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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 23 Dec 2019 18:29:39 -0800
Message-ID: <CALCETrWyek-xatdsUFbFvHzT+bps_bjtQoVq8MAFsS1rcZCtEA@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>,
        Arnd Bergmann <arnd@arndb.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Andrew Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        "open list:MIPS" <linux-mips@vger.kernel.org>,
        X86 ML <x86@kernel.org>
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On Mon, Dec 23, 2019 at 6:31 AM 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.

I'm okay with it, mainly because I don't expect many workloads to have
more than one copy of the code hot at the same time.