Subject: Re: [PATCH v2] cpuidle: Fix last_residency division

From: Arnd Bergmann <[email-masked]>
Date: Fri, 24 Jun 2016 21:43:13 +0200

On Friday, June 24, 2016 9:31:35 PM CEST Shreyas B Prabhu wrote:
> > If those functions are called less often than cpuidle_enter_state(),
> > we could just move the division there. Since the divisor is constant,
> > do_div() can convert it into a multiply and shift, or we could use
> > your the code you suggest above, or use a 32-bit division most of
> > the time:
> > 
> >       if (diff <= UINT_MAX)
> >               diff_32 = (u32)diff / NSECS_PER_USEC;
> >       else
> >               diff_32 = div_u64(diff, NSECS_PER_USEC;
> > 
> > which gcc itself will turn into a multiplication or series of
> > shifts on CPUs on which that is faster.
> >
> I'm not sure which division method of the three suggested here to use.
> Does anyone have a strong preference?
> 

It depends on how accurate we want it and how long we expect
the times to be. The optimization for the 4.2 second cutoff
for doing a 32-bit division only makes sense if the majority
of the sleep times are below that.

	Arnd