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Date: Thu, 7 Apr 2011 14:06:11 +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>,
David Sharp <dhsharp@...gle.com>,
Michael Rubin <mrubin@...gle.com>,
Ken Chen <kenchen@...gle.com>, 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 08:17:33PM -0700, Vaibhav Nagarnaik wrote:
> On Wed, Apr 6, 2011 at 6:33 PM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > 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.
> >
>
> The perf event cgroup awareness comes from creating a different hierarchy for
> perf events. When the events and the current task's cgroup match, the events
> are logged. So the changes are pretty specific to the perf events.
>
> Even in the case where changes are made to handle trace events, the interface
> files are still needed. The interface used to specify perf events uses the
> perf_event syscall which isn't available to specify trace events.
>
> This is based on my limited understanding of the perf_events cgroup awareness
> patch. Please correct me if I am missing anything.
Ah but perf events can do much more than counting and sampling
hardware events. Trace events can be used as perf events too.
List the events:
perf list -e tracepoints
List of pre-defined events (to be used in -e):
skb:kfree_skb [Tracepoint event]
skb:consume_skb [Tracepoint event]
skb:skb_copy_datagram_iovec [Tracepoint event]
net:net_dev_xmit [Tracepoint event]
net:net_dev_queue [Tracepoint event]
net:netif_receive_skb [Tracepoint event]
net:netif_rx [Tracepoint event]
napi:napi_poll [Tracepoint event]
scsi:scsi_dispatch_cmd_start [Tracepoint event]
scsi:scsi_dispatch_cmd_error [Tracepoint event]
scsi:scsi_dispatch_cmd_done [Tracepoint event]
scsi:scsi_dispatch_cmd_timeout [Tracepoint event]
scsi:scsi_eh_wakeup [Tracepoint event]
drm:drm_vblank_event [Tracepoint event]
drm:drm_vblank_event_queued [Tracepoint event]
drm:drm_vblank_event_delivered [Tracepoint event]
block:block_rq_abort [Tracepoint event]
block:block_rq_requeue [Tracepoint event]
block:block_rq_complete [Tracepoint event]
block:block_rq_insert [Tracepoint event]
etc...
Trace sched switch events:
perf record -e sched:sched_switch -a
^C
Print them:
perf script
swapper 0 [000] 1132.964598: sched_switch: prev_comm=swapper prev_pid=0 prev_prio=120 prev_state=R ==> next_comm
kworker/0:1 4358 [000] 1132.964641: sched_switch: prev_comm=kworker/0:1 prev_pid=4358 prev_prio=120 prev_state=S ==> ne
syslogd 2703 [000] 1132.964720: sched_switch: prev_comm=syslogd prev_pid=2703 prev_prio=120 prev_state=D ==> next_c
swapper 0 [000] 1132.965100: sched_switch: prev_comm=swapper prev_pid=0 prev_prio=120 prev_state=R ==> next_comm
perf 4725 [001] 1132.965178: sched_switch: prev_comm=perf prev_pid=4725 prev_prio=120 prev_state=D ==> next_comm
swapper 0 [001] 1132.965227: sched_switch: prev_comm=kworker/0:0 prev_pid=0 prev_prio=120 prev_state=R ==> next_
perf 4725 [001] 1132.965246: sched_switch: prev_comm=perf prev_pid=4725 prev_prio=120 prev_state=D ==> next_comm
etc...
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