RE: [PATCH v2] cpuidle: Fix last_residency division

From: Arnd Bergmann

Sent: 24 June 2016 20:43
On Friday, June 24, 2016 9:31:35 PM CEST Shreyas B Prabhu wrote:

quoted

quoted

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 <=3D UINT_MAX)
              diff_32 =3D (u32)diff / NSECS_PER_USEC;
      else
              diff_32 =3D 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?

=20
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.

It also depends if the code actually cares about the length of 'long' sleep=
s.
I'd guess that for cpu idle 4.2 seconds is 'a long time', so the div_u64()
result could be treated as 4.2 seconds without causing grief.

Actually the cost of a 64bit divide after a 4 second sleep will be noise.
OTOH a 64bit divide after a sleep that lasted a few ns will be significant.

	David