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    Subject[PATCH v2 1/5] fs: Use a percpu_counter to track nr_dentry
    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

    (socket8 bench result : 27.5s to 25s)

    Signed-off-by: Eric Dumazet <dada1@cosmosbay.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 a1d86c7..46d5d1e 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 0dcdd94..c5e7aa5 100644
    --- a/include/linux/fs.h
    +++ b/include/linux/fs.h
    @@ -2216,6 +2216,8 @@ 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 9d048fa..eebddef 100644
    --- a/kernel/sysctl.c
    +++ b/kernel/sysctl.c
    @@ -1243,7 +1243,7 @@ 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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