Re: [RFC PATCH 1/2] sched/update_avg: avoid negative time

[Michael wang <wangyun@linux.vnet.ibm.com>]

Hi, Alex

On 01/21/2014 01:33 PM, Alex Shi wrote:
> rq->avg_idle try to reflect the average idle time between the cpu idle
> and first wakeup. But in the function, it maybe get a negative value
> if old avg_idle is too small. Then this negative value will be double
> counted in next time calculation. Guess that is not the original purpose,
> so recalibrate it to zero.
> 
> Signed-off-by: Alex Shi <alex.shi@linaro.org>
> ---
>  kernel/sched/core.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 30eb011..af9121c6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1358,6 +1358,9 @@ static void update_avg(u64 *avg, u64 sample)
>  {
>  	s64 diff = sample - *avg;
>  	*avg += diff >> 3;
> +
> +	if (*avg < 0)
> +		*avg = 0;

This seems like won't happen...

if 'diff' is negative, it's absolute value won't bigger than '*avg', not
to mention we only use 1/8 of it.

Regards,
Michael Wang

>  }
>  #endif
>