Re: disabling group leader perf_event

[Avi Kivity <avi@redhat.com>]

  On 09/06/2010 02:54 PM, Peter Zijlstra wrote:
>
>> Basically, to read() all events in one go.  I have many of them.
>>
>> My current problem is that I have an event (kvm_exit) which I want to
>> drill down by looking at a field (exit_reason).  So I create lots of
>> separate perf_events with a filter for each reason:
>> kvm_exit(exit_reason==0), kvm_exit(exit_reason==1), etc.  But filters
>> are fairly slow (can have ~60 such events on AMD), so I want to make
>> this drill-down optional.
> Yeah, filters suck.

Any idea why?  I saw nothing obvious in the code, except that there is 
lots of it.

> So what you're basically trying to do is create some histogram of
> exit_reason?

Yes, exactly.

> Being able to make histograms in-kernel has been on the todo list for a
> long while, its just that I never could come up with a sane
> interface.. :/

Interesting, I thought it was just me.

One option is to keep the existing filter interface, but recognize those 
cases and optimize the implementation.  Sort of like a compiler can 
optimize a large dense switch statement to a jump table.

>> Current plan is to have a group for the basic events and another group
>> for the drilldown events (each per-cpu), and activate the drilldown
>> group on user request.  perf will be able to schedule both groups
>> concurrently since they only contain tracepoints, yes?
> More or less, yeah (the scheduling of software and hardware events isn't
> properly separated atm -- am working on that). Software events have no
> scheduling constraints and should always get scheduled.

Great, thanks.

(one other issue - right now I'm using cpu events.  If I switch to task 
events, I lose events generated by workqueues, yes?)

-- 
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