Re: [PATCH 3/7] perf_counter: ioctl(PERF_COUNTER_IOC_RESET)

[Ingo Molnar <mingo@elte.hu>]

* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Tue, 2009-05-05 at 11:31 -0700, Corey Ashford wrote:
> > Hi Peter,
> > 
> > Peter Zijlstra wrote:
> > 
> > > +static void perf_counter_reset(struct perf_counter *counter)
> > > +{
> > > +	atomic_set(&counter->count, 0);
> > > +}
> > > +
> > 
> > 
> > Thanks for posting a patch for this issue.
> > 
> > As Ingo said, I think the hardware counter needs to be reset as well as 
> > the value saved in the perf_counter struct.
> > 
> 
> I don't think that's needed, we calculate a delta between prev_count and
> the current read and use that to increment counter->count. Therefore
> when we reset counter->count we should not need to touch the hardware
> counter.
> 
> However, I do think we need the below, first read the hardware counter
> to ensure that delta spoken of above is as close to zero as possible
> when we reset.
> 
> And update the user-page bits.
> 
> ---
> Index: linux-2.6/kernel/perf_counter.c
> ===================================================================
> --- linux-2.6.orig/kernel/perf_counter.c
> +++ linux-2.6/kernel/perf_counter.c
> @@ -1299,7 +1299,9 @@ static unsigned int perf_poll(struct fil
>  
>  static void perf_counter_reset(struct perf_counter *counter)
>  {
> +	(void)perf_counter_read(counter);
>  	atomic_set(&counter->count, 0);
> +	perf_counter_update_userpage(counter);
>  }

Mind sending a changelogged patch? Thanks,

	Ingo