Subject: Re: [PATCH v4] cpuidle: Fix last_residency division

From: Balbir Singh <[email-masked]>
Date: Fri, 01 Jul 2016 22:41:13 +1000

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.