Re: disabling group leader perf_event

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2010-09-06 at 14:34 +0300, Avi Kivity wrote:
> On 09/06/2010 02:24 PM, Peter Zijlstra wrote:
> > On Mon, 2010-09-06 at 12:12 +0300, Avi Kivity wrote:
> >> If I read the code correctly, disabling a group leader perf_event will
> >> disable the entire group.
> >>
> >> Is this correct?
> > Yeah, pretty much.
> 
> Well, I never liked group_leader style APIs.  I like different types for 
> the container and the contained.  But such is not unix.
> 
> >> If so, how can I disable just the event itself?  Can I allocate a dummy
> >> invent for the group leader so I can enable and disable each perf_event
> >> in the group individually?
> > Which makes me wonder why you use groups in the first place.
> 
> 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.

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

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.. :/

> 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.