linux-kernel - Re: perf_counter: resetting a event counter

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Date:	Tue, 5 May 2009 08:52:06 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: perf_counter: resetting a event counter


* Corey Ashford <cjashfor@...ux.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
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