linux-kernel - Re: Surprising code generated for vdso_read

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Date:   Wed, 19 Feb 2020 09:45:45 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Arnd Bergmann <arnd@...db.de>,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>
Subject: Re: Surprising code generated for vdso_read_begin()



Le 16/02/2020 à 19:10, Arnd Bergmann a écrit :
> On Sat, Jan 11, 2020 at 12:33 PM Segher Boessenkool
> <segher@...nel.crashing.org> wrote:
>>
>> On Fri, Jan 10, 2020 at 07:45:44AM +0100, Christophe Leroy wrote:
>>> Le 09/01/2020 à 21:07, Segher Boessenkool a écrit :
>>>> It looks like the compiler did loop peeling.  What GCC version is this?
>>>> Please try current trunk (to become GCC 10), or at least GCC 9?
>>>
>>> It is with GCC 5.5
>>>
>>> https://mirrors.edge.kernel.org/pub/tools/crosstool/ doesn't have more
>>> recent than 8.1
>>
>> Arnd, can you update the tools?  We are at 8.3 and 9.2 now :-)  Or is
>> this hard and/or painful to do?
> 
> To follow up on this older thread, I have now uploaded 6.5, 7.5, 8.3 and 9.2
> binaries, as well as a recent 10.0 snapshot.
> 

Thanks Arnd,

I have built the VDSO with 9.2, I get less performant result than with 
8.2 (same performance as with 5.5).

After a quick look, I see:
- Irrelevant NOPs to align loops and stuff, allthough -mpcu=860 should 
avoid that.
- A stack frame is set for saving r31 in __c_kernel_clock_gettime. GCC 
8.1 don't need that, all VDSO functions are frameless with 8.1

Christophe