linux-kernel - Re: [RFC PATCH v2 05/10] lib: vdso: inline do

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Message-ID: <87o8vbrpej.fsf@nanos.tec.linutronix.de>
Date:   Fri, 10 Jan 2020 22:07:48 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Arnd Bergmann <arnd@...db.de>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andy Lutomirski <luto@...nel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list\:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [RFC PATCH v2 05/10] lib: vdso: inline do_hres()

Arnd Bergmann <arnd@...db.de> writes:
> On Mon, Dec 23, 2019 at 3:31 PM Christophe Leroy
> <christophe.leroy@....fr> wrote:
>>
>> do_hres() is called from several places, so GCC doesn't inline
>> it at first.
>>
>> do_hres() takes a struct __kernel_timespec * parameter for
>> passing the result. In the 32 bits case, this parameter corresponds
>> to a local var in the caller. In order to provide a pointer
>> to this structure, the caller has to put it in its stack and
>> do_hres() has to write the result in the stack. This is suboptimal,
>> especially on RISC processor like powerpc.
>>
>> By making GCC inline the function, the struct __kernel_timespec
>> remains a local var using registers, avoiding the need to write and
>> read stack.
>>
>> The improvement is significant on powerpc.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>
> Good idea, I can see how this ends up being an improvement
> for most of the callers.
>
> Acked-by: Arnd Bergmann <arnd@...db.de>

  https://lore.kernel.org/r/20191112012724.250792-3-dima@arista.com

On the way to be applied.

Thanks,

        tglx