Re: clocksource.c "margin"

Re: clocksource.c "margin"

[John Stultz]

On Wed, 2011-11-30 at 19:09 -0500, Yang Honggang(Joseph) wrote:
> Hi John Stultz,
> I'm very sorry to trouble you. A problem puzzled me when I reading your 
> code in kernel/time/clocksource.c.
> In the following functions, your said in the comments that >>5 was used 
> to leave a margin of 12.5%.
> I don't know how to get the result. I think is should be >> 3 in order 
> to leave a margin of 12.5%.
> The operation of >> 5 only left 3.125% margin.
> 
> __clocksource_updatefreq_scale
> ...
> sec = (cs->mask - (cs->mask >> 5));
> ...
> clocksource_max_deferment
> ...
>      /*
>       * To ensure that the clocksource does not wrap whilst we are idle,
>       * limit the time the clocksource can be deferred by 12.5%. Please
>       * note a margin of 12.5% is used because this can be computed with
>       * a shift, versus say 10% which would require division.
>       */
>      return max_nsecs - (max_nsecs >> 5);
> ...

Indeed. This looks wrong.

Would you like to send a patch in correcting this issue? If not, let me
know and I'll do so.

thanks
-john

clocksource.c "margin"

[Yang Honggang(Joseph)]

Hi John Stultz,
I'm very sorry to trouble you. A problem puzzled me when I reading your 
code in kernel/time/clocksource.c.
In the following functions, your said in the comments that >>5 was used 
to leave a margin of 12.5%.
I don't know how to get the result. I think is should be >> 3 in order 
to leave a margin of 12.5%.
The operation of >> 5 only left 3.125% margin.

__clocksource_updatefreq_scale
...
sec = (cs->mask - (cs->mask >> 5));
...
clocksource_max_deferment
...
     /*
      * To ensure that the clocksource does not wrap whilst we are idle,
      * limit the time the clocksource can be deferred by 12.5%. Please
      * note a margin of 12.5% is used because this can be computed with
      * a shift, versus say 10% which would require division.
      */
     return max_nsecs - (max_nsecs >> 5);
...

Best regards,
Joseph