Re: [PATCH v3 1/3] vdso/math64: Add and use __iter_div64_u64_rem()

[David Laight <david.laight.linux@gmail.com>]

On Tue, 1 Sep 2026 10:35:30 +0200
Thomas Weißschuh <thomas.weissschuh@linutronix.de> wrote:

> Hi Zhan,
> 
> thanks for the new version. However please slow down a bit with sending new
> revisions to give people time to respond.
> 
> On Tue, Sep 01, 2026 at 10:06:34AM +0800, Zhan Xusheng wrote:
> > The vDSO basetimes for CLOCK_MONOTONIC and CLOCK_BOOTTIME are kept in the
> > scaled nanoseconds of tkr_mono, so normalising them means dividing by
> > NSEC_PER_SEC << shift, which does not fit the u32 divisor of
> > __iter_div_u64_rem().
> > 
> > update_vdso_time_data() therefore open-codes the iterative division twice.
> > Turning the loops into a plain modulo is not an option either, as the vDSO
> > has no 64-bit division helpers on 32-bit.  
> 
> This is not vDSO userspace code, but on the kernel side.
> The problem is that divisions are unnecessarily slow.

Until you get to (on x86) zen3 or cannon lake when they drop to ~15 clocks.
(I think someone finally allocated a bit of silicon to integer divide.)

> Also a module alone would not be enough, as we need the division result.

Isn't the divisor constant? (or rather a constant shifted left some).
In that case you can do a 'multiply by reciprocal'.
The simple 'multiply an shift right 32' will give slightly low quotient.
But you need to calculate the remainder - so can fixup the overlarge
remainder it can generate.

Of course, even that is only worthwhile if the quotient is more than
(a guess) 4.

David