Messages in this thread | | | Date | Tue, 5 May 2009 09:42:11 +0200 | From | Ingo Molnar <> | Subject | Re: perf_counter: resetting a event counter |
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* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> On Tue, 2009-05-05 at 08:52 +0200, Ingo Molnar wrote: > > * 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. > > 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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