Re: [RFC PATCH 1/9] ipvs network name space aware

To: paulmck@xxxxxxxxxxxxxxxxxx
Subject: Re: [RFC PATCH 1/9] ipvs network name space aware
Cc: Hans Schillstrom <hans.schillstrom@xxxxxxxxxxxx>, Daniel Lezcano <daniel.lezcano@xxxxxxx>, "lvs-devel@xxxxxxxxxxxxxxx" <lvs-devel@xxxxxxxxxxxxxxx>, "netdev@xxxxxxxxxxxxxxx" <netdev@xxxxxxxxxxxxxxx>, "netfilter-devel@xxxxxxxxxxxxxxx" <netfilter-devel@xxxxxxxxxxxxxxx>, "horms@xxxxxxxxxxxx" <horms@xxxxxxxxxxxx>, "ja@xxxxxx" <ja@xxxxxx>, "wensong@xxxxxxxxxxxx" <wensong@xxxxxxxxxxxx>
From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Date: Thu, 21 Oct 2010 10:58:24 +0200
> You said that there were a lot of "stepi" commands to get through
> rcu_read_lock() on x86_64.  This is quite surprising, especially if you
> built with CONFIG_RCU_TREE.  Even if you built with CONFIG_PREEMPT_RCU_TREE,
> you should only see something like the following from rcu_read_lock():
> 000000b7 <__rcu_read_lock>:
>       b7:     55                      push   %ebp
>       b8:     64 a1 00 00 00 00       mov    %fs:0x0,%eax
>       be:     ff 80 80 01 00 00       incl   0x180(%eax)
>       c4:     89 e5                   mov    %esp,%ebp
>       c6:     5d                      pop    %ebp
>       c7:     c3                      ret    
> Unless you have some sort of debugging options turned on.  Or unless
> six instructions counts for "quite many" stepi commands.  ;-)

Paul, this should be inlined, dont you think ?

Also, I dont understand why we use ACCESS_ONCE() in rcu_read_lock()


Apparently, some compilers are a bit noisy here.

mov    0x1b0(%rdx),%eax
inc    %eax
mov    %eax,0x1b0(%rdx)

instead of :

incl   0x1b0(%rax)

So if the ACCESS_ONCE() is needed, we might add a comment, because it's
not obvious ;)

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