linux-kernel - Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal due to a kernel NULL pointer dereference)

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Message-ID: <CA+icZUX=0qtftp+y1RdeyAZrZuiNMHKk=jhpWiB2sGqsJtcWuQ@mail.gmail.com>
Date:	Mon, 14 Sep 2015 09:49:36 +0200
From:	Sedat Dilek <sedat.dilek@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Baoquan He <bhe@...hat.com>, Denys Vlasenko <dvlasenk@...hat.com>,
	Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Graf <tgraf@...g.ch>, Ingo Molnar <mingo@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal due
 to a kernel NULL pointer dereference)

On Mon, Sep 14, 2015 at 9:12 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sun, Sep 13, 2015 at 04:33:39AM +0200, Sedat Dilek wrote:
>> > It looks like an inline-optimization bug in CLANG when the compiler's
>> > optimization-level is higher than -O2.
>
>> > [1] http://lists.linuxfoundation.org/pipermail/llvmlinux/2015-September/001355.html
>>
>> After some discussion on #llvm it turned out to be a known issue in LLVMLinux!
>>
>> Unfortunately, an existing patch [1] got archived which is still
>> required to build x86_64 correctly.
>
>> [1] http://git.linuxfoundation.org/?p=llvmlinux.git;a=blob_plain;f=arch/x86_64/patches/ARCHIVE/0029-Fix-ARCH_HWEIGHT-for-compilation-with-clang.patch;hb=HEAD
>
> As long as LLVM cannot do things like that and requires full function
> calls I cannot see it being a sensible compiler to use from a
> performance POV.
>
> There's a fairly large difference between an inline POPCNT instruction
> and a full out-of-line function call.
>
> /me goes back to ignoring LLVM for the time being.

Can you give an example or describe a test-case to check the performance?

I have here diverse Linux v4.2 kernels (all have the same kernel-config)...

[ llvmlinux-patched ]

#1: Compiled with CLANG v3.7 from a self-built llvm-toolchain v3.7.0
#2: Compiled with GCC v4.9

[ unpatched ]

#3: Compiled with GCC v4.9

Can you also comment on the effects of CONFIG_CC_OPTIMIZE_FOR_SIZE in
case of performance?
It is only to reduce binary size or does it also do some "speed" optimization?

Thanks in advance.

- Sedat -
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