On 04/18/17 17:07, Frank Rowand wrote:

On 04/17/17 17:32, Tyrel Datwyler wrote:

quoted

This patch introduces event tracepoints for tracking a device_nodes
reference cycle as well as reconfig notifications generated in response
to node/property manipulations.

With the recent upstreaming of the refcount API several device_node
underflows and leaks have come to my attention in the pseries (DLPAR) dynamic
logical partitioning code (ie. POWER speak for hotplugging virtual and physcial
resources at runtime such as cpus or IOAs). These tracepoints provide a
easy and quick mechanism for validating the reference counting of
device_nodes during their lifetime.

Further, when pseries lpars are migrated to a different machine we
perform a live update of our device tree to bring it into alignment with the
configuration of the new machine. The of_reconfig_notify trace point
provides a mechanism that can be turned for debuging the device tree
modifications with out having to build a custom kernel to get at the
DEBUG code introduced by commit 00aa3720.

I do not like changing individual (or small groups of) printk() style
debugging information to tracepoint style.

As far as I know, there is no easy way to combine trace data and printk()
style data to create a single chronology of events.  If some of the
information needed to debug an issue is trace data and some is printk()
style data then it becomes more difficult to understand the overall
situation.

And of course the other issue with using tracepoints is the extra space
required to hold the tracepoint info.  With the pr_debug() approach, the
space usage can be easily removed for a production kernel via a config
option.

Tracepoints are wonderful technology, but not always the proper tool to
use for debug info.

If Rob wants to convert printk() style data to trace data (and I can't
convince him otherwise) then I will have further comments on this specific
patch.

-Frank

quoted

The following trace events are provided: of_node_get, of_node_put,
of_node_release, and of_reconfig_notify. These trace points require a kernel
built with ftrace support to be enabled. In a typical environment where
debugfs is mounted at /sys/kernel/debug the entire set of tracepoints
can be set with the following:

  echo "of:*" > /sys/kernel/debug/tracing/set_event

or

  echo 1 > /sys/kernel/debug/tracing/of/enable

The following shows the trace point data from a DLPAR remove of a cpu
from a pseries lpar:

cat /sys/kernel/debug/tracing/trace | grep "POWER8@10"

cpuhp/23-147   [023] ....   128.324827:
	of_node_put: refcount=5, dn->full_name=/cpus/PowerPC,POWER8@10
cpuhp/23-147   [023] ....   128.324829:
	of_node_put: refcount=4, dn->full_name=/cpus/PowerPC,POWER8@10
cpuhp/23-147   [023] ....   128.324829:
	of_node_put: refcount=3, dn->full_name=/cpus/PowerPC,POWER8@10
cpuhp/23-147   [023] ....   128.324831:
	of_node_put: refcount=2, dn->full_name=/cpus/PowerPC,POWER8@10
   drmgr-7284  [009] ....   128.439000:
	of_node_put: refcount=1, dn->full_name=/cpus/PowerPC,POWER8@10
   drmgr-7284  [009] ....   128.439002:
	of_reconfig_notify: action=DETACH_NODE, dn->full_name=/cpus/PowerPC,POWER8@10,
			    prop->name=null, old_prop->name=null
   drmgr-7284  [009] ....   128.439015:
	of_node_put: refcount=0, dn->full_name=/cpus/PowerPC,POWER8@10
   drmgr-7284  [009] ....   128.439016:
	of_node_release: dn->full_name=/cpus/PowerPC,POWER8@10, dn->_flags=4

Signed-off-by: Tyrel Datwyler <redacted>
---
 drivers/of/dynamic.c      | 30 ++++++---------
 include/trace/events/of.h | 93 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 105 insertions(+), 18 deletions(-)
 create mode 100644 include/trace/events/of.h

< snip >