From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753306AbeDRN5G (ORCPT ); Wed, 18 Apr 2018 09:57:06 -0400 Received: from 178.115.242.59.static.drei.at ([178.115.242.59]:34065 "EHLO mail.osadl.at" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1751383AbeDRN5F (ORCPT ); Wed, 18 Apr 2018 09:57:05 -0400 Date: Wed, 18 Apr 2018 13:49:51 +0000 From: Nicholas Mc Guire To: Philipp Klocke Cc: lukas.bulwahn@gmail.com, kernelnewbies@kernelnewbies.org, llvmlinux@lists.linuxfoundation.org, sil2review@lists.osadl.org, Ingo Molnar , Peter Zijlstra , linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched/fair: Change sched_feat(x) in !CONFIG_SCHED_DEBUG case Message-ID: <20180418134951.GA7289@osadl.at> References: <20180416085426.24157-1-Phil_K97@gmx.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180416085426.24157-1-Phil_K97@gmx.de> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 16, 2018 at 10:54:26AM +0200, Philipp Klocke wrote: > Make sched_feat(x) return 1 or 0 instead of 2**x or 0 when > sysctl_sched_features is constant, by changing the left shift of the > mask-bit to a right shift of the bitmap. The behaviour of the code > remains unchanged. > We prove this by showing that the compiler can evaluate this shift > during compile time, which results in generating the same > machine code as before. > > This patch is motivated by the clang warning Wconstant-logical-operand, > issued when logically comparing a variable to a constant integer that is > neither 1 nor 0. It happens for sched_feat(x) when sysctl_sched_features > is constant, i.e., CONFIG_SCHED_DEBUG is not set. > > kernel/sched/fair.c:3927:14: warning: use of logical '&&' with constant operand [-Wconstant-logical-operand] > if (initial && sched_feat(START_DEBIT)) > ^ ~~~~~~~~~~~~~~~~~~~~~~~ > kernel/sched/fair.c:3927:14: note: use '&' for a bitwise operation > if (initial && sched_feat(START_DEBIT)) > ^~ > & > kernel/sched/fair.c:3927:14: note: remove constant to silence this warning > if (initial && sched_feat(START_DEBIT)) > ~^~~~~~~~~~~~~~~~~~~~~~~~~~ > > This resolves the warning, leaves the code base largely as is. > > Tested with gcc 7.3.1 and clang 6.0.0 on x86_64, comparing resulting > binaries with diff. > Applicable to all modern compilers and architectures > > Signed-off-by: Philipp Klocke > Suggested-by: Lukas Bulwahn Reviewed-by: Nicholas Mc Guire > --- > kernel/sched/sched.h | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index fb5fc458547f..d2aedee6ab0f 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -1305,7 +1305,11 @@ static const_debug __maybe_unused unsigned int sysctl_sched_features = > 0; > #undef SCHED_FEAT > > +#ifdef CONFIG_SCHED_DEBUG > #define sched_feat(x) (sysctl_sched_features & (1UL << __SCHED_FEAT_##x)) > +#else > +#define sched_feat(x) ((sysctl_sched_features >> __SCHED_FEAT_##x) & 1UL) > +#endif > The changed sched_feat(y) line is fine but I do not get/like the of the ifdef - keeping the change minimal is ok if there is a relevant impact but here there is no effective difference (you write the object code is the same for the !CONFIG_SCHED_DEBUG case) So I think the ifdef should be kicked here and the proposed change seems fine to me. > #endif /* SCHED_DEBUG && HAVE_JUMP_LABEL */ > > -- > 2.17.0 >