creating custom kernel modules to register probe functions using the event
tracing infrastructure.
-Simplistically, tracepoints will represent an important event that when can
-be taken in conjunction with other tracepoints to build a "Big Picture" of
+Simplistically, tracepoints represent important events that can be
+taken in conjunction with other tracepoints to build a "Big Picture" of
what is going on within the system. There are a large number of methods for
gathering and interpreting these events. Lacking any current Best Practises,
this document describes some of the methods that can be used.
will give a fair indication of the number of events available.
-2.2 PCL
+2.2 PCL (Performance Counters for Linux)
-------
-Discovery and enumeration of all counters and events, including tracepoints
+Discovery and enumeration of all counters and events, including tracepoints,
are available with the perf tool. Getting a list of available events is a
-simple case of
+simple case of:
$ perf list 2>&1 | grep Tracepoint
ext4:ext4_free_inode [Tracepoint event]
[ .... remaining output snipped .... ]
-2. Enabling Events
+3. Enabling Events
==================
-2.1 System-Wide Event Enabling
+3.1 System-Wide Event Enabling
------------------------------
See Documentation/trace/events.txt for a proper description on how events
can be enabled system-wide. A short example of enabling all events related
-to page allocation would look something like
+to page allocation would look something like:
$ for i in `find /sys/kernel/debug/tracing/events -name "enable" | grep mm_`; do echo 1 > $i; done
-2.2 System-Wide Event Enabling with SystemTap
+3.2 System-Wide Event Enabling with SystemTap
---------------------------------------------
In SystemTap, tracepoints are accessible using the kernel.trace() function
print_count()
}
-2.3 System-Wide Event Enabling with PCL
+3.3 System-Wide Event Enabling with PCL
---------------------------------------
By specifying the -a switch and analysing sleep, the system-wide events
Similarly, one could execute a shell and exit it as desired to get a report
at that point.
-2.4 Local Event Enabling
+3.4 Local Event Enabling
------------------------
Documentation/trace/ftrace.txt describes how to enable events on a per-thread
basis using set_ftrace_pid.
-2.5 Local Event Enablement with PCL
+3.5 Local Event Enablement with PCL
-----------------------------------
-Events can be activate and tracked for the duration of a process on a local
+Events can be activated and tracked for the duration of a process on a local
basis using PCL such as follows.
$ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free_direct \
0.973913387 seconds time elapsed
-3. Event Filtering
+4. Event Filtering
==================
Documentation/trace/ftrace.txt covers in-depth how to filter events in
ftrace. Obviously using grep and awk of trace_pipe is an option as well
as any script reading trace_pipe.
-4. Analysing Event Variances with PCL
+5. Analysing Event Variances with PCL
=====================================
Any workload can exhibit variances between runs and it can be important
-to know what the standard deviation in. By and large, this is left to the
+to know what the standard deviation is. By and large, this is left to the
performance analyst to do it by hand. In the event that the discrete event
occurrences are useful to the performance analyst, then perf can be used.
aggregation of discrete events, then a script would need to be developed.
Using --repeat, it is also possible to view how events are fluctuating over
-time on a system wide basis using -a and sleep.
+time on a system-wide basis using -a and sleep.
$ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free_direct \
-e kmem:mm_pagevec_free \
1.002251757 seconds time elapsed ( +- 0.005% )
-5. Higher-Level Analysis with Helper Scripts
+6. Higher-Level Analysis with Helper Scripts
============================================
When events are enabled the events that are triggering can be read from
o Reading information from /proc for the PID that triggered the event
o Deriving a higher-level event from a series of lower-level events.
- o Calculate latencies between two events
+ o Calculating latencies between two events
Documentation/trace/postprocess/trace-pagealloc-postprocess.pl is an example
script that can read trace_pipe from STDIN or a copy of a trace. When used
-on-line, it can be interrupted once to generate a report without existing
+on-line, it can be interrupted once to generate a report without exiting
and twice to exit.
Simplistically, the script just reads STDIN and counts up events but it
processes, the parent process responsible for creating all the helpers
can be identified
-6. Lower-Level Analysis with PCL
+7. Lower-Level Analysis with PCL
================================
-There may also be a requirement to identify what functions with a program
+There may also be a requirement to identify what functions within a program
were generating events within the kernel. To begin this sort of analysis, the
-data must be recorded. At the time of writing, this required root
+data must be recorded. At the time of writing, this required root:
$ perf record -c 1 \
-e kmem:mm_page_alloc -e kmem:mm_page_free_direct \
# (For more details, try: perf report --sort comm,dso,symbol)
#
-According to this, the vast majority of events occured triggered on events
-within the VDSO. With simple binaries, this will often be the case so lets
+According to this, the vast majority of events triggered on events
+within the VDSO. With simple binaries, this will often be the case so let's
take a slightly different example. In the course of writing this, it was
-noticed that X was generating an insane amount of page allocations so lets look
-at it
+noticed that X was generating an insane amount of page allocations so let's look
+at it:
$ perf record -c 1 -f \
-e kmem:mm_page_alloc -e kmem:mm_page_free_direct \
# (For more details, try: perf report --sort comm,dso,symbol)
#
-So, almost half of the events are occuring in a library. To get an idea which
-symbol.
+So, almost half of the events are occurring in a library. To get an idea which
+symbol:
$ perf report --sort comm,dso,symbol
# Samples: 27666
0.01% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 [.] get_fast_path
0.00% Xorg [kernel] [k] ftrace_trace_userstack
-To see where within the function pixmanFillsse2 things are going wrong
+To see where within the function pixmanFillsse2 things are going wrong:
$ perf annotate pixmanFillsse2
[ ... ]
git.openpandora.org / pandora-kernel.git / commitdiff