linux-kernel - Re: [RFC] tracing: a proc file to automatically release ring buffer memory

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Message-ID: <AANLkTinPq0hNMnS9_VLUXOQNeQCUAKvoxZfiBoNCzsnX@mail.gmail.com>
Date:	Fri, 18 Mar 2011 15:59:53 -0700
From:	David Sharp <dhsharp@...gle.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>, mingo@...e.hu,
	fweisbec@...il.com, lizf@...fujitsu.com,
	Michael Rubin <mrubin@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] tracing: a proc file to automatically release ring buffer memory

On Thu, Mar 17, 2011 at 2:36 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 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.

Thanks for the feedback Steve. An ioctl on buffer_size_kb sounds fine to me.
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