Re: perf_counter: resetting a event counter

[Ingo Molnar <mingo@elte.hu>]

* Corey Ashford <cjashfor@linux.vnet.ibm.com> wrote:

> Hi,
>
> In implementing the PAPI_reset function, whose purpose is to reset 
> all of the counters in an event set, I found that [it appears] 
> there is no straight-forward way to implement this function using 
> "Performance Couunters for Linux".  My current implementation just 
> closes the counters and reopens them again.  This is not a elegant 
> solution, nor is the other alternative that occurred to me: 
> maintain a "virtual" counter in user space, maintained using a 
> base count, which is subtracted off of the current perf_counter 
> value of a particular counter.
>
> Is there a way that I missed to reset an event counter?  If not, 
> I'd like to request that a new ioctl command be added to support 
> this ability.

We already have such ioctl actions:

        case PERF_COUNTER_IOC_ENABLE:
        case PERF_COUNTER_IOC_DISABLE:
        case PERF_COUNTER_IOC_REFRESH:

It would be a pretty natural addition to also have a reset method 
there. Would you like to take a stab at it and send a patch? A 
first-level approximation would be to do something like:

	perf_counter_disable(counter);
	atomic64_set(&counter->count, 0);
	perf_counter_enable(counter);

btw, the reset code should probably take the counter->mutex lock as 
well, because parallel resets done from multiple contexts are 
otherwise not well-defined.

	Ingo