[PATCH v3] cpuidle: Fix last_residency division

[Nicolas Pitre]

On Wed, 29 Jun 2016, Daniel Lezcano wrote:

> On 06/29/2016 09:06 AM, Shreyas B. Prabhu wrote:
> > diff --git a/drivers/cpuidle/cpuidle.h b/drivers/cpuidle/cpuidle.h
> > index f87f399..c8ea5ad 100644
> > --- a/drivers/cpuidle/cpuidle.h
> > +++ b/drivers/cpuidle/cpuidle.h
> > @@ -68,4 +68,27 @@ static inline void
> > cpuidle_coupled_unregister_device(struct cpuidle_device *dev)
> >   }
> >   #endif
> >
> > +/*
> > + * Used for calculating last_residency in usec. Optimized for case
> > + * where last_residency in nsecs is < INT_MAX/2 by using faster
> > + * approximation. Approximated value has less than 1% error.
> > + */
> > +static inline int convert_nsec_to_usec(u64 nsec)
> > +{
> > +	if (likely(nsec < INT_MAX / 2)) {
> 
> UINT_MAX ?

Actually this can be better than that.

> > +		int usec = (int)nsec;

First, you'll want an unsigned type. Given the provided argument is u64, 
we can assume there won't be any negative values here.

Then it would be wise to use a type with an explicit width, like U32.

> > +		usec += usec >> 5;
> > +		usec = usec >> 10;
> > +		return usec;

And now you want to maximize the available range. So not to overflow the 
first addition, we must respect:

	usec + (usec >> 5) <= 0xffffffff 
	usec + usec/32 <= 0xffffffff
	usec <= (0xffffffff * 32) / 33

Therefore:

	nsec <= 0xf83e0f82

This is much better than INT_MAX/2.

> > +	} else {
> > +		u64 usec = div_u64(nsec, 1000);
> > +
> > +		if (usec > INT_MAX)
> > +			usec = INT_MAX;
> > +		return (int)usec;
> > +	}
> > +}
> 
> 
> 
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