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

From: Shreyas B Prabhu <[email-masked]>
Date: Fri, 24 Jun 2016 21:31:35 +0530

On 06/24/2016 03:41 PM, Arnd Bergmann wrote:
> 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).
> 

Updating show_state_time() to convert ns to us gets bit messy since
show_state_##_name which is used for disable, usage and time simply
returns state_usage->_name. And state_usage->time updation happens in
cpuidle_enter_state() itself.

> 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?

--Shreyas