linux-kernel - Re: [RFC] tracing: Adding cgroup aware tracing functionality

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Date:	Thu, 7 Apr 2011 03:33:53 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>
Cc:	Paul Menage <menage@...gle.com>, Li Zefan <lizf@...fujitsu.com>,
	Stephane Eranian <eranian@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, containers@...ts.linux-foundation.org
Subject: Re: [RFC] tracing: Adding cgroup aware tracing functionality

On Wed, Apr 06, 2011 at 11:50:21AM -0700, Vaibhav Nagarnaik wrote:
> All
> The cgroup functionality is being used widely in different scenarios. It also
> is being integrated with other parts of kernel to take advantage of its
> features. One of the areas that is not yet aware of cgroup functionality is
> the ftrace framework.
> 
> Although ftrace provides a way to filter based on PIDs of tasks to be traced,
> it is restricted to specific tracers, like function tracer. Also it becomes
> difficult to keep track of all PIDs in a dynamic environment with processes
> being created and destroyed in a short amount of time.
> 
> An application that creates many processes/tasks is convenient to track and
> control with cgroups, but it is difficult to track these processes for the
> purposes of tracing. And if child processes are moved to another cgroup, it
> makes sense to trace only the original cgroup.
> 
> This proposal is to create a file in the tracing directory called
> set_trace_cgroup to which a user can write the path of an active cgroup, one
> at a time. If no cgroups are specified, no filtering is done and all tasks are
> traced. When a cgroup path is added in, it sets a boolean tracing_enabled for
> the enabled cgroup in all the hierarchies, which enables tracing for all the
> assigned tasks under the specified cgroup.
> 
> Though creating a new file in the directory is not desirable, but this
> interface seems the most appropriate change required to implement the new
> feature.
> 
> This tracing_enabled flag is also exported in the cgroupfs directory structure
> which can be turned on/off for a specific hierarchy/cgroup combination. This
> gives control to enable/disable tracing over a cgroup in a specific hierarchy
> only.
> 
> This gives more fine-grained control over the tasks being traced. I would like
> to know your thoughts on this interface and the approach to make tracing
> cgroup aware.

So I have to ask, why can't you use perf events to do tracing limited on cgroups?
It has this cgroup context awareness.
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