Messages in this thread | | | Subject | Re: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation | From | Christophe Leroy <> | Date | Wed, 29 Jan 2020 08:26:39 +0100 |
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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
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