Subject: Re: [PATCH v3 1/7] fs: Use a percpu_counter to track nr_dentry

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 16 Dec 2008 13:04:33 -0800

On Thu, Dec 11, 2008 at 11:38:56PM +0100, Eric Dumazet wrote:
> Adding a percpu_counter nr_dentry avoids cache line ping pongs
> between cpus to maintain this metric, and dcache_lock is
> no more needed to protect dentry_stat.nr_dentry
> 
> We centralize nr_dentry updates at the right place :
> - increments in d_alloc()
> - decrements in d_free()
> 
> d_alloc() can avoid taking dcache_lock if parent is NULL
> 
> ("socketallocbench -n8" result : 27.5s to 25s)

Looks good!  (At least once I realised that nr_dentry was global rather
than per-dentry!!!)

Reviewed-by: Paul E. McKenney <[email-masked]>

> Signed-off-by: Eric Dumazet <dada1-fPLkHRcR87vqlBn2x/[email-masked]>
> ---
>  fs/dcache.c        |   49 +++++++++++++++++++++++++------------------
>  include/linux/fs.h |    2 +
>  kernel/sysctl.c    |    2 -
>  3 files changed, 32 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index fa1ba03..f463a81 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -61,12 +61,31 @@ static struct kmem_cache *dentry_cache __read_mostly;
>  static unsigned int d_hash_mask __read_mostly;
>  static unsigned int d_hash_shift __read_mostly;
>  static struct hlist_head *dentry_hashtable __read_mostly;
> +static struct percpu_counter nr_dentry;
> 
>  /* Statistics gathering. */
>  struct dentry_stat_t dentry_stat = {
>  	.age_limit = 45,
>  };
> 
> +/*
> + * Handle nr_dentry sysctl
> + */
> +#if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
> +int proc_nr_dentry(ctl_table *table, int write, struct file *filp,
> +		   void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	dentry_stat.nr_dentry = percpu_counter_sum_positive(&nr_dentry);
> +	return proc_dointvec(table, write, filp, buffer, lenp, ppos);
> +}
> +#else
> +int proc_nr_dentry(ctl_table *table, int write, struct file *filp,
> +		   void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	return -ENOSYS;
> +}
> +#endif
> +
>  static void __d_free(struct dentry *dentry)
>  {
>  	WARN_ON(!list_empty(&dentry->d_alias));
> @@ -82,8 +101,7 @@ static void d_callback(struct rcu_head *head)
>  }
> 
>  /*
> - * no dcache_lock, please.  The caller must decrement dentry_stat.nr_dentry
> - * inside dcache_lock.
> + * no dcache_lock, please.
>   */
>  static void d_free(struct dentry *dentry)
>  {
> @@ -94,6 +112,7 @@ static void d_free(struct dentry *dentry)
>  		__d_free(dentry);
>  	else
>  		call_rcu(&dentry->d_u.d_rcu, d_callback);
> +	percpu_counter_dec(&nr_dentry);
>  }
> 
>  /*
> @@ -172,7 +191,6 @@ static struct dentry *d_kill(struct dentry *dentry)
>  	struct dentry *parent;
> 
>  	list_del(&dentry->d_u.d_child);
> -	dentry_stat.nr_dentry--;	/* For d_free, below */
>  	/*drops the locks, at that point nobody can reach this dentry */
>  	dentry_iput(dentry);
>  	if (IS_ROOT(dentry))
> @@ -619,7 +637,6 @@ void shrink_dcache_sb(struct super_block * sb)
>  static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
>  {
>  	struct dentry *parent;
> -	unsigned detached = 0;
> 
>  	BUG_ON(!IS_ROOT(dentry));
> 
> @@ -678,7 +695,6 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
>  			}
> 
>  			list_del(&dentry->d_u.d_child);
> -			detached++;
> 
>  			inode = dentry->d_inode;
>  			if (inode) {
> @@ -696,7 +712,7 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
>  			 * otherwise we ascend to the parent and move to the
>  			 * next sibling if there is one */
>  			if (!parent)
> -				goto out;
> +				return;
> 
>  			dentry = parent;
> 
> @@ -705,11 +721,6 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
>  		dentry = list_entry(dentry->d_subdirs.next,
>  				    struct dentry, d_u.d_child);
>  	}
> -out:
> -	/* several dentries were freed, need to correct nr_dentry */
> -	spin_lock(&dcache_lock);
> -	dentry_stat.nr_dentry -= detached;
> -	spin_unlock(&dcache_lock);
>  }
> 
>  /*
> @@ -943,8 +954,6 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
>  	dentry->d_flags = DCACHE_UNHASHED;
>  	spin_lock_init(&dentry->d_lock);
>  	dentry->d_inode = NULL;
> -	dentry->d_parent = NULL;
> -	dentry->d_sb = NULL;
>  	dentry->d_op = NULL;
>  	dentry->d_fsdata = NULL;
>  	dentry->d_mounted = 0;
> @@ -959,16 +968,15 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
>  	if (parent) {
>  		dentry->d_parent = dget(parent);
>  		dentry->d_sb = parent->d_sb;
> +		spin_lock(&dcache_lock);
> +		list_add(&dentry->d_u.d_child, &parent->d_subdirs);
> +		spin_unlock(&dcache_lock);
>  	} else {
> +		dentry->d_parent = NULL;
> +		dentry->d_sb = NULL;
>  		INIT_LIST_HEAD(&dentry->d_u.d_child);
>  	}
> -
> -	spin_lock(&dcache_lock);
> -	if (parent)
> -		list_add(&dentry->d_u.d_child, &parent->d_subdirs);
> -	dentry_stat.nr_dentry++;
> -	spin_unlock(&dcache_lock);
> -
> +	percpu_counter_inc(&nr_dentry);
>  	return dentry;
>  }
> 
> @@ -2282,6 +2290,7 @@ static void __init dcache_init(void)
>  {
>  	int loop;
> 
> +	percpu_counter_init(&nr_dentry, 0);
>  	/* 
>  	 * A constructor could be added for stable state like the lists,
>  	 * but it is probably not worth it because of the cache nature
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 4a853ef..114cb65 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2217,6 +2217,8 @@ static inline void free_secdata(void *secdata)
>  struct ctl_table;
>  int proc_nr_files(struct ctl_table *table, int write, struct file *filp,
>  		  void __user *buffer, size_t *lenp, loff_t *ppos);
> +int proc_nr_dentry(struct ctl_table *table, int write, struct file *filp,
> +		   void __user *buffer, size_t *lenp, loff_t *ppos);
> 
>  int get_filesystem_list(char * buf);
> 
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 3d56fe7..777bee7 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1246,7 +1246,7 @@ static struct ctl_table fs_table[] = {
>  		.data		= &dentry_stat,
>  		.maxlen		= 6*sizeof(int),
>  		.mode		= 0444,
> -		.proc_handler	= &proc_dointvec,
> +		.proc_handler	= &proc_nr_dentry,
>  	},
>  	{
>  		.ctl_name	= FS_OVERFLOWUID,