[dpdk-dev] [PATCH v7 4/8] vhost: rxtx: use queue id instead of constant ring index

[Bruce Richardson]

On 21/10/2015 16:47, Stephen Hemminger wrote:
> On Wed, 21 Oct 2015 09:38:37 +0000
> "Ananyev, Konstantin" <konstantin.ananyev at intel.com> wrote:
>
>>>> minor nits:
>>>>   * this doesn't need to be marked as always inline,
>>>>      that is as they say in English "shooting a fly with a bazooka"
>>> Stephen:
>>> always_inline "forces" the compiler to inline this function, like a macro.
>>> When should it be used or is it not preferred at all?
>> I also don't understand what's wrong with using 'always_inline' here.
>> As I understand the author wants compiler to *always inline* that function.
>> So seems perfectly ok to use it here.
>> As I remember just 'inline' is sort of recommendation that compiler is free to ignore.
>> Konstantin
> I follow Linux/Linus advice and resist the urge to add strong inlining.
> The compiler does a good job of deciding to inline, and many times
> the reason it chooses for not inlining are quite good like:
>    - the code is on an unlikely branch
>    - register pressure means inlining would mean the code would be worse
>
> Therefore my rules are:
>    * only use inline for small functions. Let compiler decide on larger static funcs
>    * write code where most functions are static (localized scope) where compiler
>      can decide
>    * reserve always inline for things that access hardware and would break if not inlined.
>
On the other hand, there are cases where we know the compiler will 
likely inline, but we also know that not inlining could have a high 
performance penalty, and in that case marking as "always inline" would 
be appropriate - even though it is likely unnecessary for most 
compilers. In such a case, I would expect the verification check to be: 
explicitly mark the function as *not* to be inlined, and see what the 
perf drop is. If it's a noticable drop, marking as always-inline is an 
ok precaution against future compiler changes.

Also, we need to remember that compilers cannot know whether a function 
is data path or not, and also whether a function will be called 
per-packet or per-burst. That's only something the programmer will know, 
and functions called per-packet on the datapath generally need to be 
inlined for performance.

/Bruce

/Bruce