unlikely compiler flag propagation

[Greg KH]

On Wed, Feb 18, 2015 at 07:35:37PM +0100, Matthias Brugger wrote:
> 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?

There is no "propagation" of a likely/unlikely mark as it is just a hint
for that specific comparison test.

thanks,

greg k-h