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Date: Tue, 16 Dec 2008 13:04:33 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Eric Dumazet <dada1@...mosbay.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Ingo Molnar <mingo@...e.hu>,
Christoph Hellwig <hch@...radead.org>,
David Miller <davem@...emloft.net>,
"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org,
"kernel-testers@...r.kernel.org >> Kernel Testers List"
<kernel-testers@...r.kernel.org>, Mike Galbraith <efault@....de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Linux Netdev List <netdev@...r.kernel.org>,
Christoph Lameter <cl@...ux-foundation.org>,
linux-fsdevel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH v3 1/7] fs: Use a percpu_counter to track nr_dentry
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 <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
> ---
> 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,
--
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