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

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Message-ID: <20171128072250.GA10757@kroah.com>
Date:   Tue, 28 Nov 2017 08:22:50 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] schedule: use unlikely()

On Mon, Nov 27, 2017 at 07:05:22PM -0500, Mikulas Patocka wrote:
> 
> 
> 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.

If you can not measure it, you should not use it.  You are forgetting
about the testing that was done a few years ago that found that some
huge percentage (80? 75? 90?) of all of these markings were wrong and
harmful or did absolutely nothing.

> > 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).

If you think so, great, but prove it, otherwise you are adding markup
that is not needed or could be harmful. :)

thanks,

greg k-h