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

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Message-ID: <20140908131230.GE6758@twins.programming.kicks-ass.net>
Date:	Mon, 8 Sep 2014 15:12:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
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

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.



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