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

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Message-ID: <20171208142939.GG7793@amd>
Date:   Fri, 8 Dec 2017 15:29:39 +0100
From:   Pavel Machek <pavel@....cz>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] schedule: use unlikely()

On Tue 2017-11-28 08:22:50, Greg KH wrote:
> 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.

If Mikulas has enough data that particular if() is usually taken or
not, that should be enough.
								Pavel
								-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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