linux-kernel - Re: [PATCH v4 02/22] perf: Add AUX area to ring buffer for raw data streams

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Date:	Tue, 07 Oct 2014 00:52:49 +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,
	Robert Richter <rric@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>, kan.liang@...el.com
Subject: Re: [PATCH v4 02/22] perf: Add AUX area to ring buffer for raw data streams

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

> On Mon, Oct 06, 2014 at 12:08:19PM +0300, Alexander Shishkin wrote:
>> Peter Zijlstra <peterz@...radead.org> writes:
>> 
>> > On Mon, Sep 08, 2014 at 03:55:11PM +0300, Alexander Shishkin wrote:
>> >
>> >> Fair enough. Then I'd like to disable the ACTIVE ones before freeing AUX
>> >> stuff and then re-enabling them since perf_event_{en,dis}able() already
>> >> provide the convenient cross-cpu calls, which would also avoid
>> >> concurrency between pmu::{add,del} callbacks and this unmap path. Makes
>> >> sense?
>> >
>> > But why? The buffer stuff is RCU freed, so if the hardware observes
>> > pages and does get_page_unless_zero() on them its good. The memory will
>> > not be freed from underneath the hardware writer because of the
>> > get_page().
>> >
>> > Then when the buffer is full and we 'swap', we'll find there is no next
>> > buffer. At that point we can not provide a new buffer, effectively
>> > stopping the hardware writes and release the old buffer, freeing the
>> > memory.
>> 
>> There are several problems with this. Firstly, aux buffers can be quite
>> large, which means that we have to do get_page() on thousands of pages
>> on every pmu::add, which is a hot path and free_page() again in
>> pmu::del.
>> 
>> Secondly, all the sg bookkeeping that the driver keeps in aux_priv needs
>> to be refcounted. Right now, in the mmap_close path we just free
>> everything. But if we want to free the aux_pages in pmu::del, we need to
>> keep a list of these pages still around after mmap_close() and same goes
>> for the actual sg tables. I can see a way of doing that on the ring
>> buffer side (as opposed to the driver side), but are you quite sure we
>> should go down this road?
>
> No, and I think I realized this after sending that email, but at the
> time I figured there was another way to do it. Of course, now that
> several weeks have passed I cannot for the life of me remember what it
> was.
>
> Lemme go over these patches again to refresh my mind and maybe I'll
> remember.

Yes, ring buffer can keep a refcount for the aux_priv object, which is
grabbed once at mmap and once at perf_aux_output_begin() and released
accordingly and whichever drops the refcount to zero calls
pmu::free_aux. No need to grab page->_count and driver is a bit simpler.

Regards,
--
Alex
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