On Fri, 2016-07-01 at 10:06 +0200, Daniel Lezcano wrote:
> On 06/30/2016 05:37 PM, Nicolas Pitre wrote:
> >
> > On Thu, 30 Jun 2016, Daniel Lezcano wrote:
> [ ... ]
>
> >
> > >
> > > >
> > > > + if (likely(nsec < DIV_APPROXIMATION_THRESHOLD)) {
> > > > + u32 usec = nsec;
> > > > +
> > > > + usec += usec >> 5;
> > > > + usec = usec >> 10;
> > > > +
> > > > + /* Can safely cast to int since usec is < INT_MAX */
> > > > + return usec;
> > > > + } else {
> > > > + u64 usec = div_u64(nsec, 1000);
> > > > +
> > > > + if (usec > INT_MAX)
> > > > + usec = INT_MAX;
> > > > +
> > > > + /* Can safely cast to int since usec is < INT_MAX */
> > > > + return usec;
> > > > + }
> > > > +}
> > >
> > > What bothers me with this division is the benefit of adding an extra ultra
> > > optimized division by 1000 in cpuidle.h while we have already ktime_divns
> > > which is optimized in ktime.h.
> > It is "optimized" but still much heavier than what is presented above as
> > it provides maximum precision.
> >
> > It all depends on how important the performance gain from the original
> > shift by 10 was in the first place.
> Actually the original shift was there because it was convenient as a
> simple ~div1000 operation. But against all odds, the approximation
> introduced a regression on a very specific use case on PowerPC.
>
> We are not in the hot path and I think we can live with a ktime_divns
> without problem. That would simplify the fix I believe.
>
I would tend to agree with this and there are better ways to do
multiplicative inverses if we care
Balbir Singh.
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