From: Philipp Klocke <Phil_K97@gmx.de> To: Peter Zijlstra <peterz@infradead.org> Cc: sil2review@lists.osadl.org, kernelnewbies@kernelnewbies.org, llvmlinux@lists.linuxfoundation.org, linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>, der.herr@hofr.at, lukas.bulwahn@gmail.com Subject: Re: [PATCH] sched/fair: Change sched_feat(x) in !CONFIG_SCHED_DEBUG case Date: Fri, 20 Apr 2018 18:29:07 +0200 [thread overview] Message-ID: <34572fee-36d0-36e1-ba6d-f098b145aba4@gmx.de> (raw) In-Reply-To: <20180420075717.GB4064@hirez.programming.kicks-ass.net> On 20.04.2018 09:57, Peter Zijlstra wrote: > On Mon, Apr 16, 2018 at 10:54:26AM +0200, Philipp Klocke wrote: > >> 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)) >> ~^~~~~~~~~~~~~~~~~~~~~~~~~~ >> @@ -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 > So this is extra ugly, for no gain? The gain is stopping a warning that clutters the output log of clang. To improve readability, one can drop the ifdef-structure and just keep the right shift version, like Nicholas suggested. This will have a (very small) impact on performance in CONFIG_SCHED_DEBUG case, but when debugging, performance is no problem anyways. > WTH does clang complain about a constant? Can't you just disable that > stupid warning? There are 2 ways to disable the warning. Either disable it for this particular occurrence, which clutters the code with #pragma's. THIS is really ugly. Or disable it globally and maybe miss some important/helpful warnings. > Also, if sysctl_sched_features is a constant, the both expressions > _should_ really result in a constant and clang should still warn about > it. No, because clang only warns if the constant is neither 1 nor 0. (These being the 'best' integer representations of booleans) > I'm really not seeing why we'd want to do this. Just fix clang to not be > stupid. > _______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
WARNING: multiple messages have this Message-ID (diff)
From: Phil_K97@gmx.de (Philipp Klocke) To: kernelnewbies@lists.kernelnewbies.org Subject: [PATCH] sched/fair: Change sched_feat(x) in !CONFIG_SCHED_DEBUG case Date: Fri, 20 Apr 2018 18:29:07 +0200 [thread overview] Message-ID: <34572fee-36d0-36e1-ba6d-f098b145aba4@gmx.de> (raw) In-Reply-To: <20180420075717.GB4064@hirez.programming.kicks-ass.net> On 20.04.2018 09:57, Peter Zijlstra wrote: > On Mon, Apr 16, 2018 at 10:54:26AM +0200, Philipp Klocke wrote: > >> 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)) >> ~^~~~~~~~~~~~~~~~~~~~~~~~~~ >> @@ -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 > So this is extra ugly, for no gain? The gain is stopping a warning that clutters the output log of clang. To improve readability, one can drop the ifdef-structure and just keep the right shift version, like Nicholas suggested. This will have a (very small) impact on performance in CONFIG_SCHED_DEBUG case, but when debugging, performance is no problem anyways. > WTH does clang complain about a constant? Can't you just disable that > stupid warning? There are 2 ways to disable the warning. Either disable it for this particular occurrence, which clutters the code with #pragma's. THIS is really ugly. Or disable it globally and maybe miss some important/helpful warnings. > Also, if sysctl_sched_features is a constant, the both expressions > _should_ really result in a constant and clang should still warn about > it. No, because clang only warns if the constant is neither 1 nor 0. (These being the 'best' integer representations of booleans) > I'm really not seeing why we'd want to do this. Just fix clang to not be > stupid. >
next prev parent reply other threads:[~2018-04-20 16:29 UTC|newest] Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-04-16 8:54 [PATCH] sched/fair: Change sched_feat(x) in !CONFIG_SCHED_DEBUG case Philipp Klocke 2018-04-18 13:49 ` Nicholas Mc Guire 2018-04-18 13:49 ` Nicholas Mc Guire 2018-04-20 7:57 ` Peter Zijlstra 2018-04-20 7:57 ` Peter Zijlstra 2018-04-20 16:29 ` Philipp Klocke [this message] 2018-04-20 16:29 ` Philipp Klocke 2018-04-20 16:51 ` Peter Zijlstra 2018-04-20 16:51 ` Peter Zijlstra 2018-04-20 21:29 ` Lukas Bulwahn 2018-04-20 21:29 ` Lukas Bulwahn 2018-04-23 9:45 ` Peter Zijlstra 2018-04-23 9:45 ` Peter Zijlstra 2018-04-20 10:34 ` Peter Zijlstra 2018-04-20 10:34 ` Peter Zijlstra
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=34572fee-36d0-36e1-ba6d-f098b145aba4@gmx.de \ --to=phil_k97@gmx.de \ --cc=der.herr@hofr.at \ --cc=kernelnewbies@kernelnewbies.org \ --cc=linux-kernel@vger.kernel.org \ --cc=llvmlinux@lists.linuxfoundation.org \ --cc=lukas.bulwahn@gmail.com \ --cc=mingo@redhat.com \ --cc=peterz@infradead.org \ --cc=sil2review@lists.osadl.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes, see mirroring instructions on how to clone and mirror all data and code used by this external index.