fs: use percpu counter for nr_dentry and nr_dentry_unused

The nr_dentry stat is a globally touched cacheline and atomic operation
twice over the lifetime of a dentry. It is used for the benfit of userspace
only. Turn it into a per-cpu counter and always decrement it in d_free instead
of doing various batching operations to reduce lock hold times in the callers.

Based on an earlier patch from Nick Piggin <npiggin@suse.de>.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 32 insertions, 19 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 028753951e95..c37a656802b0 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -67,6 +67,19 @@ struct dentry_stat_t dentry_stat = {
         .age_limit = 45,
 };
 
+static struct percpu_counter nr_dentry __cacheline_aligned_in_smp;
+static struct percpu_counter nr_dentry_unused __cacheline_aligned_in_smp;
+
+#if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
+int proc_nr_dentry(ctl_table *table, int write, void __user *buffer,
+                   size_t *lenp, loff_t *ppos)
+{
+        dentry_stat.nr_dentry = percpu_counter_sum_positive(&nr_dentry);
+        dentry_stat.nr_unused = percpu_counter_sum_positive(&nr_dentry_unused);
+        return proc_dointvec(table, write, buffer, lenp, ppos);
+}
+#endif
+
 static void __d_free(struct rcu_head *head)
 {
         struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
@@ -78,13 +91,14 @@ static void __d_free(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)
 {
+        percpu_counter_dec(&nr_dentry);
         if (dentry->d_op && dentry->d_op->d_release)
                 dentry->d_op->d_release(dentry);
+
         /* if dentry was never inserted into hash, immediate free is OK */
         if (hlist_unhashed(&dentry->d_hash))
                 __d_free(&dentry->d_u.d_rcu);
@@ -125,14 +139,14 @@ static void dentry_lru_add(struct dentry *dentry)
 {
         list_add(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);
         dentry->d_sb->s_nr_dentry_unused++;
-        dentry_stat.nr_unused++;
+        percpu_counter_inc(&nr_dentry_unused);
 }
 
 static void dentry_lru_add_tail(struct dentry *dentry)
 {
         list_add_tail(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);
         dentry->d_sb->s_nr_dentry_unused++;
-        dentry_stat.nr_unused++;
+        percpu_counter_inc(&nr_dentry_unused);
 }
 
 static void dentry_lru_del(struct dentry *dentry)
@@ -140,7 +154,7 @@ static void dentry_lru_del(struct dentry *dentry)
         if (!list_empty(&dentry->d_lru)) {
                 list_del(&dentry->d_lru);
                 dentry->d_sb->s_nr_dentry_unused--;
-                dentry_stat.nr_unused--;
+                percpu_counter_dec(&nr_dentry_unused);
         }
 }
 
@@ -149,7 +163,7 @@ static void dentry_lru_del_init(struct dentry *dentry)
         if (likely(!list_empty(&dentry->d_lru))) {
                 list_del_init(&dentry->d_lru);
                 dentry->d_sb->s_nr_dentry_unused--;
-                dentry_stat.nr_unused--;
+                percpu_counter_dec(&nr_dentry_unused);
         }
 }
 
@@ -168,7 +182,6 @@ 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))
@@ -314,7 +327,6 @@ int d_invalidate(struct dentry * dentry)
 EXPORT_SYMBOL(d_invalidate);
 
 /* This should be called _only_ with dcache_lock held */
-
 static inline struct dentry * __dget_locked(struct dentry *dentry)
 {
         atomic_inc(&dentry->d_count);
@@ -534,7 +546,7 @@ static void prune_dcache(int count)
 {
         struct super_block *sb, *p = NULL;
         int w_count;
-        int unused = dentry_stat.nr_unused;
+        int unused = percpu_counter_sum_positive(&nr_dentry_unused);
         int prune_ratio;
         int pruned;
 
@@ -699,20 +711,13 @@ 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;
-
                 } while (list_empty(&dentry->d_subdirs));
 
                 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);
 }
 
 /*
@@ -896,12 +901,16 @@ EXPORT_SYMBOL(shrink_dcache_parent);
  */
 static int shrink_dcache_memory(struct shrinker *shrink, int nr, gfp_t gfp_mask)
 {
+        int nr_unused;
+
         if (nr) {
                 if (!(gfp_mask & __GFP_FS))
                         return -1;
                 prune_dcache(nr);
         }
-        return (dentry_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
+
+        nr_unused = percpu_counter_sum_positive(&nr_dentry_unused);
+        return (nr_unused / 100) * sysctl_vfs_cache_pressure;
 }
 
 static struct shrinker dcache_shrinker = {
@@ -968,9 +977,10 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
         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;
 }
 EXPORT_SYMBOL(d_alloc);
@@ -2417,6 +2427,9 @@ static void __init dcache_init(void)
 {
         int loop;
 
+        percpu_counter_init(&nr_dentry, 0);
+        percpu_counter_init(&nr_dentry_unused, 0);
+
         /* 
          * A constructor could be added for stable state like the lists,
          * but it is probably not worth it because of the cache nature