linux-kernel - Re: [PATCH v6 1/3] trace: Make removal of ring buffer pages atomic

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Date:	Wed, 02 May 2012 21:55:18 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Laurent Chavey <chavey@...gle.com>,
	Justin Teravest <teravest@...gle.com>,
	David Sharp <dhsharp@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/3] trace: Make removal of ring buffer pages atomic

On Wed, 2012-04-25 at 14:18 -0700, Vaibhav Nagarnaik wrote:
> This patch adds the capability to remove pages from a ring buffer
> without destroying any existing data in it.
> 
> This is done by removing the pages after the tail page. This makes sure
> that first all the empty pages in the ring buffer are removed. If the
> head page is one in the list of pages to be removed, then the page after
> the removed ones is made the head page. This removes the oldest data
> from the ring buffer and keeps the latest data around to be read.
> 
> To do this in a non-racey manner, tracing is stopped for a very short
> time while the pages to be removed are identified and unlinked from the
> ring buffer. The pages are freed after the tracing is restarted to
> minimize the time needed to stop tracing.
> 
> The context in which the pages from the per-cpu ring buffer are removed
> runs on the respective CPU. This minimizes the events not traced to only
> NMI trace contexts.
> 
> Signed-off-by: Vaibhav Nagarnaik <vnagarnaik@...gle.com>

Hmm, something in this patch breaks buffers_size_kb and friends.

-- Steve


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