Re: [RFC PATCH 0/4] perf: Custom contexts

[Frederic Weisbecker]

On Fri, Mar 25, 2011 at 04:03:53PM +0100, Peter Zijlstra wrote:
> On Fri, 2011-03-25 at 15:47 +0100, Frederic Weisbecker wrote:
> 
> > > Furthermore, there is a definite possibility for weird behaviour in
> > > there, in that if you're trying to measure a similar event to the one
> > > that is used to enable/disable it, it very much depends on the order of
> > > the demux lists as to which is processed first.
> > 
> > Do you have an example of that? I have some trouble to figure out the
> > issue.
> 
> Suppose you for some reason or other, gate irq_enter counting with
> irq_enter/irq_exit (pretty daft example but hey), they irq_enter will
> have two events associated, one trigger and one counter gated by the
> trigger.
> 
> Now when irq_enter happens, the software event code in
> do_perf_sw_event() will iterate the events interested in the event (yay
> for naming). If the trigger comes first it will have enabled the gate
> and the event will count, if the event comes first then the gate will
> still be disabled and we'll not count.

Ok I see. So demux order problem only happen if the trigger is the same
kind of event than the target?

But that's an inherent problem of such event tree, I'm not sure we can
do much about it. Perhaps forcing some order somehow, no idea, we can
probably deal with that leater.

> 
> > > The simply scheme I came up with is having these events be part of the
> > > event_group and add only one field:
> > >
> > >  pause_ops : 2
> > >
> > > with:
> > >
> > > enum perf_event_pause_ops {
> > >  PERF_PAUSE_OP_NOP = 0,
> > >  PERF_PAUSE_OP_INC,
> > >  PERF_PAUSE_OP_DEC,
> > >  PERF_PAUSE_OP_TOGGLE,
> > > };
> > >
> > > and have INC increment the parent pause field and clip at INT_MAX, DEC
> > > decrement the pause field and clip at 0, and TOGGLE do ^1.
> > >
> > > That however doesn't allow for these full expression trees, so we need
> > > to come up with something else. It does however do away with the
> > > ioctl()s, that redundant flag and the weird event separation.
> > 
> > That's a great idea! It indeed utterly simplifies the implementation
> > and the interface
> > and reuse what we already habe.
> > This makes me double reconsider the recursive issue now.
> > 
> > But I'm still worried. I don't know how useful the event tree could be. But
> > I have the feeling we are losing a nice and very flexible feature if
> > we get rid of it.
> > This can be something we solve later if we accept recursive groups may
> > be, although
> > things may be even more complicated if we take that path.
> 
> Yeah, like I said I'm not sure my proposal is any good because of the
> limited functionality it provides, but its a different way of looking at
> the problem so I provided it.
> 
> We'll have to try and explode the issue a bit more to see if there's
> anything else we can come up with.

So, at least we can factorize the notion of starter/stopper into a
single trigger attr, which has some inc/dec properties, like you
proposed. That still requires a silly ioctl though.

About the inherit part, I thought about making the trigger only apply to
events inside a same group. That may simplify a bit the inherit implementation
by pushing a bit deeper the mess. OTOH that doesn't solve the fact
we have to deal with a kind of duplicate event list.

Thoughts?
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