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    SubjectRe: [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu_head
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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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