[PATCH] knfsd: make nfsd readahead params cache SMP-friendly

Make the nfsd read-ahead params cache more SMP-friendly by changing the single
global list and lock into a fixed 16-bucket hashtable with per-bucket locks.
This reduces spinlock contention in nfsd_read() on read-heavy workloads on
multiprocessor servers.

Testing was on a 4 CPU 4 NIC Altix using 4 IRIX clients each doing 1K
streaming reads at full line rate.  The server had 128 nfsd threads, which
sizes the RA cache at 256 entries, of which only a handful were used.  Flat
profiling shows nfsd_read(), including the inlined nfsd_get_raparms(), taking
10.4% of each CPU.  This patch drops the contribution from nfsd() to 1.71% for
each CPU.

Signed-off-by: Greg Banks <gnb@melbourne.sgi.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index bfd36e5..137565a 100644 (file)
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -54,6 +54,7 @@
 #include <linux/nfsd_idmap.h>
 #include <linux/security.h>
 #endif /* CONFIG_NFSD_V4 */
+#include <linux/jhash.h>
 
 #include <asm/uaccess.h>
 
@@ -81,10 +82,19 @@ struct raparms {
        dev_t                   p_dev;
        int                     p_set;
        struct file_ra_state    p_ra;
+       unsigned int            p_hindex;
 };
 
+struct raparm_hbucket {
+       struct raparms          *pb_head;
+       spinlock_t              pb_lock;
+} ____cacheline_aligned_in_smp;
+
 static struct raparms *                raparml;
-static struct raparms *                raparm_cache;
+#define RAPARM_HASH_BITS       4
+#define RAPARM_HASH_SIZE       (1<<RAPARM_HASH_BITS)
+#define RAPARM_HASH_MASK       (RAPARM_HASH_SIZE-1)
+static struct raparm_hbucket   raparm_hash[RAPARM_HASH_SIZE];
 
 /* 
  * Called from nfsd_lookup and encode_dirent. Check if we have crossed 
@@ -743,16 +753,20 @@ nfsd_sync_dir(struct dentry *dp)
  * Obtain the readahead parameters for the file
  * specified by (dev, ino).
  */
-static DEFINE_SPINLOCK(ra_lock);
 
 static inline struct raparms *
 nfsd_get_raparms(dev_t dev, ino_t ino)
 {
        struct raparms  *ra, **rap, **frap = NULL;
        int depth = 0;
+       unsigned int hash;
+       struct raparm_hbucket *rab;
+
+       hash = jhash_2words(dev, ino, 0xfeedbeef) & RAPARM_HASH_MASK;
+       rab = &raparm_hash[hash];
 
-       spin_lock(&ra_lock);
-       for (rap = &raparm_cache; (ra = *rap); rap = &ra->p_next) {
+       spin_lock(&rab->pb_lock);
+       for (rap = &rab->pb_head; (ra = *rap); rap = &ra->p_next) {
                if (ra->p_ino == ino && ra->p_dev == dev)
                        goto found;
                depth++;
@@ -761,7 +775,7 @@ nfsd_get_raparms(dev_t dev, ino_t ino)
        }
        depth = nfsdstats.ra_size*11/10;
        if (!frap) {    
-               spin_unlock(&ra_lock);
+               spin_unlock(&rab->pb_lock);
                return NULL;
        }
        rap = frap;
@@ -769,15 +783,16 @@ nfsd_get_raparms(dev_t dev, ino_t ino)
        ra->p_dev = dev;
        ra->p_ino = ino;
        ra->p_set = 0;
+       ra->p_hindex = hash;
 found:
-       if (rap != &raparm_cache) {
+       if (rap != &rab->pb_head) {
                *rap = ra->p_next;
-               ra->p_next   = raparm_cache;
-               raparm_cache = ra;
+               ra->p_next   = rab->pb_head;
+               rab->pb_head = ra;
        }
        ra->p_count++;
        nfsdstats.ra_depth[depth*10/nfsdstats.ra_size]++;
-       spin_unlock(&ra_lock);
+       spin_unlock(&rab->pb_lock);
        return ra;
 }
 
@@ -853,11 +868,12 @@ nfsd_vfs_read(struct svc_rqst *rqstp, struct svc_fh *fhp, struct file *file,
 
        /* Write back readahead params */
        if (ra) {
-               spin_lock(&ra_lock);
+               struct raparm_hbucket *rab = &raparm_hash[ra->p_hindex];
+               spin_lock(&rab->pb_lock);
                ra->p_ra = file->f_ra;
                ra->p_set = 1;
                ra->p_count--;
-               spin_unlock(&ra_lock);
+               spin_unlock(&rab->pb_lock);
        }
 
        if (err >= 0) {
@@ -1833,11 +1849,11 @@ nfsd_permission(struct svc_export *exp, struct dentry *dentry, int acc)
 void
 nfsd_racache_shutdown(void)
 {
-       if (!raparm_cache)
+       if (!raparml)
                return;
        dprintk("nfsd: freeing readahead buffers.\n");
        kfree(raparml);
-       raparm_cache = raparml = NULL;
+       raparml = NULL;
 }
 /*
  * Initialize readahead param cache
@@ -1846,19 +1862,31 @@ int
 nfsd_racache_init(int cache_size)
 {
        int     i;
+       int     j = 0;
+       int     nperbucket;
 
-       if (raparm_cache)
+
+       if (raparml)
                return 0;
+       if (cache_size < 2*RAPARM_HASH_SIZE)
+               cache_size = 2*RAPARM_HASH_SIZE;
        raparml = kmalloc(sizeof(struct raparms) * cache_size, GFP_KERNEL);
 
        if (raparml != NULL) {
                dprintk("nfsd: allocating %d readahead buffers.\n",
                        cache_size);
+               for (i = 0 ; i < RAPARM_HASH_SIZE ; i++) {
+                       raparm_hash[i].pb_head = NULL;
+                       spin_lock_init(&raparm_hash[i].pb_lock);
+               }
+               nperbucket = cache_size >> RAPARM_HASH_BITS;
                memset(raparml, 0, sizeof(struct raparms) * cache_size);
                for (i = 0; i < cache_size - 1; i++) {
-                       raparml[i].p_next = raparml + i + 1;
+                       if (i % nperbucket == 0)
+                               raparm_hash[j++].pb_head = raparml + i;
+                       if (i % nperbucket < nperbucket-1)
+                               raparml[i].p_next = raparml + i + 1;
                }
-               raparm_cache = raparml;
        } else {
                printk(KERN_WARNING
                       "nfsd: Could not allocate memory read-ahead cache.\n");