Re: [PATCH v1 03/11] perf: Allow for multiple ring buffers per event

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> How about something like this for the itrace thing?
> 
> You would mmap() the regular buffer; when write ->aux_{offset,size} 
> in the control page. After which you can do a second mmap() with the 
> .pgoff matching the aux_offset you gave and .length matching the 
> aux_size you gave.
> 
> This way the mmap() content still looks like a single linear file 
> (could be sparse if you leave a hole, although we could require the 
> aux_offset to match the end of the data section).
> 
> And there is still the single event->rb, not more.
> 
> Then, when data inside that aux data store changes they should 
> inject an PERF_RECORD_AUX to indicate this did happen, which ties it 
> back into the normal event flow.
> 
> With this there should be no difficult page table tricks or 
> anything.
> 
> The patch is way incomplete but should sketch enough of the idea..
> 
> So the aux_head/tail values should also be in the file space and not 
> start at 0 again, similar for the offsets in the AUX record.

This looks like a pretty good concept to me, to support the buffering 
quirks/constraints that itrace CPUs apparently have.

Thanks,

	Ingo