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Subject[PATCH] [DCACHE] : avoid RCU for never hashed dentries
Date
Some dentries dont need to be globally visible in dentry hashtable. (pipes & 
sockets)

Such dentries dont need to wait for a RCU grace period at delete time. Being
able to free them permits a better CPU cache use (hot cache)

This patch combined with (dont insert pipe dentries into dentry_hashtable)
reduced time of { pipe(p); close(p[0]); close(p[1]);} on my UP machine (1.6
GHz Pentium-M) from 3.23 us to 2.86 us
(But this patch does not depend on other patches, only bench results)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
--- linux-2.6.19-rc6/fs/dcache.c 2006-11-22 17:33:33.000000000 +0100
+++ linux-2.6.19-rc6-ed/fs/dcache.c 2006-11-22 17:33:58.000000000 +0100
@@ -68,15 +68,19 @@ struct dentry_stat_t dentry_stat = {
.age_limit = 45,
};

-static void d_callback(struct rcu_head *head)
+static void __d_free(struct dentry *dentry)
{
- struct dentry * dentry = container_of(head, struct dentry, d_u.d_rcu);
-
if (dname_external(dentry))
kfree(dentry->d_name.name);
kmem_cache_free(dentry_cache, dentry);
}

+static void d_callback(struct rcu_head *head)
+{
+ struct dentry * dentry = container_of(head, struct dentry, d_u.d_rcu);
+ __d_free(dentry);
+}
+
/*
* no dcache_lock, please. The caller must decrement dentry_stat.nr_dentry
* inside dcache_lock.
@@ -85,7 +89,11 @@ static void d_free(struct dentry *dentry
{
if (dentry->d_op && dentry->d_op->d_release)
dentry->d_op->d_release(dentry);
- call_rcu(&dentry->d_u.d_rcu, d_callback);
+ /* if dentry was never inserted into hash, immediate free is OK */
+ if (dentry->d_hash.pprev == NULL)
+ __d_free(dentry);
+ else
+ call_rcu(&dentry->d_u.d_rcu, d_callback);
}

/*
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