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Date: Wed, 7 May 2014 21:25:14 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Frederic Weisbecker <fweisbec@gmail.com>,
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        Stephane Eranian <eranian@google.com>, Andi Kleen <ak@linux.intel.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Matt Fleming <matt.fleming@intel.com>
Subject: Re: [PATCH v1 03/11] perf: Allow for multiple ring buffers per event
Message-ID: <20140507192514.GB23409@gmail.com>
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* 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