Messages in this thread | | | Subject | Re: [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu_head | From | Eric Dumazet <> | Date | Tue, 01 Mar 2011 10:20:13 +0100 |
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Le mardi 01 mars 2011 à 16:53 +0800, Lai Jiangshan a écrit : > On 03/01/2011 04:16 PM, David Miller wrote: > > From: Lai Jiangshan <laijs@cn.fujitsu.com> > > Date: Tue, 01 Mar 2011 16:03:44 +0800 > > > >> > >> struct dst_entry assumes the size of struct rcu_head as 2 * sizeof(long) > >> and manually adds pads for aligning for "__refcnt". > >> > >> When the size of struct rcu_head is changed, these manual padding > >> is wrong. Use __attribute__((aligned (64))) instead. > >> > >> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> > > > > We don't want to use the align if it's going to waste lots of space. > > > > Instead we want to rearrange the structure so that the alignment comes > > more cheaply. > > Subject: [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu_head > > struct dst_entry assumes the size of struct rcu_head as 2 * sizeof(long) > and manually adds pads for aligning for "__refcnt". > > When the size of struct rcu_head is changed, these manual padding > are hardly suit for the changes. So we rearrange the structure, > and move the seldom access rcu_head to the end of the structure. > > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> > --- > > diff --git a/include/net/dst.h b/include/net/dst.h > index 93b0310..d8c5296 100644 > --- a/include/net/dst.h > +++ b/include/net/dst.h > @@ -37,7 +37,6 @@ > struct sk_buff; > > struct dst_entry { > - struct rcu_head rcu_head; > struct dst_entry *child; > struct net_device *dev; > short error; > @@ -78,6 +77,13 @@ struct dst_entry { > __u32 __pad2; > #endif > > + unsigned long lastuse; > + union { > + struct dst_entry *next; > + struct rtable __rcu *rt_next; > + struct rt6_info *rt6_next; > + struct dn_route __rcu *dn_next; > + }; > > /* > * Align __refcnt to a 64 bytes alignment > @@ -92,13 +98,7 @@ struct dst_entry { > */ > atomic_t __refcnt; /* client references */ > int __use; > - unsigned long lastuse; > - union { > - struct dst_entry *next; > - struct rtable __rcu *rt_next; > - struct rt6_info *rt6_next; > - struct dn_route __rcu *dn_next; > - }; > + struct rcu_head rcu_head; > }; > > #ifdef __KERNEL__
Nope...
"lastuse" and "next" must be in this place, or this introduce false sharing we wanted to avoid in the past.
I suggest you leave this code as is, we will address the problem when rcu_head changes (assuming we can test a CONFIG_RCU_HEAD_DEBUG or something)
First part of "struct dst_entry" is mostly read, while part beginning after refcnt is often written.
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