linux-kernel - RE: [PATCH RFC] netfilter: cacheline aligning in struct netns

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D0F6DB542@AcuExch.aculab.com>
Date:	Wed, 12 Mar 2014 09:36:36 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Zhouyi Zhou' <zhouzhouyi@...il.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"pablo@...filter.org" <pablo@...filter.org>,
	"jbenc@...hat.com" <jbenc@...hat.com>,
	"brouer@...hat.com" <brouer@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	Zhouyi Zhou <yizhouzhou@....ac.cn>
Subject: RE: [PATCH RFC] netfilter: cacheline aligning in struct netns_ct

From: Zhouyi Zhou
> not frequently changing components should share same cachelines
> 
> Signed-off-by: Zhouyi Zhou <yizhouzhou@....ac.cn>
> ---
>  include/net/netns/conntrack.h |   12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/include/net/netns/conntrack.h b/include/net/netns/conntrack.h
> index fbcc7fa..69d2d58 100644
> --- a/include/net/netns/conntrack.h
> +++ b/include/net/netns/conntrack.h
> @@ -65,6 +65,12 @@ struct nf_ip_net {
>  struct netns_ct {
>  	atomic_t		count;
>  	unsigned int		expect_count;
> +	struct hlist_nulls_head	unconfirmed;
> +	struct hlist_nulls_head	dying;
> +	struct hlist_nulls_head tmpl;
> +
> +	/*  not frequently changing components should share same cachelines */
> +	struct nf_ip_net	nf_ct_proto ____cacheline_aligned_in_smp;

I'm not sure this has the effect you are trying to achieve.
Adding an __attribute__((aligned(n))) to a structure member not only
adds padding before the member, but also pads the member to a multiple
of the specified size.

With large cache lines you probably want neither.
IIRC one of the ppc systems has something like 256 byte cache lines
(and if it doesn't now, it might have soon).
So arbitrarily cache-line aligning structure members may not be a good idea.

In any case reducing the number of cache lines accessed, and the number
dirtied is probably more important than putting 'readonly' data in its
own cache lines.

	David



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