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
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