Zhiqiang,
On Tue, 25 Jun 2019, Zhiqiang Liu wrote:
> I have a doubt about _msecs_to_jiffies funcs, especially when input m is
> equal to 0.
>
> For different HZ setttings, different _msecs_to_jiffies funcs will be
> chosen for msecs_to_jiffies func. However, the performance of different
> _msecs_to_jiffies is inconsistent with input m is equal to 0.
>
> If HZ satisfies the condition: HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC %
> HZ), the return value of _msecs_to_jiffies func with m=0 is different
> with different HZ setting.
> ------------------------------------
> | HZ | MSEC_PER_SEC / HZ | return |
> ------------------------------------
> |1000| 1 | 0 |
> |500 | 2 | 1 |
> |200 | 5 | 1 |
> |100 | 10 | 1 |
> ------------------------------------
>
> Why only the return value of HZ=1000 is equal to 0 with m=0 ?
I don't know how you tested that, but obviously all four HZ values use
this variant:
> #if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
> static inline unsigned long _msecs_to_jiffies(const unsigned int m)
> {
> return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
> }
and for all four HZ values the result is 0. Why?
For m = 0 the calculation reduces to:
((MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ)
i.e.
(x - 1) / x where x = [1, 2, 5, 10]
which is guaranteed to be 0 for integer math. If not, you have a compiler
problem.
Thanks,
tglx
