netdev - Re: hackbench regression due to commit 9dfc6e68bfe6e

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Date:	Wed, 7 Apr 2010 11:43:21 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Pekka Enberg <penberg@...helsinki.fi>, alex.shi@...el.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Ma, Ling" <ling.ma@...el.com>,
	"Chen, Tim C" <tim.c.chen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: hackbench regression due to commit 9dfc6e68bfe6e

On Wed, 7 Apr 2010, Zhang, Yanmin wrote:

> I collected retired instruction, dtlb miss and LLC miss.
> Below is data of LLC miss.
>
> Kernel 2.6.33:
>     20.94%        hackbench  [kernel.kallsyms]                                       [k] copy_user_generic_string
>     14.56%        hackbench  [kernel.kallsyms]                                       [k] unix_stream_recvmsg
>     12.88%        hackbench  [kernel.kallsyms]                                       [k] kfree
>      7.37%        hackbench  [kernel.kallsyms]                                       [k] kmem_cache_free
>      7.18%        hackbench  [kernel.kallsyms]                                       [k] kmem_cache_alloc_node
>      6.78%        hackbench  [kernel.kallsyms]                                       [k] kfree_skb
>      6.27%        hackbench  [kernel.kallsyms]                                       [k] __kmalloc_node_track_caller
>      2.73%        hackbench  [kernel.kallsyms]                                       [k] __slab_free
>      2.21%        hackbench  [kernel.kallsyms]                                       [k] get_partial_node
>      2.01%        hackbench  [kernel.kallsyms]                                       [k] _raw_spin_lock
>      1.59%        hackbench  [kernel.kallsyms]                                       [k] schedule
>      1.27%        hackbench  hackbench                                               [.] receiver
>      0.99%        hackbench  libpthread-2.9.so                                       [.] __read
>      0.87%        hackbench  [kernel.kallsyms]                                       [k] unix_stream_sendmsg
>
> Kernel 2.6.34-rc3:
>     18.55%        hackbench  [kernel.kallsyms]                                                     [k] copy_user_generic_str
> ing
>     13.19%        hackbench  [kernel.kallsyms]                                                     [k] unix_stream_recvmsg
>     11.62%        hackbench  [kernel.kallsyms]                                                     [k] kfree
>      8.54%        hackbench  [kernel.kallsyms]                                                     [k] kmem_cache_free
>      7.88%        hackbench  [kernel.kallsyms]                                                     [k] __kmalloc_node_track_
> caller

Seems that the overhead of __kmalloc_node_track_caller was increased. The
function inlines slab_alloc().

>      6.54%        hackbench  [kernel.kallsyms]                                                     [k] kmem_cache_alloc_node
>      5.94%        hackbench  [kernel.kallsyms]                                                     [k] kfree_skb
>      3.48%        hackbench  [kernel.kallsyms]                                                     [k] __slab_free
>      2.15%        hackbench  [kernel.kallsyms]                                                     [k] _raw_spin_lock
>      1.83%        hackbench  [kernel.kallsyms]                                                     [k] schedule
>      1.82%        hackbench  [kernel.kallsyms]                                                     [k] get_partial_node
>      1.59%        hackbench  hackbench                                                             [.] receiver
>      1.37%        hackbench  libpthread-2.9.so                                                     [.] __read

I wonder if this is not related to the kmem_cache_cpu structure straggling
cache line boundaries under some conditions. On 2.6.33 the kmem_cache_cpu
structure was larger and therefore tight packing resulted in different
alignment.

Could you see how the following patch affects the results. It attempts to
increase the size of kmem_cache_cpu to a power of 2 bytes. There is also
the potential that other per cpu fetches to neighboring objects affect the
situation. We could cacheline align the whole thing.

---
 include/linux/slub_def.h |    5 +++++
 1 file changed, 5 insertions(+)

Index: linux-2.6/include/linux/slub_def.h
===================================================================
--- linux-2.6.orig/include/linux/slub_def.h	2010-04-07 11:33:50.000000000 -0500
+++ linux-2.6/include/linux/slub_def.h	2010-04-07 11:35:18.000000000 -0500
@@ -38,6 +38,11 @@ struct kmem_cache_cpu {
 	void **freelist;	/* Pointer to first free per cpu object */
 	struct page *page;	/* The slab from which we are allocating */
 	int node;		/* The node of the page (or -1 for debug) */
+#ifndef CONFIG_64BIT
+	int dummy1;
+#endif
+	unsigned long dummy2;
+
 #ifdef CONFIG_SLUB_STATS
 	unsigned stat[NR_SLUB_STAT_ITEMS];
 #endif
--
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