Re: [PATCH 2/6] RFC perf_counter: singleshot support

[Paul Mackerras <paulus@samba.org>]

Ingo Molnar writes:

> Sure. One sidenote - the precision of sw clocks has dropped a bit 
> lately:
> 
> aldebaran:~/linux/linux/Documentation/perf_counter> ./perfstat -e 
> 1:0 -e 1:0 -e 1:0 -e 1:0 -e 1:0 sleep 1
> 
>  Performance counter stats for 'sleep':
> 
>        0.762664  cpu clock ticks      (msecs)
>        0.761440  cpu clock ticks      (msecs)
>        0.760977  cpu clock ticks      (msecs)
>        0.760587  cpu clock ticks      (msecs)
>        0.760287  cpu clock ticks      (msecs)
> 
>  Wall-clock time elapsed:  1003.139373 msecs
> 
> See that slight but noticeable skew? This used to work fine and we 
> had the exact same value everywhere. Can we fix that while still 
> keeping the code nice?

I suggest basing the software clock on counter->ctx->time_now, and
make get_context_time use cpu_clock() always.  That way we will only
call cpu_clock() once even if we have multiple cpu clock counters, and
that will eliminate the skew as well as being more efficient.

Paul.