Re: [PATCH] schedule: use unlikely()

[Mikulas Patocka]

On Sat, 25 Nov 2017, Greg KH wrote:

> On Mon, Nov 13, 2017 at 02:00:45PM -0500, Mikulas Patocka wrote:
> > A small patch for schedule(), so that the code goes straght in the common
> > case.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> 
> Was this a measurable difference?  If so, great, please provide the
> numbers and how you tested in the changelog.  If it can't be measured,
> then it is not worth it to add these markings

It is much easier to make microoptimizations (such as using likely() and 
unlikely()) than to measure their effect.

If a programmer were required to measure performance every time he uses 
likely() or unlikely() in his code, he wouldn't use them at all.

> as the CPU/compiler almost always knows better.
> 
> thanks,
> 
> greg k-h

The compiler assumes that pointers are usually not NULL - but in this 
case, they are usually NULL. The compiler can't know better (unless 
profile feedback is used).

Mikulas