netdev - Re: [RFC][PATCH] Turn part of SNMP accounting macros into functions

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Date:	Thu, 28 Aug 2008 14:51:20 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Pavel Emelyanov <xemul@...nvz.org>
Cc:	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [RFC][PATCH] Turn part of SNMP accounting macros into functions

Pavel Emelyanov a écrit :
> Eric Dumazet wrote:
>>
>> I dont know, but passing all those "struct net *net" to every 
>> network function in the kernel sounds overkill, especially for
>> !CONFIG_NET_NS users. This is pure bloat.
>>
>> We could define two macros so that function prototypes dont include
>> useless pointers, especially on arches where only first and second
>> parameter is passed in eax and edx register ;)
>>
>> #ifdef CONFIG_NET_NS
>> # define VNETPTR ,struct net *net
>> # define NETPTR  net
>> #else
>> # define VNETPTR
>> # defint NETPTR  &init_net
>> #endif
>>
>> ...
>> void TCP_DEC_STATS(int field VNETPTR);
>> ...
>> void TCP_DEC_STATS(int field VNETPTR)
>> {
>> 	SNMP_DEC_STATS((NETPTR)->mib.tcp_statistics, field);
>> }
>> ...
> 
> I agree with this, but I've checked whether compiling out the first argument
> of XXX_STATS would help and found out that there's almost no difference (-9
> bytes) in the net/ipv4/built-in.o for NET_NS=n case. 
> 
> After turning the macros into functions, compiling the first argument out gives
> us 500 more bytes out. Good catch, but I have a question - the way you proposed
> to implement the function itself is rather nice, but how would you implement
> the *call* to that function in order to make it variable-arguments depending on
> the config option?
> 
> I see the similar way:
> 
> +#ifdef CONFIG_NET_NS
> +#define NET_ARG(net) net,
> +#else
> +#define NET_ARG(net)
> +#endif
> ...
> -	TCP_INC_STATS(sock_net(sk), field);
> +	TCP_INC_STATS(NET_ARG(sock_net(sk) field);
> 
> but do these 500 bytes compensate for this ugly style?

500 bytes here, but what about other uses in kernel ?

Sometime I wonder if a *global* pointer ('current' pointer for example is not passed to every kernel function that want access to current struture) would save many kbytes in vmlinux text. This way, !CONFIG_NET_NS overhead would completely vanish.

Oh well...




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