[PATCH 00/10] String hash improvements

[PATCH 00/10] String hash improvements

[George Spelvin]

On Tue, 17 May 2016 at 09:32, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Tue, May 17, 2016 at 6:41 AM, George Spelvin <linux@horizon.com> wrote:
>> I had assumed that since they weren't fully baked when the window opened,
>> they weren't eligible, but I'll try.

> Hey, if they aren't ready, they aren't.

Well, they're close, and I can and did *get* them ready.

> How about just the minimal set of patches that you'er happy with as-is?

The things are a bit interdependent.  I can't fix hash_64() on 32-bit systems until
I get rid of hash_string()'s need for it to return 64 bits, which requires work
on the dcache hashes to make them suitable replacements...

The real fun has come from TPTB deciding to sell the horizon.com domain,
and it turns out that updating rDNS takes the ISP a whole freaking week,
during which time outgoing mail trips everyone's spam filters.

That finally got fixed, just in time for me to put my dominant hand through
a piece of glass.  It's been a week. :-(


Anyway, the patches...

This series does several related things:
1) Gets rid of the string hashes in <linux/sunrpc/svcauth.h>,
   and uses the dcache hash (fs/namei.c) instead.
2) Avoid 64-bit multiplies in hash_64() on 32-bit platforms.
   Two 32-bit multiplies will do well enough.
3) Rids the world of the bad hash multipliers in hash_32.
   This finishes the job started in 689de1d6ca.
   The vast majority of Linux architectures have hardware support
   for 32x32-bit multiply and so derive no benefit from "simplified"
   multipliers.
   The few processors that do not (68000, h8/300 and some models of
   Microblaze) have arch-specific implementations added.  Those patches
   are last in the series so they can go through the relevant arch
   maintainers.
4) Overhauls the dcache hash mixing.
   The patch in 2bf0b16954 was an off-the-cuff suggestion.  Replaced with
   a much more careful design that's simultaneously faster and better.
   (My own invention, as there was noting suitable in the literature I
   could find.  Comments welcome!)

Things I thought about but can wait for now:
5) Modify the hash_name() loop to skip the initial HASH_MIX().
   That would let us salt the hash if we ever wanted to.
6) Modify bytemask_from_count to handle inputs of 1..sizeof(long)
   rather than 0..sizeof(long)-1.  This would simplify all its users
   including full_name_hash.
7) Sort out partial_name_hash().
   The hash function is declared as using a long state, even though
   it's truncated to 32 bits at the end and the extra internal state
   contributes nothing to the result.  And some callers do odd things:
   * fs/hfs/string.c only allocates 32 bits of state
   * fs/hfsplus/unicode.c uses it to hash 16-bit unicode symbols not bytes

I'm not particularly fond of the names of the header files I created,
but if anyone has a better idea please talk fast!

George Spelvin (10):
  Pull out string hash to <linux/stringhash.h>
  fs/namei.c: Add hash_string() function.
  <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string()
  Change hash_64() return value to 32 bits.
  Eliminate bad hash multipliers from hash_32() and hash_64().
  fs/namei.c: Improve dcache hash function
  <linux/hash.h>: Add support for architecture-specific functions
  m68k: Add <asm/archhash.h>
  microblaze: Add <asm/archhash.h>
  h8300: Add <asm/archhash.h>

 arch/Kconfig                           |   8 ++
 arch/h8300/Kconfig                     |   1 +
 arch/h8300/include/asm/archhash.h      |  52 ++++++++++++
 arch/m68k/Kconfig                      |   1 +
 arch/m68k/include/asm/archhash.h       |  67 +++++++++++++++
 arch/microblaze/Kconfig                |   1 +
 arch/microblaze/include/asm/archhash.h |  80 ++++++++++++++++++
 fs/namei.c                             | 149 +++++++++++++++++++++++++--------
 include/linux/dcache.h                 |  27 +-----
 include/linux/hash.h                   | 111 ++++++++++++------------
 include/linux/stringhash.h             |  76 +++++++++++++++++
 include/linux/sunrpc/svcauth.h         |  36 ++------
 12 files changed, 464 insertions(+), 145 deletions(-)
 create mode 100644 arch/h8300/include/asm/archhash.h
 create mode 100644 arch/m68k/include/asm/archhash.h
 create mode 100644 arch/microblaze/include/asm/archhash.h
 create mode 100644 include/linux/stringhash.h

-- 
2.8.1

Re: [PATCH 00/10] String hash improvements

[Geert Uytterhoeven]

Hi George,

On Wed, May 25, 2016 at 9:20 AM, George Spelvin
<linux@sciencehorizons.net> wrote:
> I'm not particularly fond of the names of the header files I created,
> but if anyone has a better idea please talk fast!

Usually this is handled through include/asm-generic/.
Put the generic default implementation in include/asm-generic/hash.h.

Architectures that need to override provide their own version, e.g.
arch/m68k/include/asm/hash.h. They may #include <asm-generic/hash.h>
if they still want to reuse parts of the generic implementation.

Other architectures add "generic-y += hash.h" to their
arch/<ARCH>/include/asm/Kbuild.

<linux/hash.h> includes <asm/hash.h> t.

>  arch/h8300/include/asm/archhash.h      |  52 ++++++++++++
>  arch/m68k/include/asm/archhash.h       |  67 +++++++++++++++
>  arch/microblaze/include/asm/archhash.h |  80 ++++++++++++++++++
>  include/linux/hash.h                   | 111 ++++++++++++------------
>  include/linux/stringhash.h             |  76 +++++++++++++++++

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH 00/10] String hash improvements

[George Spelvin]

Geert Uytterhoeven wrote:
> Usually this is handled through include/asm-generic/.
> Put the generic default implementation in include/asm-generic/hash.h.
>
> Architectures that need to override provide their own version, e.g.
> arch/m68k/include/asm/hash.h. They may #include <asm-generic/hash.h>
> if they still want to reuse parts of the generic implementation.
>
> Other architectures add "generic-y += hash.h" to their
> arch/<ARCH>/include/asm/Kbuild.

I thought about that, but then I'd have to edit *every* architecture,
and might need acks from all the maintainers.

I was looking for something that was a total no-op on most architectures.

But if this is preferred, it's not technically difficult at all.


If asm-generic were in the <asm/*.h> search path, it would magically
Just Work, but leftover files from a broken checkout would be a big
potential problem.

Re: [PATCH 00/10] String hash improvements

[Geert Uytterhoeven]

On Wed, May 25, 2016 at 10:11 AM, George Spelvin
<linux@sciencehorizons.net> wrote:
> Geert Uytterhoeven wrote:
>> Usually this is handled through include/asm-generic/.
>> Put the generic default implementation in include/asm-generic/hash.h.
>>
>> Architectures that need to override provide their own version, e.g.
>> arch/m68k/include/asm/hash.h. They may #include <asm-generic/hash.h>
>> if they still want to reuse parts of the generic implementation.
>>
>> Other architectures add "generic-y += hash.h" to their
>> arch/<ARCH>/include/asm/Kbuild.
>
> I thought about that, but then I'd have to edit *every* architecture,
> and might need acks from all the maintainers.
>
> I was looking for something that was a total no-op on most architectures.
>
> But if this is preferred, it's not technically difficult at all.

As you only include <asm/archhash.h> if CONFIG_HAVE_ARCH_HASH
is defined, you can also just call the arch-specific one <asm/hash.h>.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH 00/10] String hash improvements

[George Spelvin]

>> +#if defined(CONFIG_M68000) || defined(CONFIG_M68010)

> As I said before, I don't think you need this check, given HAVE_ARCH_HASH is
> selected by M68000, and M68010 doesn't exist.

I was going belt & suspenders on general principles, but yes, I'm happy
to leave it out.

I noticed that CONFIG_M68010 doesn't exist in Linus' tree, but you
recommended it, so I thought you might know something I don't.

> As you only include <asm/archhash.h> if CONFIG_HAVE_ARCH_HASH
> is defined, you can also just call the arch-specific one <asm/hash.h>.

Yes, that's a possibility, too.  But weren't we still discussing whether
I should use conditional #inclusion based on a symbol, or asm-generic?

If neither of us has a killer argument that convinces the other, style
issues like this are amenable to voting, so I was going to wait a little
bit for others to chime in.


Thank you very much for the comments!

Re: [PATCH 00/10] String hash improvements

[Linus Torvalds]

On Wed, May 25, 2016 at 12:20 AM, George Spelvin
<linux@sciencehorizons.net> wrote:
>
> Well, they're close, and I can and did *get* them ready.

Ok, thanks. For some odd reason all your emails in this series got
marked as spam. Every single one, including the cover letter (but not
your replies to the replies to this).

Stupidly, I unmarked them before thinking to ask google *why* they got
marked as spam.

Did you do something different to send this series than you usually do?

Would you mind sending it again (maybe you have changes due to some o
the comments, but even if not, I'd like to see what the spam filter
says)? You could do it just to me to avoid filling up the mailing list
with duplicates.

                    Linus

[PATCH v3 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hashlen_string()

[George Spelvin]

Finally, the first use of previous two patches: eliminate the
separate ad-hoc string hash functions in the sunrpc code.

Now hash_str() is a wrapper around hash_string(), and hash_mem() is
likewise a wrapper around full_name_hash().

Note that sunrpc code *does* call hash_mem() with a zero length, which
is why the previous patch needed to handle that in full_name_hash().
(Thanks, Bruce, for finding that!)

This also eliminates the only caller of hash_long which asks for
more than 32 bits of output.

The comment about the quality of hashlen_string() and full_name_hash()
is jumping the gun by a few patches; they aren't very impressive now,
but will be improved greatly later in the series.

Signed-off-by: George Spelvin <linux@sciencehorizons.net>
Tested-by: J. Bruce Fields <bfields@redhat.com>
Acked-by: J. Bruce Fields <bfields@redhat.com>
Cc: Jeff Layton <jlayton@poochiereds.net>
Cc: linux-nfs@vger.kernel.org
---
 include/linux/sunrpc/svcauth.h | 40 +++++++++-------------------------------
 1 file changed, 9 insertions(+), 31 deletions(-)

diff --git a/include/linux/sunrpc/svcauth.h b/include/linux/sunrpc/svcauth.h
index c00f53a4..91d5a5d6 100644
--- a/include/linux/sunrpc/svcauth.h
+++ b/include/linux/sunrpc/svcauth.h
@@ -16,6 +16,7 @@
 #include <linux/sunrpc/cache.h>
 #include <linux/sunrpc/gss_api.h>
 #include <linux/hash.h>
+#include <linux/stringhash.h>
 #include <linux/cred.h>
 
 struct svc_cred {
@@ -165,41 +166,18 @@ extern int svcauth_unix_set_client(struct svc_rqst *rqstp);
 extern int unix_gid_cache_create(struct net *net);
 extern void unix_gid_cache_destroy(struct net *net);
 
-static inline unsigned long hash_str(char *name, int bits)
+/*
+ * The <stringhash.h> functions are good enough that we don't need to
+ * use hash_32() on them; just extracting the high bits is enough.
+ */
+static inline unsigned long hash_str(char const *name, int bits)
 {
-	unsigned long hash = 0;
-	unsigned long l = 0;
-	int len = 0;
-	unsigned char c;
-	do {
-		if (unlikely(!(c = *name++))) {
-			c = (char)len; len = -1;
-		}
-		l = (l << 8) | c;
-		len++;
-		if ((len & (BITS_PER_LONG/8-1))==0)
-			hash = hash_long(hash^l, BITS_PER_LONG);
-	} while (len);
-	return hash >> (BITS_PER_LONG - bits);
+	return hashlen_hash(hashlen_string(name)) >> (32 - bits);
 }
 
-static inline unsigned long hash_mem(char *buf, int length, int bits)
+static inline unsigned long hash_mem(char const *buf, int length, int bits)
 {
-	unsigned long hash = 0;
-	unsigned long l = 0;
-	int len = 0;
-	unsigned char c;
-	do {
-		if (len == length) {
-			c = (char)len; len = -1;
-		} else
-			c = *buf++;
-		l = (l << 8) | c;
-		len++;
-		if ((len & (BITS_PER_LONG/8-1))==0)
-			hash = hash_long(hash^l, BITS_PER_LONG);
-	} while (len);
-	return hash >> (BITS_PER_LONG - bits);
+	return full_name_hash(buf, length) >> (32 - bits);
 }
 
 #endif /* __KERNEL__ */
-- 
2.8.1

Re: [PATCH 00/10] String hash improvements

[Fubo Chen]

On Wed, May 25, 2016 at 9:08 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> Ok, thanks. For some odd reason all your emails in this series got
> marked as spam. Every single one, including the cover letter (but not
> your replies to the replies to this).

I have added the following filter to my gmail account: "never send
e-mails with [PATCH in the subject to the spam folder". That helps a
lot.

Fubo.

[PATCH 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string()

[George Spelvin]

Finally, the first use of previous two patches: Eliminate the
separate ad-hoc string hash functions in the sunrpc code.

This also eliminates the only caller of hash_long which asks for
more than 32 bits of output.

sunrpc guys: Is it okay if I send this to Linus directly?

Signed-off-by: George Spelvin <linux@sciencehorizons.net>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Jeff Layton <jlayton@poochiereds.net>
Cc: linux-nfs@vger.kernel.org
---
 include/linux/sunrpc/svcauth.h | 36 +++++-------------------------------
 1 file changed, 5 insertions(+), 31 deletions(-)

diff --git a/include/linux/sunrpc/svcauth.h b/include/linux/sunrpc/svcauth.h
index c00f53a4..ef2b2552 100644
--- a/include/linux/sunrpc/svcauth.h
+++ b/include/linux/sunrpc/svcauth.h
@@ -16,6 +16,7 @@
 #include <linux/sunrpc/cache.h>
 #include <linux/sunrpc/gss_api.h>
 #include <linux/hash.h>
+#include <linux/stringhash.h>
 #include <linux/cred.h>
 
 struct svc_cred {
@@ -165,41 +166,14 @@ extern int svcauth_unix_set_client(struct svc_rqst *rqstp);
 extern int unix_gid_cache_create(struct net *net);
 extern void unix_gid_cache_destroy(struct net *net);
 
-static inline unsigned long hash_str(char *name, int bits)
+static inline unsigned long hash_str(char const *name, int bits)
 {
-	unsigned long hash = 0;
-	unsigned long l = 0;
-	int len = 0;
-	unsigned char c;
-	do {
-		if (unlikely(!(c = *name++))) {
-			c = (char)len; len = -1;
-		}
-		l = (l << 8) | c;
-		len++;
-		if ((len & (BITS_PER_LONG/8-1))==0)
-			hash = hash_long(hash^l, BITS_PER_LONG);
-	} while (len);
-	return hash >> (BITS_PER_LONG - bits);
+	return hash_32(hashlen_hash(hash_string(name)), bits);
 }
 
-static inline unsigned long hash_mem(char *buf, int length, int bits)
+static inline unsigned long hash_mem(char const *buf, int length, int bits)
 {
-	unsigned long hash = 0;
-	unsigned long l = 0;
-	int len = 0;
-	unsigned char c;
-	do {
-		if (len == length) {
-			c = (char)len; len = -1;
-		} else
-			c = *buf++;
-		l = (l << 8) | c;
-		len++;
-		if ((len & (BITS_PER_LONG/8-1))==0)
-			hash = hash_long(hash^l, BITS_PER_LONG);
-	} while (len);
-	return hash >> (BITS_PER_LONG - bits);
+	return hash_32(full_name_hash(buf, length), bits);
 }
 
 #endif /* __KERNEL__ */
-- 
2.8.1

[PATCH RESEND 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string()

[George Spelvin]

Finally, the first use of previous two patches: Eliminate the
separate ad-hoc string hash functions in the sunrpc code.

This also eliminates the only caller of hash_long which asks for
more than 32 bits of output.

sunrpc guys: Is it okay if I send this to Linus directly?

Signed-off-by: George Spelvin <linux@sciencehorizons.net>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Jeff Layton <jlayton@poochiereds.net>
Cc: linux-nfs@vger.kernel.org
---
I have't heard anything from the linix-NFS crowd, so this is a re-ping.

I've been having some e-mail troubles leading to increased spam scores
due to a recent domain name change, so this is a resend from a different
address in case the previous copy got binned.

I'll happily send along the entire patch series, but the tl;dr is this
is part of a patch series that improves the <linux/hash.h> and fs/namei.c
hash functions, implementing Linus's suggestion:

On Mon, 02 May 2016 at 09:24:21, Linus Torvalds wrote:
> That said, I actually think hash_str() should be killed entirely.
> Better just use what we use for pathnames: full_name_hash() (which
> gets a pointer and length) and hash_name (which gets the string).
> 
> Those functions do the "word-at-a-time" optimization, and they should
> do a good enough job. If they aren't, we should fix them, because they
> are a hell of a lot more important than anything that the svcauth code
> does.

He had a typically fulminating comment about the hard-to-predict if()
branch in the inner loop.

 include/linux/sunrpc/svcauth.h | 36 +++++-------------------------------
 1 file changed, 5 insertions(+), 31 deletions(-)

diff --git a/include/linux/sunrpc/svcauth.h b/include/linux/sunrpc/svcauth.h
index c00f53a4..ef2b2552 100644
--- a/include/linux/sunrpc/svcauth.h
+++ b/include/linux/sunrpc/svcauth.h
@@ -16,6 +16,7 @@
 #include <linux/sunrpc/cache.h>
 #include <linux/sunrpc/gss_api.h>
 #include <linux/hash.h>
+#include <linux/stringhash.h>
 #include <linux/cred.h>
 
 struct svc_cred {
@@ -165,41 +166,14 @@ extern int svcauth_unix_set_client(struct svc_rqst *rqstp);
 extern int unix_gid_cache_create(struct net *net);
 extern void unix_gid_cache_destroy(struct net *net);
 
-static inline unsigned long hash_str(char *name, int bits)
+static inline unsigned long hash_str(char const *name, int bits)
 {
-	unsigned long hash = 0;
-	unsigned long l = 0;
-	int len = 0;
-	unsigned char c;
-	do {
-		if (unlikely(!(c = *name++))) {
-			c = (char)len; len = -1;
-		}
-		l = (l << 8) | c;
-		len++;
-		if ((len & (BITS_PER_LONG/8-1))==0)
-			hash = hash_long(hash^l, BITS_PER_LONG);
-	} while (len);
-	return hash >> (BITS_PER_LONG - bits);
+	return hash_32(hashlen_hash(hash_string(name)), bits);
 }
 
-static inline unsigned long hash_mem(char *buf, int length, int bits)
+static inline unsigned long hash_mem(char const *buf, int length, int bits)
 {
-	unsigned long hash = 0;
-	unsigned long l = 0;
-	int len = 0;
-	unsigned char c;
-	do {
-		if (len == length) {
-			c = (char)len; len = -1;
-		} else
-			c = *buf++;
-		l = (l << 8) | c;
-		len++;
-		if ((len & (BITS_PER_LONG/8-1))==0)
-			hash = hash_long(hash^l, BITS_PER_LONG);
-	} while (len);
-	return hash >> (BITS_PER_LONG - bits);
+	return hash_32(full_name_hash(buf, length), bits);
 }
 
 #endif /* __KERNEL__ */
-- 
2.8.1

Re: [PATCH RESEND 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string()

[J. Bruce Fields]

On Thu, May 26, 2016 at 02:39:39PM -0400, George Spelvin wrote:
> Finally, the first use of previous two patches: Eliminate the
> separate ad-hoc string hash functions in the sunrpc code.
> 
> This also eliminates the only caller of hash_long which asks for
> more than 32 bits of output.
> 
> sunrpc guys: Is it okay if I send this to Linus directly?

Sorry for ignoring this.... Looks straightforward to me; for what it's
worth:

	Acked-by: J. Bruce Fields <bfields@redhat.com>

If you have a git branch I could fetch, I could also run my usual tests
on it.  (Which would only help in the unlikely case this breaks nfs
completely somehow--they certainly wouldn't catch some hash performance
regressoin.)

--b.

> 
> Signed-off-by: George Spelvin <linux@sciencehorizons.net>
> Cc: "J. Bruce Fields" <bfields@fieldses.org>
> Cc: Jeff Layton <jlayton@poochiereds.net>
> Cc: linux-nfs@vger.kernel.org
> ---
> I have't heard anything from the linix-NFS crowd, so this is a re-ping.
> 
> I've been having some e-mail troubles leading to increased spam scores
> due to a recent domain name change, so this is a resend from a different
> address in case the previous copy got binned.
> 
> I'll happily send along the entire patch series, but the tl;dr is this
> is part of a patch series that improves the <linux/hash.h> and fs/namei.c
> hash functions, implementing Linus's suggestion:
> 
> On Mon, 02 May 2016 at 09:24:21, Linus Torvalds wrote:
> > That said, I actually think hash_str() should be killed entirely.
> > Better just use what we use for pathnames: full_name_hash() (which
> > gets a pointer and length) and hash_name (which gets the string).
> > 
> > Those functions do the "word-at-a-time" optimization, and they should
> > do a good enough job. If they aren't, we should fix them, because they
> > are a hell of a lot more important than anything that the svcauth code
> > does.
> 
> He had a typically fulminating comment about the hard-to-predict if()
> branch in the inner loop.
> 
>  include/linux/sunrpc/svcauth.h | 36 +++++-------------------------------
>  1 file changed, 5 insertions(+), 31 deletions(-)
> 
> diff --git a/include/linux/sunrpc/svcauth.h b/include/linux/sunrpc/svcauth.h
> index c00f53a4..ef2b2552 100644
> --- a/include/linux/sunrpc/svcauth.h
> +++ b/include/linux/sunrpc/svcauth.h
> @@ -16,6 +16,7 @@
>  #include <linux/sunrpc/cache.h>
>  #include <linux/sunrpc/gss_api.h>
>  #include <linux/hash.h>
> +#include <linux/stringhash.h>
>  #include <linux/cred.h>
>  
>  struct svc_cred {
> @@ -165,41 +166,14 @@ extern int svcauth_unix_set_client(struct svc_rqst *rqstp);
>  extern int unix_gid_cache_create(struct net *net);
>  extern void unix_gid_cache_destroy(struct net *net);
>  
> -static inline unsigned long hash_str(char *name, int bits)
> +static inline unsigned long hash_str(char const *name, int bits)
>  {
> -	unsigned long hash = 0;
> -	unsigned long l = 0;
> -	int len = 0;
> -	unsigned char c;
> -	do {
> -		if (unlikely(!(c = *name++))) {
> -			c = (char)len; len = -1;
> -		}
> -		l = (l << 8) | c;
> -		len++;
> -		if ((len & (BITS_PER_LONG/8-1))==0)
> -			hash = hash_long(hash^l, BITS_PER_LONG);
> -	} while (len);
> -	return hash >> (BITS_PER_LONG - bits);
> +	return hash_32(hashlen_hash(hash_string(name)), bits);
>  }
>  
> -static inline unsigned long hash_mem(char *buf, int length, int bits)
> +static inline unsigned long hash_mem(char const *buf, int length, int bits)
>  {
> -	unsigned long hash = 0;
> -	unsigned long l = 0;
> -	int len = 0;
> -	unsigned char c;
> -	do {
> -		if (len == length) {
> -			c = (char)len; len = -1;
> -		} else
> -			c = *buf++;
> -		l = (l << 8) | c;
> -		len++;
> -		if ((len & (BITS_PER_LONG/8-1))==0)
> -			hash = hash_long(hash^l, BITS_PER_LONG);
> -	} while (len);
> -	return hash >> (BITS_PER_LONG - bits);
> +	return hash_32(full_name_hash(buf, length), bits);
>  }
>  
>  #endif /* __KERNEL__ */
> -- 
> 2.8.1
> 

Re: [PATCH RESEND 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string()

[George Spelvin]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain, Size: 1059 bytes --]

J. Bruce Fields wrote:
> Hm.  Are there some missing includes, maybe?:
> 
> In file included from include/linux/hashtable.h:12:0,
>                  from fs/btrfs/props.c:19:
> include/linux/hash.h: In function `hash_64_genericâ':
>  include/linux/hash.h:76:3: error: implicit declaration of function `BUILD_BUG_ON' [-Werror=implicit-function-declaration]
>    BUILD_BUG_ON(bits > 32 || bits == 0);
>    ^

Ooh, excellent catch!  That's a paper-bag mistake.  Thank you for
saving me from Linus's wrath.

(My lame explanation: "make allyesconfig" is annoyingly slow, and that
error was an afterthought I added after the "real work" parts.  Do you
think that check should even be in there?)

#include <linux/bug.h> and try again, or re-pull...

Oh, bugger, that revealed something else that scrolled past
in previous builds.  I'll push when I've fixed this one, too..

drivers/base/power/trace.c defines its own static hash_string()
function *and* includes <limux/dcache.h>, leading to conflicting
declarations.  Which one to rename...?


Anyway, much obliged!

Re: [PATCH RESEND 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string()

[George Spelvin]

Okay, it's now okay to re-pull.  I renamed mine to hashlen_string(),
since that's what it returns (the hash and the length), which both
solves the problem,and is frankly a better name.

Thank you for prodding me to wade through the GCC 6 warning
spam.  Thre were some problems hiding there.

I discovered that adding #include <linux/bug.h> breaks the tools.perf
build.  It makes local copies of some kernel headers under control of
a MANIFEST file, and I can't be arsed to figure it out.

So I deleted the BUILD_BUG_ON entirely, and it's now (finally!) finished
an allyesconfig build without any warnings that look new.

Re: [PATCH RESEND 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string()

[J. Bruce Fields]

On Thu, May 26, 2016 at 11:29:46PM -0400, George Spelvin wrote:
> Okay, it's now okay to re-pull.  I renamed mine to hashlen_string(),
> since that's what it returns (the hash and the length), which both
> solves the problem,and is frankly a better name.
> 
> Thank you for prodding me to wade through the GCC 6 warning
> spam.  Thre were some problems hiding there.
> 
> I discovered that adding #include <linux/bug.h> breaks the tools.perf
> build.  It makes local copies of some kernel headers under control of
> a MANIFEST file, and I can't be arsed to figure it out.
> 
> So I deleted the BUILD_BUG_ON entirely, and it's now (finally!) finished
> an allyesconfig build without any warnings that look new.

I also needed the following.

That still gets me some compiler warnings in gfs2, but looks like code
your patches don't touch, so that's probably a preexisting gfs2 problem.

But... the resulting kernel isn't booting succesfully for me.  Not sure
what's up there, I'll investigate some more.

--b.

commit 5168cb717b37ef0b183dee9b411a85831b558b9b
Author: J. Bruce Fields <bfields@redhat.com>
Date:   Fri May 27 07:10:21 2016 -0400

    XXX

diff --git a/include/linux/hash.h b/include/linux/hash.h
index 1afde47..21b8ebc 100644
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -15,6 +15,7 @@
  */
 
 #include <asm/types.h>
+#include <linux/bug.h>
 #include <linux/compiler.h>
 
 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */

Re: [PATCH RESEND 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string()

[George Spelvin]

> I also needed the following.

I just found out five mintues ago that the push last night failed
because I forgot the -f.  :-( *Now* it'll pull and work.

> But... the resulting kernel isn't booting succesfully for me.  Not sure
> what's up there, I'll investigate some more.

Thank you *so much* for the help, but do get the latest.

Re: [PATCH RESEND 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string()

[J. Bruce Fields]

On Fri, May 27, 2016 at 08:20:22AM -0400, George Spelvin wrote:
> > I also needed the following.
> 
> I just found out five mintues ago that the push last night failed
> because I forgot the -f.  :-( *Now* it'll pull and work.
> 
> > But... the resulting kernel isn't booting succesfully for me.  Not sure
> > what's up there, I'll investigate some more.
> 
> Thank you *so much* for the help, but do get the latest.

I don't see yet what the problem is, but I can tell you that as of
95d5dfa805a6344e69ef41f56bb4955269079ec7 "fs/namei.c: Improve dcache
hash function" my (Fedora 21 x86_64) VM no longer starts up reliably....

--b.

Re: [PATCH RESEND 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string()

[George Spelvin]

> I don't see yet what the problem is, but I can tell you that as of
> 95d5dfa805a6344e69ef41f56bb4955269079ec7 "fs/namei.c: Improve dcache
> hash function" my (Fedora 21 x86_64) VM no longer starts up reliably....

Just a note... the bug I explained in a previous e-mail was the
culprit, and it is now been confirmed fixed.

Here's the fixed patch, with extra comments around full_name_hash
for any future hackers.

(I also updated the self-test to prevent regressions.)


commit 0a3d46fc8bcc33f6157b3f4e72a075c4207cd673
Author: George Spelvin <linux@sciencehorizons.net>
Date:   Mon May 23 07:43:58 2016 -0400

    fs/namei.c: Improve dcache hash function
    
    Patch 0fed3ac866 improved the hash mixing, but the function is slower
    than necessary; there's a 7-instruction dependency chain (10 on x86)
    each loop iteration.
    
    Word-at-a-time access is a very tight loop (which is good, because
    link_path_walk() is one of the hottest code paths in the entire kernel),
    and the hash mixing function must not have a longer latency to avoid
    slowing it down.
    
    There do not appear to be any published fast hash functions that:
    1) Operate on the input a word at a time, and
    2) Don't need to know the length of the input beforehand, and
    3) Have a single iterated mixing function, not needing conditional
       branches or unrolling to distinguish different loop iterations.
    
    One of the algorithms which comes closest is Yann Collet's xxHash, but
    that's two dependent multiplies per word, which is too much.
    
    The key insights in this design are:
    
    1) Except for multiplies, to diffuse one input bit across 64 bits of hash
       state takes at least log2(64) = 6 sequentially dependent instructions.
       That is more cycles than we'd like.
    2) An operation like "hash ^= hash << 13" requires a second temporary
       register anyway, and on a 2-operand machine like x86, it's three
       instructions.
    3) A better use of a second register is to hold a two-word hash state.
       With careful design, no temporaries are needed at all, so it doesn't
       increase register pressure.  And this gets rid of register copying
       on 2-operand machines, so the code is smaller and faster.
    4) Using two words of state weakens the requriement for one-round mixing;
       we now have two rounds of mixing before cancellation is possible.
    5) A two-word hash state also allows operations on both halves to be
       done in parallel, so on a superscalar processor we get more mixing
       in fewer cycles.
    
    I ended up using a mixing function inspired by the ChaCha and Speck
    round functions.  It is 6 simple instructions and 3 cycles per iteration
    (assuming mutliply by 9 can be done by an "lea" isntruction):
    
    		x ^= *input++;
    	y ^= x;	x = ROL(x, K1);
    	x += y;	y = ROL(y, K2);
    	y *= 9;
    
    Not only is this reversible, two consecutive rounds are reversible:
    if you are given the initial and final states, but not the intermediate
    state, it is possible to compute both input words.  This means that at
    least 3 words of input are required to create a collision.
    
    (It also has the property, used by hash_name() to avoid a branch, that
    it hashes all-zero to all-zero.)
    
    The rotate constants K1 and K2 were found by experiment.  The search took
    a sample of random initial states (I used 1023) and considered the effect
    of flipping each of the 64 input bits on each of the 128 output bits two
    rounds later.  Each of the 8192 pairs can be considered a biased coin, and
    adding up the Shannon entropy of all of them produces a score.
    
    The best-scoring shifts also did well in other tests (flipping bits in y,
    trying 3 or 4 rounds of mixing, flipping all 64*63/2 pairs of input bits),
    so the choice was made with the additional constraint that the sum of the
    shifts is odd and not too close to the word size.
    
    The final state is then folded into a 32-bit hash value by a less carefully
    optimized multiply-based scheme.  This also has to be fast, as pathname
    components tend to be short (the most common case is one iteration!), but
    there's some room for latency, as there is a fair bit of intervening logic
    before the hash value is used for anything.
    
    (Performance verified with "bonnie++ -s 0 -n 1536:-2" on tmpfs.  I need
    a better benchmark; the numbers seem to show a slight dip in performance
    between 4.6.0 and this patch, but they're too noisy to quote.)
    
    Special thanks to Bruce fields for helping me find a nasty fencepost
    error in an earlier version of this patch.
    
    Signed-off-by: George Spelvin <linux@sciencehorizons.net>
    Thanks-to: J. Bruce Fields <bfields@redhat.com>

diff --git a/fs/namei.c b/fs/namei.c
index d43391d0..38e14ad0 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -35,6 +35,7 @@
 #include <linux/fs_struct.h>
 #include <linux/posix_acl.h>
 #include <linux/hash.h>
+#include <linux/bitops.h>
 #include <asm/uaccess.h>
 
 #include "internal.h"
@@ -1788,81 +1789,124 @@ static int walk_component(struct nameidata *nd, int flags)
 #include <asm/word-at-a-time.h>
 
 #ifdef CONFIG_64BIT
-
-static inline unsigned int fold_hash(unsigned long hash)
-{
-	return hash_64(hash, 32);
-}
+/*
+ * Register pressure in the mixing function is an issue, particularly
+ * on 32-bit x86, but almost any function requires one state value and
+ * one temporary.  Instead, use a function designed for two state values
+ * and no temporaries.
+ *
+ * This function cannot create a collision in only two iterations, so
+ * we have two iterations to achieve avalanche.  In those two iterations,
+ * we have six layers of mixing, which is enough to spread one bit's
+ * influence out to 2^6 = 64 state bits.
+ *
+ * Rotate constants are scored by considering either 64 one-bit input
+ * deltas or 64*63/2 = 2016 two-bit input deltas, and finding the
+ * probability of that delta causing a change to each of the 128 output
+ * bits, using a sample of random initial states.
+ *
+ * The Shannon entropy of the computed probabilities is then summed
+ * to produce a score.  Ideally, any input change has a 50% chance of
+ * toggling any given output bit.
+ *
+ * Mixing scores (in bits) for (12,45):
+ * Input delta: 1-bit      2-bit
+ * 1 round:     713.3    42542.6
+ * 2 rounds:   2753.7   140389.8
+ * 3 rounds:   5954.1   233458.2
+ * 4 rounds:   7862.6   256672.2
+ * Perfect:    8192     258048
+ *            (64*128) (64*63/2 * 128)
+ */
+#define HASH_MIX(x, y, a) 	\
+	( 	x ^= (a),	\
+	y ^= x,	x = rol64(x,12),\
+	x += y,	y = rol64(y,45),\
+	y *= 9			)
 
 /*
- * This is George Marsaglia's XORSHIFT generator.
- * It implements a maximum-period LFSR in only a few
- * instructions.  It also has the property (required
- * by hash_name()) that mix_hash(0) = 0.
+ * Fold two longs into one 32-bit hash value.  This must be fast, but
+ * latency isn't quite as critical, as there is a fair bit of additional
+ * work done before the hash value is used.
  */
-static inline unsigned long mix_hash(unsigned long hash)
+static inline unsigned int fold_hash(unsigned long x, unsigned long y)
 {
-	hash ^= hash << 13;
-	hash ^= hash >> 7;
-	hash ^= hash << 17;
-	return hash;
+        y ^= x * GOLDEN_RATIO_64;
+        y *= GOLDEN_RATIO_64;
+        return y >> 32;
 }
 
 #else	/* 32-bit case */
 
-#define fold_hash(x) (x)
+/*
+ * 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 long mix_hash(unsigned long hash)
+static inline unsigned int fold_hash(unsigned long x, unsigned long y)
 {
-	hash ^= hash << 13;
-	hash ^= hash >> 17;
-	hash ^= hash << 5;
-	return hash;
+	/* Use arch-optimized multiply if one exists */
+        return __hash_32(y ^ __hash_32(x));
 }
 
 #endif
 
-/* Return the hash of a string of known length */
+/*
+ * Return the hash of a string of known length.  This is carfully
+ * designed to match hash_name(), which is the more critical function.
+ * In particular, we must hash a final word containing 0..7 payload bytes,
+ * to match the way that hash_name() iterates until it finds the delimiter
+ * after the name.
+ */
 unsigned int full_name_hash(const char *name, unsigned int len)
 {
-	unsigned long a, hash = 0;
+	unsigned long a, x = 0, y = 0;
 
 	for (;;) {
 		a = load_unaligned_zeropad(name);
 		if (len < sizeof(unsigned long))
 			break;
-		hash = mix_hash(hash + a);
+		HASH_MIX(x, y, a);
 		name += sizeof(unsigned long);
 		len -= sizeof(unsigned long);
 		if (!len)
-			goto done;
+			goto done;	/* Optimize out bytemask_from_count(0) */
 	}
-	hash += a & bytemask_from_count(len);
+	x ^= a & bytemask_from_count(len);
 done:
-	return fold_hash(hash);
+	return fold_hash(x, y);
 }
 EXPORT_SYMBOL(full_name_hash);
 
 /* Return the "hash_len" (hash and length) of a null-terminated string */
 u64 hashlen_string(const char *name)
 {
-	unsigned long a, adata, mask, hash, len;
+	unsigned long a = 0, x = 0, y = 0, adata, mask, len;
 	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
 
-	hash = a = 0;
 	len = -sizeof(unsigned long);
 	do {
-		hash = mix_hash(hash + a);
+		HASH_MIX(x, y, a);
 		len += sizeof(unsigned long);
 		a = load_unaligned_zeropad(name+len);
 	} while (!has_zero(a, &adata, &constants));
 
 	adata = prep_zero_mask(a, adata, &constants);
 	mask = create_zero_mask(adata);
-	hash += a & zero_bytemask(mask);
-	len += find_zero(mask);
+	x ^= a & zero_bytemask(mask);
 
-	return hashlen_create(fold_hash(hash), len);
+	return hashlen_create(fold_hash(x, y), len + find_zero(mask));
 }
 EXPORT_SYMBOL(hashlen_string);
 
@@ -1872,13 +1916,12 @@ EXPORT_SYMBOL(hashlen_string);
  */
 static inline u64 hash_name(const char *name)
 {
-	unsigned long a, b, adata, bdata, mask, hash, len;
+	unsigned long a = 0, b, x = 0, y = 0, adata, bdata, mask, len;
 	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
 
-	hash = a = 0;
 	len = -sizeof(unsigned long);
 	do {
-		hash = mix_hash(hash + a);
+		HASH_MIX(x, y, a);
 		len += sizeof(unsigned long);
 		a = load_unaligned_zeropad(name+len);
 		b = a ^ REPEAT_BYTE('/');
@@ -1886,15 +1929,13 @@ static inline u64 hash_name(const char *name)
 
 	adata = prep_zero_mask(a, adata, &constants);
 	bdata = prep_zero_mask(b, bdata, &constants);
-
 	mask = create_zero_mask(adata | bdata);
+	x ^= a & zero_bytemask(mask);
 
-	hash += a & zero_bytemask(mask);
-	len += find_zero(mask);
-	return hashlen_create(fold_hash(hash), len);
+	return hashlen_create(fold_hash(x, y), len + find_zero(mask));
 }
 
-#else
+#else	/* !CONFIG_DCACHE_WORD_ACCESS: Slow, byte-at-a-time version */
 
 /* Return the hash of a string of known length */
 unsigned int full_name_hash(const char *name, unsigned int len)