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 08:26:39 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     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>,
        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



Le 29/01/2020 à 08:14, Thomas Gleixner a écrit :
> 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.
> 

Yes, I did it already. It spares reading d->mast, that the main advantage.

Christophe