lkml.org 
[lkml]   [2020]   [Jan]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation
From
Date


Le 29/01/2020 à 08:14, Thomas Gleixner a écrit :
> Andy Lutomirski <luto@kernel.org> writes:
>
>> On Thu, Jan 16, 2020 at 11:57 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>>>
>>> Andy Lutomirski <luto@kernel.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

\
 
 \ /
  Last update: 2020-01-29 08:26    [W:0.064 / U:1.036 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site