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From: David Howells <dhowells@redhat.com>
To: linux-cachefs@redhat.com
Cc: dhowells@redhat.com, Anna Schumaker <anna.schumaker@netapp.com>,
	Steve French <sfrench@samba.org>,
	Dominique Martinet <asmadeus@codewreck.org>,
	Jeff Layton <jlayton@redhat.com>,
	David Wysochanski <dwysocha@redhat.com>,
	linux-afs@lists.infradead.org, linux-nfs@vger.kernel.org,
	linux-cifs@vger.kernel.org, ceph-devel@vger.kernel.org,
	v9fs-developer@lists.sourceforge.net,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 10/12] fscache: Fix cookie key hashing
Date: Mon, 21 Jun 2021 22:46:58 +0100	[thread overview]
Message-ID: <162431201844.2908479.8293647220901514696.stgit@warthog.procyon.org.uk> (raw)
In-Reply-To: <162431188431.2908479.14031376932042135080.stgit@warthog.procyon.org.uk>

The current hash algorithm used for hashing cookie keys is really bad,
producing almost no dispersion (after a test kernel build, ~30000 files
were split over just 18 out of the 32768 hash buckets).

Borrow the full_name_hash() hash function into fscache to do the hashing
for cookie keys and, in the future, volume keys.

I don't want to use full_name_hash() as-is because I want the hash value to
be consistent across arches and over time as the hash value produced may
get used on disk.

I can also optimise parts of it away as the key will always be a padded
array of aligned 32-bit words.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/fscache/cookie.c   |   14 +-------------
 fs/fscache/internal.h |    2 ++
 fs/fscache/main.c     |   39 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+), 13 deletions(-)

diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index ec9bce33085f..2558814193e9 100644
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -87,10 +87,8 @@ void fscache_free_cookie(struct fscache_cookie *cookie)
 static int fscache_set_key(struct fscache_cookie *cookie,
 			   const void *index_key, size_t index_key_len)
 {
-	unsigned long long h;
 	u32 *buf;
 	int bufs;
-	int i;
 
 	bufs = DIV_ROUND_UP(index_key_len, sizeof(*buf));
 
@@ -104,17 +102,7 @@ static int fscache_set_key(struct fscache_cookie *cookie,
 	}
 
 	memcpy(buf, index_key, index_key_len);
-
-	/* Calculate a hash and combine this with the length in the first word
-	 * or first half word
-	 */
-	h = (unsigned long)cookie->parent;
-	h += index_key_len + cookie->type;
-
-	for (i = 0; i < bufs; i++)
-		h += buf[i];
-
-	cookie->key_hash = h ^ (h >> 32);
+	cookie->key_hash = fscache_hash(0, buf, bufs);
 	return 0;
 }
 
diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index 200082cafdda..a49136c63e4b 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -74,6 +74,8 @@ extern struct workqueue_struct *fscache_object_wq;
 extern struct workqueue_struct *fscache_op_wq;
 DECLARE_PER_CPU(wait_queue_head_t, fscache_object_cong_wait);
 
+extern unsigned int fscache_hash(unsigned int salt, unsigned int *data, unsigned int n);
+
 static inline bool fscache_object_congested(void)
 {
 	return workqueue_congested(WORK_CPU_UNBOUND, fscache_object_wq);
diff --git a/fs/fscache/main.c b/fs/fscache/main.c
index c1e6cc9091aa..4207f98e405f 100644
--- a/fs/fscache/main.c
+++ b/fs/fscache/main.c
@@ -93,6 +93,45 @@ static struct ctl_table fscache_sysctls_root[] = {
 };
 #endif
 
+/*
+ * Mixing scores (in bits) for (7,20):
+ * Input delta: 1-bit      2-bit
+ * 1 round:     330.3     9201.6
+ * 2 rounds:   1246.4    25475.4
+ * 3 rounds:   1907.1    31295.1
+ * 4 rounds:   2042.3    31718.6
+ * Perfect:    2048      31744
+ *            (32*64)   (32*31/2 * 64)
+ */
+#define HASH_MIX(x, y, a)	\
+	(	x ^= (a),	\
+	y ^= x,	x = rol32(x, 7),\
+	x += y,	y = rol32(y,20),\
+	y *= 9			)
+
+static inline unsigned int fold_hash(unsigned long x, unsigned long y)
+{
+	/* Use arch-optimized multiply if one exists */
+	return __hash_32(y ^ __hash_32(x));
+}
+
+/*
+ * Generate a hash.  This is derived from full_name_hash(), but we want to be
+ * sure it is arch independent and that it doesn't change as bits of the
+ * computed hash value might appear on disk.  The caller also guarantees that
+ * the hashed data will be a series of aligned 32-bit words.
+ */
+unsigned int fscache_hash(unsigned int salt, unsigned int *data, unsigned int n)
+{
+	unsigned int a, x = 0, y = salt;
+
+	for (; n; n--) {
+		a = *data++;
+		HASH_MIX(x, y, a);
+	}
+	return fold_hash(x, y);
+}
+
 /*
  * initialise the fs caching module
  */



  parent reply	other threads:[~2021-06-21 21:47 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-21 21:44 [PATCH 00/12] fscache: Some prep work for fscache rewrite David Howells
2021-06-21 21:44 ` [PATCH 01/12] fscache: Select netfs stats if fscache stats are enabled David Howells
2021-06-21 21:45 ` [PATCH 02/12] netfs: Move cookie debug ID to struct netfs_cache_resources David Howells
2021-06-21 21:45 ` [PATCH 03/12] cachefiles: Use file_inode() rather than accessing ->f_inode David Howells
2021-06-21 21:45 ` [PATCH 04/12] fscache: Add a cookie debug ID and use that in traces David Howells
2021-06-21 21:45 ` [PATCH 05/12] fscache: Procfile to display cookies David Howells
2021-06-21 21:45 ` [PATCH 06/12] fscache, cachefiles: Remove the histogram stuff David Howells
2021-06-21 21:46 ` [PATCH 07/12] fscache: Remove the object list procfile David Howells
2021-06-21 21:46 ` [PATCH 08/12] fscache: Change %p in format strings to something else David Howells
2021-06-21 21:46 ` [PATCH 09/12] cachefiles: " David Howells
2021-06-21 21:46 ` David Howells [this message]
2021-08-24 16:11   ` [PATCH 10/12] fscache: Fix cookie key hashing Jeff Layton
2021-08-25 14:04   ` David Howells
2021-06-21 21:47 ` [PATCH 11/12] fscache: Fix fscache_cookie_put() to not deref after dec David Howells
2021-08-24 14:24   ` Jeff Layton
2021-08-25 14:05   ` David Howells
2021-06-21 21:47 ` [PATCH 12/12] fscache: Use refcount_t for the cookie refcount instead of atomic_t David Howells
2021-08-24 14:25 ` [PATCH 00/12] fscache: Some prep work for fscache rewrite Jeff Layton
2021-08-27 12:31 ` [PATCH v2 04/12] fscache: Add a cookie debug ID and use that in traces David Howells
2021-08-27 12:49   ` [Linux-cachefs] " Marc Dionne

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