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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Mon, 30 Dec 2019 13:07:43 +0100
Message-ID: <CAK8P3a36OgFuY72b_i6+0xBNGnaxS1SsRid+HrgQHPZtUJp3LQ@mail.gmail.com>
Subject: Re: [RFC PATCH v2 05/10] lib: vdso: inline do_hres()
To:     Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Andy Lutomirski <luto@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>
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On Mon, Dec 23, 2019 at 3:31 PM Christophe Leroy
<christophe.leroy@c-s.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@c-s.fr>

Good idea, I can see how this ends up being an improvement
for most of the callers.

Acked-by: Arnd Bergmann <arnd@arndb.de>