From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751457AbdK3IHl (ORCPT ); Thu, 30 Nov 2017 03:07:41 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:52554 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750943AbdK3IHk (ORCPT ); Thu, 30 Nov 2017 03:07:40 -0500 Date: Thu, 30 Nov 2017 08:07:44 +0000 From: Greg KH To: Mikulas Patocka Cc: Ingo Molnar , Peter Zijlstra , linux-kernel@vger.kernel.org Subject: Re: [PATCH] schedule: use unlikely() Message-ID: <20171130080744.GA16177@kroah.com> References: <20171125085644.GA1843@kroah.com> <20171128072250.GA10757@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.1 (2017-09-22) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > > > > > > > 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