[PATCH v2] cpuidle: Fix last_residency division

[Arnd Bergmann]

On Friday, June 24, 2016 9:00:48 AM CEST David Laight wrote:
> The intent of the >> 10 was probably to avoid an expensive 64bit divide.
> So maybe something like:
>         diff = time_end - time_start;
>         if (diff >= INT_MAX/2)
>                 diff_32 = INT_MAX/2/1000;
>         else
>                 diff_32 = diff;
>                 diff_32 += diff_32 >> 6;
>                 diff_32 >>= 10;
>         }
> 
> Adding an extra 1/32 makes the division by be something slightly below 1000.

Why not change the definition of the time to nanoseconds and update
the users accordingly?

I see that cpuidle_enter_state() writes to the last_residency value,
and it is read in three places: ladder_select_state(), menu_update()
and show_state_time() (for sysfs).

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.

	Arnd