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		Using the Linux Kernel Latency Histograms


This document gives a short explanation how to enable, configure and use
latency histograms. Latency histograms are primarily relevant in the
context of real-time enabled kernels (CONFIG_PREEMPT/CONFIG_PREEMPT_RT)
and are used in the quality management of the Linux real-time
capabilities.


* Purpose of latency histograms

A latency histogram continuously accumulates the frequencies of latency
data. There are two types of histograms
- potential sources of latencies
- effective latencies


* Potential sources of latencies

Potential sources of latencies are code segments where interrupts,
preemption or both are disabled (aka critical sections). To create
histograms of potential sources of latency, the kernel stores the time
stamp at the start of a critical section, determines the time elapsed
when the end of the section is reached, and increments the frequency
counter of that latency value - irrespective of whether any concurrently
running process is affected by latency or not.
- Configuration items (in the Kernel hacking/Tracers submenu)
  CONFIG_INTERRUPT_OFF_LATENCY
  CONFIG_PREEMPT_OFF_LATENCY


* Effective latencies

Effective latencies are actually occuring during wakeup of a process. To
determine effective latencies, the kernel stores the time stamp when a
process is scheduled to be woken up, and determines the duration of the
wakeup time shortly before control is passed over to this process. Note
that the apparent latency in user space may be somewhat longer, since the
process may be interrupted after control is passed over to it but before
the execution in user space takes place. Simply measuring the interval
between enqueuing and wakeup may also not appropriate in cases when a
process is scheduled as a result of a timer expiration. The timer may have
missed its deadline, e.g. due to disabled interrupts, but this latency
would not be registered. Therefore, the offsets of missed timers are
recorded in a separate histogram. If both wakeup latency and missed timer
offsets are configured and enabled, a third histogram may be enabled that
records the overall latency as a sum of the timer latency, if any, and the
wakeup latency. This histogram is called "timerandwakeup".
- Configuration items (in the Kernel hacking/Tracers submenu)
  CONFIG_WAKEUP_LATENCY
  CONFIG_MISSED_TIMER_OFSETS


* Usage

The interface to the administration of the latency histograms is located
in the debugfs file system. To mount it, either enter

mount -t sysfs nodev /sys
mount -t debugfs nodev /sys/kernel/debug

from shell command line level, or add

nodev	/sys			sysfs	defaults	0 0
nodev	/sys/kernel/debug	debugfs	defaults	0 0

to the file /etc/fstab. All latency histogram related files are then
available in the directory /sys/kernel/debug/tracing/latency_hist. A
particular histogram type is enabled by writing non-zero to the related
variable in the /sys/kernel/debug/tracing/latency_hist/enable directory.
Select "preemptirqsoff" for the histograms of potential sources of
latencies and "wakeup" for histograms of effective latencies etc. The
histogram data - one per CPU - are available in the files

/sys/kernel/debug/tracing/latency_hist/preemptoff/CPUx
/sys/kernel/debug/tracing/latency_hist/irqsoff/CPUx
/sys/kernel/debug/tracing/latency_hist/preemptirqsoff/CPUx
/sys/kernel/debug/tracing/latency_hist/wakeup/CPUx
/sys/kernel/debug/tracing/latency_hist/wakeup/sharedprio/CPUx
/sys/kernel/debug/tracing/latency_hist/missed_timer_offsets/CPUx
/sys/kernel/debug/tracing/latency_hist/timerandwakeup/CPUx

The histograms are reset by writing non-zero to the file "reset" in a
particular latency directory. To reset all latency data, use

#!/bin/sh

TRACINGDIR=/sys/kernel/debug/tracing
HISTDIR=$TRACINGDIR/latency_hist

if test -d $HISTDIR
then
  cd $HISTDIR
  for i in `find . | grep /reset$`
  do
    echo 1 >$i
  done
fi


* Data format

Latency data are stored with a resolution of one microsecond. The
maximum latency is 10,240 microseconds. The data are only valid, if the
overflow register is empty. Every output line contains the latency in
microseconds in the first row and the number of samples in the second
row. To display only lines with a positive latency count, use, for
example,

grep -v " 0$" /sys/kernel/debug/tracing/latency_hist/preemptoff/CPU0

#Minimum latency: 0 microseconds.
#Average latency: 0 microseconds.
#Maximum latency: 25 microseconds.
#Total samples: 3104770694
#There are 0 samples greater or equal than 10240 microseconds
#usecs	         samples
    0	      2984486876
    1	        49843506
    2	        58219047
    3	         5348126
    4	         2187960
    5	         3388262
    6	          959289
    7	          208294
    8	           40420
    9	            4485
   10	           14918
   11	           18340
   12	           25052
   13	           19455
   14	            5602
   15	             969
   16	              47
   17	              18
   18	              14
   19	               1
   20	               3
   21	               2
   22	               5
   23	               2
   25	               1


* Wakeup latency of a selected process

To only collect wakeup latency data of a particular process, write the
PID of the requested process to

/sys/kernel/debug/tracing/latency_hist/wakeup/pid

PIDs are not considered, if this variable is set to 0.


* Details of the process with the highest wakeup latency so far

Selected data of the process that suffered from the highest wakeup
latency that occurred in a particular CPU are available in the file

/sys/kernel/debug/tracing/latency_hist/wakeup/max_latency-CPUx.

In addition, other relevant system data at the time when the
latency occurred are given.

The format of the data is (all in one line):
<PID> <Priority> <Latency> (<Timeroffset>) <Command> \
<- <PID> <Priority> <Command> <Timestamp>

The value of <Timeroffset> is only relevant in the combined timer
and wakeup latency recording. In the wakeup recording, it is
always 0, in the missed_timer_offsets recording, it is the same
as <Latency>.

When retrospectively searching for the origin of a latency and
tracing was not enabled, it may be helpful to know the name and
some basic data of the task that (finally) was switching to the
late real-tlme task. In addition to the victim's data, also the
data of the possible culprit are therefore displayed after the
"<-" symbol.

Finally, the timestamp of the time when the latency occurred
in <seconds>.<microseconds> after the most recent system boot
is provided.

These data are also reset when the wakeup histogram is reset.