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SubjectRe: [PATCH] netns: deinline net_generic()
On 04/14/2015 03:19 PM, Eric Dumazet wrote:
> On Tue, 2015-04-14 at 14:25 +0200, Denys Vlasenko wrote:
>> On x86 allyesconfig build:
>> The function compiles to 130 bytes of machine code.
>> It has 493 callsites.
>> Total reduction of vmlinux size: 27906 bytes.
>>
>> text data bss dec hex filename
>> 82447071 22255384 20627456 125329911 77861f7 vmlinux4
>> 82419165 22255384 20627456 125302005 777f4f5 vmlinux5
>
> This sounds a big hammer to me.
>
> These savings probably comes from the BUG_ON() that could simply be
> removed.
> The second one for sure has no purpose. First one looks defensive.
>
> For a typical (non allyesconfig) kernel, net_generic() would translate
> to :
>
> return net->gen[id - 1]

My allyesconfig, with BUG_ON's commented out:

void *net_generic(const struct net *net, int id)
{
struct net_generic *ng;
void *ptr;

rcu_read_lock();
ng = rcu_dereference(net->gen);
// BUG_ON(id == 0 || id > ng->len);
ptr = ng->ptr[id - 1];
rcu_read_unlock();

// BUG_ON(!ptr);
return ptr;
}

results in the following assembly:

net_generic:
call __fentry__
pushq %rbp #
movq %rsp, %rbp #,
pushq %r12 #
movl %esi, %r12d # id, id
pushq %rbx #
movq %rdi, %rbx # net, net
call rcu_read_lock #
movq 4784(%rbx), %rbx # MEM[(struct net_generic * const volatile *)net_2(D) + 4784B], _________p1
call debug_lockdep_rcu_enabled #
testl %eax, %eax # D.48937
je .L93 #,
cmpb $0, __warned.47396(%rip) #, __warned
jne .L93 #,
call rcu_read_lock_held #
testl %eax, %eax # D.48944
jne .L93 #,
movq $.LC6, %rdx #,
movl $51, %esi #,
movq $.LC0, %rdi #,
movb $1, __warned.47396(%rip) #, __warned
call lockdep_rcu_suspicious #
.L93:
decl %r12d # tmp68
movslq %r12d, %r12 # tmp68, tmp69
movq 24(%rbx,%r12,8), %rbx # _________p1_4->ptr, ptr
call rcu_read_unlock #
movq %rbx, %rax # ptr,
popq %rbx #
popq %r12 #
popq %rbp #
ret

This is still 112 bytes of code.


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