linux-kernel - Re: [PATCH 110/110] lib/vdso: Improve do_hres() and update vdso data unconditionally

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Message-ID: <CAAhV-H4PEcCgOBL8ksjc+4LC9VPoCRBMtuGEK6ckmdJYXjdcSg@mail.gmail.com>
Date:   Tue, 22 Oct 2019 10:42:55 +0800
From:   Huacai Chen <chenhc@...ote.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>,
        Paul Burton <paul.burton@...s.com>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 110/110] lib/vdso: Improve do_hres() and update vdso data unconditionally

Hi, Thomas,

If we use (s64)cycles < 0, then how to solve the problem that a 64bit
counter become negative?

Maybe we can change the "invalid" value from U64_MAX to 0?  I think
the performance of "cycles == 0" is better than "cycles == U64_MAX".

Huacai

On Mon, Oct 21, 2019 at 10:58 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Mon, 21 Oct 2019, Thomas Gleixner wrote:
>
> > On Mon, 21 Oct 2019, Huacai Chen wrote:
> > > @@ -50,7 +50,7 @@ static int do_hres(const struct vdso_data *vd, clockid_t clk,
> > >             cycles = __arch_get_hw_counter(vd->clock_mode);
> > >             ns = vdso_ts->nsec;
> > >             last = vd->cycle_last;
> > > -           if (unlikely((s64)cycles < 0))
> > > +           if (unlikely(cycles == U64_MAX))
> > >                     return -1;
> >
> > That used to create worse code than the weird (s64) type cast which has the
> > same effect. Did you double check that there is no change?
>
> It still does for 32bit.