include <linux/wait.h>
>  #include <linux/rwsem.h>
> +#include <linux/spinlock.h>
> +#include <linux/cache.h>
>  
>  struct file;
>  struct dentry;
> @@ -34,6 +36,40 @@ struct kernfs_fs_context;
>  struct kernfs_open_node;
>  struct kernfs_iattrs;
>  
> +/*
> + * NR_KERNFS_LOCK_BITS determines size (NR_KERNFS_LOCKS) of hash
> + * table of locks.
> + * Having a small hash table would impact scalability, since
> + * more and more kernfs_node objects will end up using same lock
> + * and having a very large hash table would waste memory.
> + *
> + * At the moment size of hash table of locks is being set based on
> + * the number of CPUs as follows:
> + *
> + * NR_CPU      NR_KERNFS_LOCK_BITS      NR_KERNFS_LOCKS
> + *   1                  1                       2
> + *  2-3                 2                       4
> + *  4-7                 4                       16
> + *  8-15                6                       64
> + *  16-31               8                       256
> + *  32 and more         10                      1024
> + */
> +#ifdef CONFIG_SMP
> +#define NR_KERNFS_LOCK_BITS (2 * (ilog2(NR_CPUS < 32 ? NR_CPUS : 32)))
> +#else
> +#define NR_KERNFS_LOCK_BITS     1
> +#endif
> +
> +#define NR_KERNFS_LOCKS     (1 << NR_KERNFS_LOCK_BITS)
> +
> +struct kernfs_open_node_lock {
> +	spinlock_t lock;
> +} ____cacheline_aligned_in_smp;
> +
> +struct kernfs_open_file_mutex {
> +	struct mutex lock;
> +} ____cacheline_aligned_in_smp;
> +
>  enum kernfs_node_type {
>  	KERNFS_DIR		= 0x0001,
>  	KERNFS_FILE		= 0x0002,
> @@ -90,6 +126,7 @@ enum kernfs_root_flag {
>  	KERNFS_ROOT_SUPPORT_USER_XATTR		= 0x0008,
>  };
>  
> +
>  /* type-specific structures for kernfs_node union members */
>  struct kernfs_elem_dir {
>  	unsigned long		subdirs;
> @@ -201,6 +238,8 @@ struct kernfs_root {
>  
>  	wait_queue_head_t	deactivate_waitq;
>  	struct rw_semaphore	kernfs_rwsem;
> +	struct kernfs_open_node_lock open_node_locks[NR_KERNFS_LOCKS];
> +	struct kernfs_open_file_mutex open_file_mutex[NR_KERNFS_LOCKS];
>  };

I think struct kernfs_root can be declared locally inside fs/kernfs/ to
keep other subsystems/files from having to see this structure, right?
That would make this a bit less of a "rebuild the world" type of change
and could be done in a patch before this one.

Overall, this looks sane to me, nice work.

Tejun, any comments?

thanks,

greg k-h