Subject: Re: [RFC] tracing: a proc file to automatically release ring
 buffer memory
From: Steven Rostedt <rostedt@goodmis.org>
To: Vaibhav Nagarnaik <vnagarnaik@google.com>
Cc: mingo@elte.hu, fweisbec@gmail.com, lizf@cn.fujitsu.com,
        Michael Rubin <mrubin@google.com>, David Sharp <dhsharp@google.com>,
        linux-kernel@vger.kernel.org
In-Reply-To: <AANLkTimXUDcD5YWAcR=SGa3EBM3+TaMRL1UtF4epxNaP@mail.gmail.com>
References: <AANLkTimXUDcD5YWAcR=SGa3EBM3+TaMRL1UtF4epxNaP@mail.gmail.com>
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Date: Thu, 17 Mar 2011 17:36:20 -0400
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On Thu, 2011-03-17 at 14:29 -0700, Vaibhav Nagarnaik wrote:
> All
> 
> The current way to release memory from ftrace ring buffer is to:
> echo 0 > /sys/kernel/debug/tracing/buffer_size_kb
> 
> Consider a scenario where ftrace is handled by a user space
> process. Normally, this tracing is done as a low priority task on the
> system. Under memory pressure for high priority tasks, tracing should
> be turned off and the trace ring buffer memory released. The buffer
> allocated is kernel memory and under OOM condition if the user space
> process is oom-killed, there is no way to release the ring buffer
> memory.
> 
> The proposal is to add a proc entry
> /sys/kernel/debug/tracing/buffer_free. Its .release() fileops function
> executes tracing_stop() and then resizes the global trace buffer to
> minimum size (2*BUF_PAGE_SIZE). The user space process opens this file
> and keeps it open. When the process gets killed, the fd gets closed
> and .release() is called. This automatically stops the tracing and
> frees up the ring buffer memory.
> 
> It is optional to use this functionality, since it won't affect
> tracing for current programs. But it provides a way to automatically
> free up memory, if needed. It can also be used on the command line to
> stop tracing and free up memory as a single operation as:
> echo > /sys/kernel/debug/tracing/buffer_free

I don't like adding new files to debug/tracing. If we do add this
feature, I rather have it done with the buffer_size_kb directly. Perhaps
add an ioctl() to the file that will do this.

You can open it and then ioctl(RINGBUF_FREE_ON_CLOSE);

If the process dies, it closes the buffer_size_kb and frees the memory.

-- Steve


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