Date:   Tue, 13 Dec 2016 17:23:09 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xemul@...nvz.org" <xemul@...nvz.org>
Subject: Re: [PATCH 3/3] netns: fix net_generic() "id - 1" bloat

On Wed, Dec 7, 2016 at 1:49 PM, David Laight <David.Laight@...lab.com> wrote:
> From: Alexey Dobriyan
>> Sent: 05 December 2016 14:48
>> On Mon, Dec 5, 2016 at 3:49 PM, David Laight <David.Laight@...lab.com> wrote:
>> > From: Alexey Dobriyan
>> >> Sent: 02 December 2016 01:22
>> >> net_generic() function is both a) inline and b) used ~600 times.
>> >>
>> >> It has the following code inside
>> >>
>> >>               ...
>> >>       ptr = ng->ptr[id - 1];
>> >>               ...
>> >>
>> >> "id" is never compile time constant so compiler is forced to subtract 1.
>> >> And those decrements or LEA [r32 - 1] instructions add up.
>> >>
>> >> We also start id'ing from 1 to catch bugs where pernet sybsystem id
>> >> is not initialized and 0. This is quite pointless idea (nothing will
>> >> work or immediate interference with first registered subsystem) in
>> >> general but it hints what needs to be done for code size reduction.
>> >>
>> >> Namely, overlaying allocation of pointer array and fixed part of
>> >> structure in the beginning and using usual base-0 addressing.
>> >>
>> >> Ids are just cookies, their exact values do not matter, so lets start
>> >> with 3 on x86_64.
>> > ...
>> >>  struct net_generic {
>> >> -     struct {
>> >> -             unsigned int len;
>> >> -             struct rcu_head rcu;
>> >> -     } s;
>> >> -
>> >> -     void *ptr[0];
>> >> +     union {
>> >> +             struct {
>> >> +                     unsigned int len;
>> >> +                     struct rcu_head rcu;
>> >> +             } s;
>> >> +
>> >> +             void *ptr[0];
>> >> +     };
>> >>  };
>> >
>> > That union is an accident waiting to happen.
>>
>> I kind of disagree. Module authors should not be given matches,
>> but it is hard to screw up if net_generic() is all you're given.
>>
>> > What might work is to offset the Ids by
>> > (offsetof(struct net_generic, ptr)/sizeof (void *)) instead of by 1.
>> > The subtract from the offset will then counter the structure offset
>> > - which is what you are trying to achieve.
>>
>> If you suggest this layout
>>
>> struct net_generic {
>>         struct {
>>         } s;
>>         void *ptr[0];
>> };
>>
>> then is it not optimal because offset of "ptr" needs to be somewhere in code
>> either in some LEA or imm8 of the final MOV which is 1 byte more bloaty.
>>
>> Here is test program
>>
>> struct ng1 {
>>         union {
>>                 struct {
>>                         unsigned int len;
>>                 } s;
>>                 void *ptr[0];
>>         };
>> };
>> struct ng2 {
>>         struct {
>>                 unsigned int len;
>>         } s;
>>         void *ptr[0];
>> };
>> struct net {
>>         int x;
>>         struct ng1 *gen1;
>>         struct ng2 *gen2;
>> };
>> void *ng1(const struct net *net, unsigned int id)
>> {
>>         return net->gen1->ptr[id];
>> }
>> void *ng2(const struct net *net, unsigned int id)
>> {
>>         return net->gen2->ptr[id];
>> }
>>
>>
>> 0000000000000000 <ng1>:
>>    0:   48 8b 47 08             mov    rax,QWORD PTR [rdi+0x8]
>>    4:   89 f6                   mov    esi,esi
>>    6:   48 8b 04 f0             mov    rax,QWORD PTR [rax+rsi*8]
>>    a:   c3                      ret
>>
>>
>> 0000000000000010 <ng2>:
>>   10:   48 8b 47 10             mov    rax,QWORD PTR [rdi+0x10]
>>   14:   89 f6                   mov    esi,esi
>>   16:   48 8b 44 f0 [[[08]]]          mov    rax,QWORD PTR [rax+rsi*8+0x8]
>>   1b:   c3                      ret
>
> On x86 that will make ~0 difference since the offset (in that sequence)
> doesn't require an extra instruction.

Well, the point of the patch is to save .text, so might as well save
as much as possible. Any form other than "ptr[id]" is going
to be either bigger or bigger and slower and "ptr" should be the first field.

> However if you offset the 'id' values so that only
> values 2 up are valid the code becomes:
>         return net->gen2->ptr[id - 2];
> which will be exactly the same code as:
>         return net->gen1->ptr[id];
> but it is much more obvious that 'id' values must be >= 2.
>
> The '2' should be generated from the structure offset, but with my method
> is doesn't actually matter if it is wrong.

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