Re: [PATCH 4/4][RFC] perf_counter: Allow sharing of output channels

[Ingo Molnar]

* stephane eranian <eranian@xxxxxxxxxxxxxx> wrote:

> On Wed, Aug 19, 2009 at 2:56 PM, Peter Zijlstra<a.p.zijlstra@xxxxxxxxx> wrote:
> > On Wed, 2009-08-19 at 22:36 +1000, Paul Mackerras wrote:
> >> Peter Zijlstra writes:
> >>
> >> > Provide the ability to configure a counter to send 
> >> > its output to another (already existing) counter's 
> >> > output stream.
> >>
> >> What sort of thing might this be useful for?
> >
> > Some people complained that its tedious to mmap() for 
> > every counter and would like to share the mmap() 
> > output buffer between counters.
> >
> 
> > This saves on address space and mlock budget and I 
> > guess fd management logic.
>
> Interesting to see, you seem to have changed your mind 
> on this. I recall pointing this out in my early 
> comments.

Btw., did we strongly oppose the notion? The mmap output 
buffers were always multi-event in essence - just by 
virtue of the automatic events such as task creation, 
mmap, etc.

> But anyway, here are some more comments:
> 
>    - how does this work with the remapped counts?
>      Probably only see the count for the target, i.e., 
>      output, event

if you want to recover the momentary index for RDPC you 
need to mmap the counter(s) you are interested in, to get 
to the header->index value.

So you can create these counters with 1 pages only (one 
is the header) - and then remap the output to the primary 
counter's output buffer.

>    - if samples from multiple events end up in the same 
>      buffer, how do I tell them apart, i.e., how do I 
>      know sample X came from event A, sample X from 
>      event B? This may be useful to detect patterns.

Via PERF_SAMPLE_ID. That saves the counter ID into the 
event so it can be demultiplexed later on.

	Ingo
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