Re: [PATCH 2/3] perf: Remove dead code in buildin-record.c

[Peter Zijlstra]

On Sun, 2010-07-18 at 21:06 +0100, Matt Fleming wrote:
> On Sat, 26 Jun 2010 16:14:46 +0100, Matt Fleming <matt@xxxxxxxxxxxxxxxxx> wrote:
> > On Fri, 25 Jun 2010 16:48:05 +0200, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > 
> > > Ah, yes, it would be nice for SH (which doesn't have a PMI) to couple a
> > > software timer with their hardware counter to get samples.
> > > 
> > > Never got around to actually implementing that though, maybe Paul has a
> > > minion interested in making that work?
> > 
> > I'll take a look at it.
> 
> How does the 'group_fd' parameter relate to the lack of PMI? Is the idea
> to have one hrtimer that, when it fires, we sample all the counters? So
> the first counter to be created is the group leader, which starts the
> hrtimer, and all other counters are linked to this one? I had a go at
> using a hrtimer per counter (minus any weighting of samples) and it
> worked OK and seemed sensible given that we may want to sample counters
> at different frequencies.
> 
> Is this what you had in mind with the 'group_fd' paramter, Peter? That
> there'd be only one hrtimer?

Right. So sys_perf_event_open() creates a stand alone event, but if you
supply the group_fd param it will attach the newly created one to the
leader indicated by group_fd.

Groups have the properly that they will always be scheduled together,
and read/sample can access/provide data of all of them.

So you can have a sampling leader report the counts of its siblings.

You can make multiple groups, like {hrtimer, cycles} and {hrtimer,
instructions} and {hrtimer, dcache-miss} and perf will schedule the
stuff. If you'd try to do {hrtimer, cycles, insns, dcache-miss} but only
have 2 hardware counters the group creation should fail because the
implementation should dis-allow creating groups that cannot be
scheduled 


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