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

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Date:   Thu, 30 Nov 2017 08:07:44 +0000
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 Thu, Nov 30, 2017 at 02:04:01AM -0500, Mikulas Patocka wrote:
> 
> 
> On Tue, 28 Nov 2017, 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.
> 
> The whole kernel has 19878 likely/unlikely tags.

And most of them are wrong.  Don't add new ones unless you can prove it
is correct.

> Do you have benchmark proving efficiency for each of them? :-)

Yes, people have done this work in the past, see the archives.

greg k-h