unlikely compiler flag propagation

[Matthias Brugger]

Hi Greg, hi all,

2015-02-18 19:24 GMT+01:00 Greg KH <greg at kroah.com>:
> On Wed, Feb 18, 2015 at 07:09:47PM +0100, Matthias Brugger wrote:
>> Hi all,
>>
>> I have a question about the unlikely compiler flag.
>> When a called function is only returns an error with the unlikely flag
>> set, should I set the unlikely compiler flag for the return value
>> check in the callee as well?
>>
>> For example:
>>
>> int function_one(int *list, int num_elements)
>> {
>>     int i;
>>     for (i =0; i < num_elements; i++) {
>>         if (unlikely(check_element(list + i)))
>>            return 1;
>>     }
>>
>> [...]
>>
>>     return 0;
>> }
>>
>> int function_two(...)
>> {
>> [...]
>>
>>      if (function_one(list, num))
>>          return -1;
>> }
>>
>>
>> So my question is, if function_two should instead implement:
>> if (unlikely(function_one(list, num))
>>
>> Or does the unlikely compiler flag propagate to calling functions?
>
> NEVER use unlikely/likely unless you can actually measure that it
> matters if you use it.  The compiler and processor is almost always
> better at making these types of guesses and predictions, so let it do
> the work instead.
>
> As proof of this, there was a test of the kernel a year or so ago that
> measured the placement of the existing likely/unlikely markers in the
> kernel and 90% of the usages were wrong and actually slowed down the
> processor.
>
> So just don't use it, unless you can measure it.

I was just asking about the propagation because I found that
get_page_from_freelist [0] in mm/page_alloc.c calls prep_new_page [1]
which in place returns an error when entering an unlikely marked
branch.
As this happens in the fast path of a page allocation, I hope the
marker is properly set :)

So is it needed to set the unlikely in the get_page_from_freelist as
well, or will the compiler propagate the mark?

Cheers,
Matthias

[0] https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/tree/mm/page_alloc.c#n2171
[1] https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/tree/mm/page_alloc.c#n966

>
> thanks,
>
> greg k-h



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