Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 09, 2016 at 06:11:17AM +0100, Peter Zijlstra wrote:
> On Thu, Dec 08, 2016 at 08:49:39PM -0000, Thomas Gleixner wrote:
> 
> > +/*
> > + * Enabled when timekeeping is supposed to deal with virtualization keeping
> > + * VMs long enough scheduled out that the 64 * 32 bit multiplication in
> > + * timekeeping_delta_to_ns() overflows 64bit.
> > + */
> > +#ifdef CONFIG_TIMEKEEPING_USE_128BIT_MATH
> > +
> > +#if defined(CONFIG_ARCH_SUPPORTS_INT128) && defined(__SIZEOF_INT128__)
> > +static inline u64 timekeeping_delta_to_ns(struct tk_read_base *tkr, u64 delta)
> > +{
> > +	unsigned __int128 nsec;
> > +
> > +	nsec = ((unsigned __int128)delta * tkr->mult) + tkr->xtime_nsec;
> > +	return (u64) (nsec >> tkr->shift);
> > +}
> > +#else
> > +static inline u64 timekeeping_delta_to_ns(struct tk_read_base *tkr, u64 delta)
> > +{
> > +	u32 dh, dl;
> > +	u64 nsec;
> > +
> > +	dl = delta;
> > +	dh = delta >> 32;
> > +
> > +	nsec = ((u64)dl * tkr->mult) + tkr->xtime_nsec;
> > +	nsec >>= tkr->shift;
> > +	if (unlikely(dh))
> > +		nsec += ((u64)dh * tkr->mult) << (32 - tkr->shift);
> > +	return nsec;
> > +}
> > +#endif
> > +
> > +#else /* CONFIG_TIMEKEEPING_USE_128BIT_MATH */
> 
> xtime_nsec confuses me, contrary to its name, its not actually in nsec,
> its in shifted nsec units for some reason (and that might well be a good
> reason, but I don't know).
> 
> In any case, it needing to be inside the shift is somewhat unfortunate
> in that it doesn't allow you to use the existing mul_u64_u32_shr()

Wouldn't something like:

	nsec = mul_u64_u32_shr(delta, tkr->mult, tkr->shift);
	nsec += tkr->xtime_nsec >> tkr->shift;

Be good enough? Sure you have a slight rounding error, which results in
a few jaggies in the actual timeline, but it would still be monotonic.

That is, we'll observe the ns rollover 'late', but given its ns, does
anybody really care?