linux-kernel - Re: [PATCH] schedule: use unlikely()

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Date:   Mon, 27 Nov 2017 19:05:22 -0500 (EST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Greg KH <gregkh@...uxfoundation.org>
cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] schedule: use unlikely()

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@...hat.com>
> 
> 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