linux-kernel - Re: [RFC PATCH 1/6] perf: Move mlock accounting to ring buffer allocation

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Date:   Fri, 23 Sep 2016 18:58:57 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        tglx@...utronix.de, ak@...ux.intel.com
Subject: Re: [RFC PATCH 1/6] perf: Move mlock accounting to ring buffer allocation

Peter Zijlstra <peterz@...radead.org> writes:

> On Fri, Sep 23, 2016 at 05:27:22PM +0300, Alexander Shishkin wrote:
>> > Afaict there's no actual need to hide the AUX buffer for this sampling
>> > stuff; the user knows about all this and can simply mmap() the AUX part.
>> 
>> Yes, you're right here. We could also re-use the AUX record, adding a
>> new flag for this. It may be even better if I can work out the
>> inheritance (the current code doesn't handle inheritance at the moment
>> in case we decide to scrap it).
>
> What is the exact problem with inheritance? You can inherit PT (and
> other) events just fine, and their output redirects to the original
> (AUX) buffer too.
>
> Is the problem untangling which part of the AUX buffer belongs to which
> task upon sample?

Tasks we can figure out from id samples on RECORD_AUX (assuming we're
using those), but which event (if you have multiple) does a sample
belong to is trickier.

Cutting out samples becomes more interesting as normally RECORD_AUX
don't overlap, we can keep it that way and then the samples will
naturally be non-overlapping, but will all be different sizes. And there
is a question of waking up the consumer often enough to copy out all the
samples before the buffer overwrites itself. Let me think a bit.

Regards,
--
Alex