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[PATCH] futex: eliminate cache miss from futex_hash()

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  [PATCH] futex: eliminate cache miss from futex_hash() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-09-09 23:40 +0200
    Re: [PATCH] futex: eliminate cache miss from futex_hash() Davidlohr Bueso <dave@stgolabs.net> - 2015-09-10 12:30 +0200
      Re: [PATCH] futex: eliminate cache miss from futex_hash() Ingo Molnar <mingo@kernel.org> - 2015-09-12 12:00 +0200

#1221744 — [PATCH] futex: eliminate cache miss from futex_hash()

futex_hash() references two global variables: the base pointer
futex_queues and the size of the array futex_hashsize. The latter is
marked __read_mostly, while the former is not, so they are likely to
end up very far from each other. This means that futex_hash() is
likely to encounter two cache misses.

We could mark futex_queues as __read_mostly as well, but that doesn't
guarantee they'll end up next to each other (and even if they do, they
may still end up in different cache lines). So put the two variables
in a small singleton struct with sufficient alignment and mark that as
__read_mostly.

A diff of the disassembly shows what I'd expect:

 :      31 d1                   xor    %edx,%ecx
 :      c1 ca 12                ror    $0x12,%edx
 :      29 d1                   sub    %edx,%ecx
-:      48 8b 15 25 c8 e5 00    mov    0xe5c825(%rip),%rdx        # ffffffff81f149c8 <futex_hashsize>
+:      48 8b 15 35 c8 e5 00    mov    0xe5c835(%rip),%rdx        # ffffffff81f149d8 <__futex_data+0x8>
 :      31 c8                   xor    %ecx,%eax
 :      c1 c9 08                ror    $0x8,%ecx
 :      29 c8                   sub    %ecx,%eax
 :      48 83 ea 01             sub    $0x1,%rdx
 :      48 21 d0                and    %rdx,%rax
 :      48 c1 e0 06             shl    $0x6,%rax
-:      48 03 05 e4 5e 02 01    add    0x1025ee4(%rip),%rax        # ffffffff820de0a0 <futex_queues>
+:      48 03 05 14 c8 e5 00    add    0xe5c814(%rip),%rax        # ffffffff81f149d0 <__futex_data>
 :      c3                      retq
 :      0f 1f 00                nopl   (%rax)

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
Resending since this was never picked up - and I assume it's actually
ok. Also, this time the alignment is spelled 2*sizeof(long) to avoid
wasting 8 bytes on 32bit.

 kernel/futex.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 6e443efc65f4..dfc86e93c31d 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -255,9 +255,18 @@ struct futex_hash_bucket {
 	struct plist_head chain;
 } ____cacheline_aligned_in_smp;
 
-static unsigned long __read_mostly futex_hashsize;
+/*
+ * The base of the bucket array and its size are always used together
+ * (after initialization only in hash_futex()), so ensure that they
+ * reside in the same cacheline.
+ */
+static struct {
+	struct futex_hash_bucket *queues;
+	unsigned long            hashsize;
+} __futex_data __read_mostly __aligned(2*sizeof(long));
+#define futex_queues   (__futex_data.queues)
+#define futex_hashsize (__futex_data.hashsize)
 
-static struct futex_hash_bucket *futex_queues;
 
 /*
  * Fault injections for futexes.
-- 
2.1.3

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#1222094

On Wed, 09 Sep 2015, Rasmus Villemoes wrote:

>futex_hash() references two global variables: the base pointer
>futex_queues and the size of the array futex_hashsize. The latter is
>marked __read_mostly, while the former is not, so they are likely to
>end up very far from each other. This means that futex_hash() is
>likely to encounter two cache misses.
>
>We could mark futex_queues as __read_mostly as well, but that doesn't
>guarantee they'll end up next to each other (and even if they do, they
>may still end up in different cache lines). So put the two variables
>in a small singleton struct with sufficient alignment and mark that as
>__read_mostly.

This really doesn't have much practical effect -- not even on larger
boxes, where such things matter. For instance, I ran the patch on a
60-core IvyBridge with 'perf-bench futex', for which futex-hash
particularly benefits in good data layout (ie our current smp alignment).

http://linux-scalability.org/futex-__futex_data/

I think we should leave it as is.

Thanks,
Davidlohr
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#1223363

* Davidlohr Bueso <dave@stgolabs.net> wrote:

> On Wed, 09 Sep 2015, Rasmus Villemoes wrote:
> 
> >futex_hash() references two global variables: the base pointer
> >futex_queues and the size of the array futex_hashsize. The latter is
> >marked __read_mostly, while the former is not, so they are likely to
> >end up very far from each other. This means that futex_hash() is
> >likely to encounter two cache misses.
> >
> >We could mark futex_queues as __read_mostly as well, but that doesn't
> >guarantee they'll end up next to each other (and even if they do, they
> >may still end up in different cache lines). So put the two variables
> >in a small singleton struct with sufficient alignment and mark that as
> >__read_mostly.
> 
> This really doesn't have much practical effect -- not even on larger
> boxes, where such things matter. For instance, I ran the patch on a
> 60-core IvyBridge with 'perf-bench futex', for which futex-hash
> particularly benefits in good data layout (ie our current smp alignment).
> 
> http://linux-scalability.org/futex-__futex_data/
> 
> I think we should leave it as is.

But ... given that these are shared-cached values (cached on all CPUs), this 
change would only be measurable in such a benchmark if the cache footprint of the 
test is just about to overflow the size of the CPU cache and the one extra cache 
line would cause cache trashing. That is very unlikely.

So such a change seems to make sense unless you can argue that it's _bad_ to move 
them closer to each other.

Thanks,

	Ingo
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