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

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Date:	Tue, 5 May 2009 09:42:11 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>
Subject: Re: perf_counter: resetting a event counter


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

> On Tue, 2009-05-05 at 08:52 +0200, Ingo Molnar wrote:
> > * 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.
> 
> I would suggest simply bailing when the counter is active when 
> trying to reset if anything.
> 
> A plain: atomic64_set(&counter->count, 0), sounds attractive too.
> 
> The trouble with putting in those disable/enable calls is that you 
> cannot use the ioctl on an already disabled call, since it will 
> immediately enable it. Also, using it on an active counter is racy 
> in nature so the disable/enable cycle (or the proposed mutex) 
> doesn't buy you anything.

Yes, it all seems a bit racy - but straightforward.

I dont think we should restrict the ioctl to disabled state alone - 
we should disable it if it's not disabled - reset the counter - then 
re-enable-it if it was enabled before.

Btw., hw_counter->prev_counter needs to be reset too (if it's a hw 
counter not a sw counter), right?

	Ingo
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