linux-kernel - Re: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation

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Date:   Wed, 29 Jan 2020 08:14:27 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, nathanl@...ux.ibm.com,
        Arnd Bergmann <arnd@...db.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "open list\:MIPS" <linux-mips@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation

Andy Lutomirski <luto@...nel.org> writes:

> On Thu, Jan 16, 2020 at 11:57 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> Andy Lutomirski <luto@...nel.org> writes:
>> > On Thu, Jan 16, 2020 at 9:58 AM Christophe Leroy
>> >
>> > Would mul_u64_u64_shr() be a good alternative?  Could we adjust it to
>> > assume the shift is less than 32?  That function exists to benefit
>> > 32-bit arches.
>>
>> We'd want mul_u64_u32_shr() for this. The rules for mult and shift are:
>>
>
> That's what I meant to type...

Just that it does not work. The math is:

     ns = d->nsecs;   // That's the nsec value shifted left by d->shift

     ns += ((cur - d->last) & d->mask) * mult;

     ns >>= d->shift;

So we cannot use mul_u64_u32_shr() because we need the addition there
before shifting. And no, we can't drop the fractional part of
d->nsecs. Been there, done that, got sporadic time going backwards
problems as a reward. Need to look at that again as stuff has changed
over time.

On x86 we enforce that mask is 64bit, so the & operation is not there,
but due to the nasties of TSC we have that conditional

    if (cur > last)
       return (cur - last) * mult;
    return 0;

Christophe, on PPC the decrementer/RTC clocksource masks are 64bit as
well, so you can spare that & operation there too.

Thanks,

        tglx