From: eLinux.org
Using Kernel Function Trace Ver. 0.1.1 -- 2007-04-26 Most material provided by Sony
This document describes how to use Kernel Function Trace with the Linux kernel. It assumes you have already applied the KFT patch to your kernel, or that it was otherwise previously integrated with your kernel source code.
Kernel Function Trace (KFT) is a kernel function tracing system, which examines every function entry and exit in the Linux kernel. The KFT system provides for capturing a subset of these events along with timing and other details. KFT is different from other kernel tracing systems in that it is designed to be able to filter the events by the duration of the function calls. Thus, KFT is good for finding out where time is spent in functions and sub-routines in the kernel. When used in unfiltered mode, KFT is very useful to collect information about the flow of control in the kernel, which can help with debugging or optimizing kernel code.
The main mode of operation with KFT is by running a "dynamic" trace.
That is, you start the kernel as usual, then, using the /proc/kft
interface, configure a trace, start it, and retrieve the trace data
immediately.
However, another special mode of operation is available for performing bootup time tracing. In this mode, the configuration for a trace is compiled statically into the kernel. This is sometimes referred to as "static" mode. This mode is useful for getting a trace of the kernel during system bootup, before user space is running and before any services are available to configure and start a trace. This mode is particularly helpful to find problems with kernel bootup time.
In either case, you specify a KFT configuration for the trace run. The configuration tells how to automatically start and stop the trace, whether to include interrupts as part of the trace, and whether to filter the event data by various criteria (for minimum function duration, only certain listed functions, etc.)
When a trace is complete, the event data collected during the trace is
retrieved by reading from /proc/kft_data.
Finally, KFT provides tools to process and analyze the data in a KFT trace.
Quick overview for using KFT in dynamic mode:
/proc/kft/proc/kft_dataaddr2sym to convert addresses to function nameskd to analyze trace dataQuick overview for using KFT during bootup:
<linux_src>/kernel/kftstatic.conf/proc/kft_dataaddr2sym to convert addresses to function nameskd to analyze trace dataConfigure your kernel to support KFT by editing the kernel configuration (.config) file.
For example, if you are using 'make menuconfig', set the following option under the "Kernel Hacking" menu.
Kernel Hacking --->
[*] Kernel Function Trace
Save this configuration. This will set the option CONFIG_KFT=y in your kernel .config file.
If you wish to perform a trace during kernel bootup time, also configure for KFT static mode.
For example, if you are using 'make menuconfig', set the following option under the "Kernel Hacking" menu.
Kernel Hacking --->
[*] Kernel Function Trace
[*] Static function tracing configuration
Save this configuration. This will set the following options in your kernel .config file:
CONFIG_KFT=y
CONFIG_KFT_STATIC_RUN=y
If you are performing a "static" trace, edit the file
kernel/kftstatic.conf to set the configuration for the trace run you
wish to perform at system boot. (see the next section "Configuring the
trace run" for details on the trace configuration syntax and options.)
Note that even if you perform or bootup time trace, you can still
perform dynamic traces any time while the system is running.
Build the kernel, and install it to boot on your target machine.
Make sure to save the System.map file from this build, since it will
be used later when processing the trace data.
If you get an error compiling the kernel, see the next section on trouble-shooting configuration problems.
To configure your trace, you write a trace configuration file. This file specifies when to start and stop the trace, and what events to save as part of the trace data.
Here is a sample configuration file, commonly used during bootup:
begin
trigger start entry start_kernel
trigger stop entry to_userspace
filter mintime 500
end
This trace says to:
The function "start_kernel" is the first C function executed by the
kernel on startup. The function "to_userspace" is a function called
immediately before execution is transferred to the first user space
program (usually /sbin/init). This trace configuration says to start
tracing immediately when the kernel starts executing, and stop tracing
right before the first user space program runs. It will only save in the
trace buffer a record of functions that took longer than 500
microseconds to execute.
Triggers, in the configuration, are used to start and stop the data collection of the trace system. Triggers can be based on a function entry or exit event, or on the passage of time. The stop trigger is used to control the amount of data collected. The trace will automatically stop if the buffer runs out of space for trace data.
Time values are expressed in decimal microseconds. The start time is relative to booting, or to the initialization of the clock used for tracing (usually, whatever clock is being used by the internal kernel function sched_clock(). A stop time is relative to the start time.)
Here are some examples:
statement: trigger start entry start_kernel
statement: trigger stop exit do_fork
statement: trigger start time 10000000
statement: trigger stop time 5000
Filters control what data is collected during the trace. Since every kernel function entry and exit is a possible candidate for trace event recording, KFT can potentially generate a LOT of data. To control how much data is recorded, it is customary to set filters used during the trace.
You can filter by function duration, by interrupt context, or limit the trace to specific functions. Times in a filter statement are expressed in microseconds. Functions in a filter function list can be expressed by name in a static configuration, but must be expressed by addressed in a dynamic configuration.
Here are some examples:
statement: filter mintime 100
statement: filter maxtime 5000000
statement: filter noints
statement: filter onlyints
statement: filter funclist do_fork sys_read fend
For other commands you can include in the trace configuration, see Appendix A
You may get an error linking the kernel if you reference certain
functions in the kftstatic.conf that are not visible globally. If you
see a linker error like the following: "undefined reference to
`foo_func'", then you can resolve this by making 'foo_func' visible.
Usually, this means finding the declaration of 'foo_func', and
removing the 'static' keyword from its declaration.
If you are running in static mode, upon booting the kernel, the trace
should be initiated and run automatically, depending on the trigger and
filter settings in kernel/kftstatic.conf).
If you are running in dynamic mode, then you initiate a run by writing a
KFT configuration to /proc/kft, then "priming" the run.
Traces go through a state machine (a series of event transitions) in
order to actually start collecting data. This is to allow trace
collection to be separated from trace setup and preparation. The trace
configuration specifies a start trigger, which will initiate the
collection of data. When the configuration is written to /proc/kft, it
is not ready to run yet. Making the trace ready to run is called
"priming" it.
Therefore, the normal sequence of events for a trace run is:
. The user writes the configuration file, usually using an editor and creating the file in the local filesystem. Helper scripts can be used to auto-generate simple configurations for common tasks.
The user writes the configuration to KFT (via /proc/kft)
cat /tmp/trace.config >/proc/kftIf needed, the user prepares for trace by setting up programs to run.
The user primes the trace
echo "prime" >/proc/kftA kernel event occurs which starts the trace (the start trigger fires)
It is possible to force the start or end of a trace using the
/proc/kft interface. This overrides steps 5 or 7, which are normally
performed by triggers in the trace configuration.
echo "start" >/proc/kftecho "stop" >/proc/kftYou can get the status of the current trace by reading /proc/kft.
To see the status of the currently configured trace:
cat /proc/kftWhen the trace is running, the trace data is accumulated in a buffer
inside the kernel. Once the trace data is collected, you can retrieve if
from the kernel by copying the data from /proc/kft_data. Usually, you
will want to save the data to a file for later analysis.
Here is an example:
cat /proc/kft_data >/tmp/kft.logCopy the kft.log file from the target to your host development system
(on which the kernel source resides), for example, into the /tmp
directory on your host machine.
The raw kft.log file will only have numeric function addresses. To
translate these addresses to symbols, use the addr2sym program, along
with the System.map file which was produced when you built the kernel.
Change directory to your kernel source top-level directory and run
scripts/addr2sym to translate addresses to symbols:
Example:
$ scripts/addr2sym /tmp/kft.log -m System.map > /tmp/kft.lst
Here is an example fragment of output from addr2sym on a TI OMAP
Innovator. Entry and Delta value are times in microseconds. The Entry
time is the time time since machine boot, and the Delta time is the time
between the function entry and exit.
Entry Delta PID Function Called At
-------- -------- ----- ------------------------- --------------------------
23662 1333 0 con_init console_init+0x78
25375 209045 0 calibrate_delay start_kernel+0xf0
234425 106067 0 mem_init start_kernel+0x130
234432 105278 0 free_all_bootmem_node mem_init+0xc8
234435 105270 0 free_all_bootmem_core free_all_bootmem_node+0x28
340498 4005 0 kmem_cache_sizes_init start_kernel+0x134
In the above, calibrate_delay took about 209 milliseconds.
mem_init took 106 msecs, the majority of which (105 msecs) was in
free_all_bootmem_core (which is called by free_all_bootmem_node,
which is called by mem_init).
If you just look at the function duration, it may appear that lots of time is being spent in certain functions, when in reality those functions are "thin", and the real time-consuming function is one of its children. Thus, rather than look just at the function Delta (or duration), you should look at the function entry times. If there is a big leap in the function entry times, that means a lot of time was consumed in the function right before the leap.
In the example above, there is a leap from 234435 to 340498 (about 100
milliseconds) between the Entry times for free_all_bootmem_core and
kmem_cache_sizes_init. No other functions Entries (lasting more than
500 microseconds, based on the KFT configuration used) were recorded
during this time, so this means that this time was spent in
free_all_bootmem_core.
CPU-yielding functions like schedule_timeout, switch_to, kernel_thread, etc. can have large Delta values due intervening scheduling activity, but these can often be quickly filtered out by following the "leaps in the entry times in the Entry column" above.
You can use the program "kd" to further process the data. It is very
helpful at this point to have resolved the names of the functions in the
log file, but it is not strictly necessary. The kd program function
reads a KFT log file and determines the time spent locally in a function
versus the time spent in sub-routines. It sorts the functions by the
total time spent in the function, and can display various extra pieces
of information about each function (number of times called, average call
time, etc.)
Also kd can be used to re-generate a function call trace from the
trace log. This can be very helpful to see the sequence of execution
(including interrupts, context switches and returns) of the code that
was traced.
Use "./kd -h" for more usage help.
As of this writing, KFT and kd do not correctly account for scheduling jumps. The time reported by KFT for function duration is just wall time from entry to exit.
For examples of what kd can show, try the following commands on the sample kft output file:
[show all functions sorted by time]
$ ./kd kftsample.lst | less
[show only 10 top time-consuming functions]
$ ./kd -n 10 kftsample.lst
[show only functions lasting longer than 100 milliseconds]
$ ./kd -t 100000 kftsample.lst
[show each function's most time-consuming child, and the number of times it was called. (You may want to make your terminal wider for this output.)]
$ ./kd -f Fcatlmn kftsample.lst
[show call traces]
$ ./kd -c kftsample.lst
[show call traces with timing data, and functions interlaced]
$ ./kd -c -l -i kftsample.lst
Note that the call trace mode may not produce accurate results if weird filtering was used in the trace config (routines that are part of the call tree may be missing, which will confuse kd).
KFT includes the following helper scripts which are located in the
kernel scripts directory:
The use of most these are described elsewhere in this document. But this list is here for the sake of completeness.
[ should provide usage for each command? ]
(searching child functions for local time)
Here's a presentation about KFT usage: (Actually, the presentation covers KFT's predecessor KFI, but all the information is basically the same.)
This appendix describes the language for specifying a KFT trace run. Is
it used for both static mode (kftstatic.conf), and dynamic mode
(written to /proc/kft).
NOTE that for parameters referencing functions, you can use the function
name in kftstatic.conf (that is, when you using a static
configuration). However, you have to use the function address when
setting the configuration via the /proc/kft interface. The reason for
this is that kernel symbols are always available at compile-time, but
may not be available in the kernel at runtime, depending on your kernel
configuration.
To convert a function name to an address, you can look up the address
for the symbol in the System.map file for the current kernel. There is a
helper program provided called sym2addr which you can use to convert
the function names in a configuration file into addresses. To do this
manually, use:
e.g. grep do_fork System.map
c001d804 T do_fork
In this case, you would put 0xc001d804 in place of the function name in the configuration file. (Note the leading '0x'.)
To use the helper function sym2addr, do the following:
sym2addr trace_do_fork.conf System.map >trace_do_fork.conf2
cat trace_do_fork.conf2 >/proc/kft
The configuration for a single run is inside a block that starts with 'begin' and ends with 'end'. Inside the block are triggers, filters, and miscellaneous entries. By convention, each configuration entry is placed on its own line. When writing the configuration to /proc/kft, then the keyword "new" should appear before the block 'begin' keyword.
trigger:
either "start" or "stop", and then one of:
entry <funcname>
exit <funcname>
time <time-in-usecs>
syntax:
trigger start|stop entry|exit|time <arg>
Start time is relative to booting. Stop time is relative to trace start time.
filters
maxtime <max-time>
mintime <min-time>
noints
onlyints
funclist <func1> <func2> fend
syntax:
filter noints|onlyints|maxtime|mintime|funclist <args> fend
The funclist specifies a list of functions which will be traced. When a funclist is specified, only those functions are traced, and all other functions are ignored.
When specifying a configuration via /proc/kft, the 'fend' keyword must be used to indicated the end of the function list. When the configuration is specified via kftstatic.conf, no 'fend' keyword should be used.
watches
stack <low-water-threshold>
worst-stack <starting-low-water-threshold>
syntax:
watch stack|worst-stack <threshold>
A watch is used to have KFT monitor the trace for a particular condition, and act on the condition (usually preserve extra data to help debug that condition). The only supported watches currently are for monitoring the stack depth.
For a "stack" watch, while the trace is running the current position of the stack pointer is checked upon entry to every function. If the stack position is lower than the specified threshold, the current call stack of functions is preserved in the log (no matter whether the functions match other KFT filtering criteria or not), and the function durations are marked with a -2 value, to highlight them in the log. This operation (saving the call stack) is performed every time the stack position underflows the threshold. In this mode, an arbitrary number of call stacks can be recorded in the log (up to the limit of the log size).
For a "worst-stack" watch, the same monitoring is performed as with a "stack" watch. However, every time the condition is met, the threshold (worst stack left) is set to the new low stack value. In this mode, a call stack is preserved for each new low-water condition. The last such set of marked functions in the log will record the most stack-consuming call stack seen during the trace. Note also that the lowest recorded stack position is available in the KFT status information (from /proc/kft).
logentries <num-entries>
specify the maximum number entries for the log for this run
autorepeat
Repeat trace indefinitely. That is, on trace trigger stop, prime the trace to run again, but leave the data in the buffer. The trace will start again when the start trigger is matched, and stop again when the stop trigger is matched. The trace will stop autorepeating when the buffer becomes full.
# Other options that may be supported in the future:
# overwrite
# Overwrite old data in the trace buffer. This converts the trace buffer to
# a circular buffer, and does not stop the trace when the buffer becomes full.
# In overwrite mode, the end of the trace is available if the buffer is
# not large enough to hold the entire trace. In NOT overwrite mode (regular
# mode) the beginning of the trace is available if the buffer is not large
# enough to hold the entire trace.
# untimed
# Do not time function duration. Normally, the log contains only function
# entry events, with the start time and duration of the function. In
# untimed mode, the log contains entry AND exit events, with the start
# time for each event. Calculation of function duration must be done by
# a log post-processing tool.
# prime
# Immediately prime the trace for execution. "Priming" a trace means making
# it ready to run. A trace loaded without the "prime" command will not be
# enabled until the user issues a separate "prime" command through the
# /proc interface.
# prime entry ??
# primt exit ??
# prime time ??
Here are some configuration samples:
Record all functions longer that 500 microseconds, during bootup. Don't include functions executed inside interrupts.
new
begin
trigger start entry start_kernel
trigger stop exit to_userspace
filter mintime 500
filter maxtime 0
filter noints
end
Record all functions longer that 500 microseconds, for 5 seconds after the next fork don't worry about interrupts
Assuming 'do_fork' is at address 0xc001d804
new
begin
trigger start entry 0xc001d804
trigger stop time 5000000
filter mintime 500
filter maxtime 0
filter noints
end
new
begin
trigger start entry do_fork
trigger stop exit do_fork
filter mintime 10
filter maxtime 400
filter noints
logentries 500
end
new
begin
trigger start time 5000000
trigger stop time 5000
filter onlyints
end
kftstatic.conf version:
begin
trigger start time 10000000
filter funclist schedule fend
end
/proc/kft version, assuming schedule is at c02cb754
new
begin
trigger start time 10000000
filter funclist 0xc02cb754 fend
end
Here is an excerpt from a KFI log trace (processed with addr2sym). It shows all functions which lasted longer than 500 microseconds, from when the kernel entered start_kernel() to when it entered to_userspace().
Kernel Instrumentation Run ID 0
Logging started at 6785045 usec by entry to function start_kernel
Logging stopped at 8423650 usec by entry to function to_userspace
Filters:
500 usecs minimum execution time
Filter Counters:
Execution time filter count = 896348
Total entries filtered = 896348
Entries not found = 24
Number of entries after filters = 1757
Entry Delta PID Function Called At
-------- -------- ----- ------------------------- -------------------------
1 0 0 start_kernel L6+0x0
14 8687 0 setup_arch start_kernel+0x35
39 891 0 setup_memory setup_arch+0x2a8
53 872 0 register_bootmem_low_pages setup_memory+0x8f
54 871 0 free_bootmem register_bootmem_low_pages+0x95
54 871 0 free_bootmem_core free_bootmem+0x34
930 7432 0 paging_init setup_arch+0x2af
935 7427 0 zone_sizes_init paging_init+0x4e
935 7427 0 free_area_init zone_sizes_init+0x83
935 7427 0 free_area_init_node free_area_init+0x4b
935 3759 0 __alloc_bootmem_node free_area_init_node+0xc5
935 3759 0 __alloc_bootmem_core __alloc_bootmem_node+0x43
4694 3668 0 free_area_init_core free_area_init_node+0x75
4817 3535 0 memmap_init_zone free_area_init_core+0x2bd
8807 266911 0 time_init start_kernel+0xb6
8807 261404 0 get_cmos_time time_init+0x1c
270211 5507 0 select_timer time_init+0x41
270211 5507 0 init_tsc select_timer+0x45
270211 5507 0 calibrate_tsc init_tsc+0x6c
275718 1638 0 console_init start_kernel+0xbb
275718 1638 0 con_init console_init+0x59
275954 733 0 vgacon_save_screen con_init+0x288
277376 6730 0 mem_init start_kernel+0xf8
277376 1691 0 free_all_bootmem mem_init+0x52
277376 1691 0 free_all_bootmem_core free_all_bootmem+0x24
284118 25027 0 calibrate_delay start_kernel+0x10f
293860 770 0 __delay calibrate_delay+0x62
293860 770 0 delay_tsc __delay+0x26
294951 1534 0 __delay calibrate_delay+0x62
294951 1534 0 delay_tsc __delay+0x26
297134 1149 0 __delay calibrate_delay+0xbe
297134 1149 0 delay_tsc __delay+0x26
.
.
.
1638605 0 145 filemap_nopage do_no_page+0xef
1638605 0 145 __lock_page filemap_nopage+0x286
1638605 0 145 io_schedule __lock_page+0x95
1638605 0 145 schedule io_schedule+0x24
1638605 0 5 schedule worker_thread+0x217
1638605 0 1 to_userspace init+0xa6
The log is attached here: Media:Kfiboot-9.lst A Delta value of 0 usually means the exit from the routine was not seen.
Below is a kd dump of the data from the above log.
For the purpose of finding areas of big time in the kernel, the
functions with high "Local" time are important. For example,
delay_tsc() is called 156 times, resulting in 619 milliseconds of
duration. Other time-consuming routines were: isapnp_isolate(),
get_cmos_time(), default_idle().
The top line showing schedule() called 192 times and lasting over 5 seconds, is accounted wrong due to the switch in execution control inside the schedule routine. (The count of 192 calls is correct, but the duration is wrong.)
$ ~/work/kft/kft/kd -n 30 kftboot-9.lst
Function Count Time Average Local
------------------------- ----- -------- -------- --------
schedule 192 5173790 26946 5173790
do_basic_setup 1 1159270 1159270 14
do_initcalls 1 1159256 1159256 627
__delay 156 619322 3970 0
delay_tsc 156 619322 3970 619322
__const_udelay 146 608427 4167 0
probe_hwif 8 553972 69246 126
do_probe 31 553025 17839 68
ide_delay_50ms 103 552588 5364 0
isapnp_init 1 383138 383138 18
isapnp_isolate 1 383120 383120 311629
ide_init 1 339778 339778 22
probe_for_hwifs 1 339756 339756 103
ide_scan_pcibus 1 339653 339653 13
init_setup_piix 2 339640 169820 0
ide_scan_pcidev 2 339640 169820 0
piix_init_one 2 339640 169820 0
ide_setup_pci_device 2 339640 169820 242
probe_hwif_init 4 339398 84849 40
time_init 1 266911 266911 0
get_cmos_time 1 261404 261404 261404
ide_generic_init 1 214614 214614 0
ideprobe_init 1 214614 214614 0
wait_for_completion 6 194573 32428 0
default_idle 183 192589 1052 192589
io_schedule 18 171313 9517 0
__wait_on_buffer 14 150369 10740 141
i8042_init 1 137210 137210 295
i8042_port_register 2 135318 67659 301
__serio_register_port 2 135017 67508 0
Below is a kd -c trace of the data from a log taken from a PPC440g
platform, from a (dynamic) trace of the function do_fork().
Here is the configuration file that was used:
new
begin
trigger start entry do_fork
trigger stop exit do_fork
end
Here is the first part of the trace in nested call format: Times (Entry, Duration and Local) are in micro-seconds. Note the timer interrupt during the routine.
Entry Duration Local Pid Trace
---------- ---------- ---------- ------- ---------------------------------
4 20428 209 33 do_fork
7 6 6 33 | alloc_pidmap
18 2643 84 33 | copy_process
21 114 19 33 | | dup_task_struct
24 8 6 33 | | | prepare_to_copy
27 2 2 33 | | | | sub_preempt_count
35 22 9 33 | | | kmem_cache_alloc
38 2 2 33 | | | | __might_sleep
43 11 9 33 | | | | cache_alloc_refill
49 2 2 33 | | | | | sub_preempt_count
60 65 6 33 | | | __get_free_pages
63 59 14 33 | | | | __alloc_pages
65 3 3 33 | | | | | __might_sleep
71 3 3 33 | | | | | zone_watermark_ok
77 37 17 33 | | | | | buffered_rmqueue
80 4 4 33 | | | | | | __rmqueue
86 3 3 33 | | | | | | sub_preempt_count
92 3 3 33 | | | | | | bad_range
98 2 2 33 | | | | | | __mod_page_state
103 8 5 33 | | | | | | prep_new_page
106 3 3 33 | | | | | | | set_page_refs
117 2 2 33 | | | | | zone_statistics
141 25 4 33 | | do_posix_clock_monotonic_gettime
143 21 6 33 | | | do_posix_clock_monotonic_get
146 15 6 33 | | | | do_posix_clock_monotonic_gettime_parts
149 9 6 33 | | | | | getnstimeofday
152 3 3 33 | | | | | | do_gettimeofday
169 3 3 33 | | copy_semundo
174 41 17 33 | | copy_files
177 19 9 33 | | | kmem_cache_alloc
180 2 2 33 | | | | __might_sleep
185 8 5 33 | | | | cache_alloc_refill
188 3 3 33 | | | | | sub_preempt_count
200 3 3 33 | | | count_open_files
209 2 2 33 | | | sub_preempt_count
218 19 8 33 | | kmem_cache_alloc
220 2 2 33 | | | __might_sleep
225 9 6 33 | | | cache_alloc_refill
229 3 3 33 | | | | sub_preempt_count
241 2 2 33 | | sub_preempt_count
246 216 9 33 | | kmem_cache_alloc
249 199 199 33 | | | __might_sleep
----------- !!!! start --------------
253 151 63 33 timer_interrupt
256 8 6 -1 ! profile_tick
259 2 2 -1 ! ! profile_hit
267 61 15 -1 ! update_process_times
270 8 5 -1 ! ! account_system_time
273 3 3 -1 ! ! ! update_mem_hiwater
281 8 5 -1 ! ! run_local_timers
284 3 3 -1 ! ! ! raise_softirq
293 27 16 -1 ! ! scheduler_tick
.
.
.
* configure CONFIG_KFI_SAVE_SP (if saving the stack pointer as part of trace data)
*
KFT uses the "-finstrument-functions" capability of the gcc compiler to add instrumentation callouts to every kernel function entry and exit. This generates a large amount of overhead during kernel execution, even if a trace is not active. For this reason, KFT is turned off in the default configuration for your target board.
This high overhead means that using KFT may interfere with time-sensitive operations on your device. You should be careful when interpreting performance results on you device when KFT is configured on in your kernel, whether the results are obtained from KFT or from some other performance measurement tool. KFT is great at providing data for relative performance comparisons, but not for absolute performance timings.
Performance: KFT adds a fair amount of overhead to kernel execution. The reason for this is that the compiler adds instrumentation hooks to the start and end of every function. These hooks take additional time to execute. When a trace is active, even more time is used as events are compared against triggers and filters, and as events are logged to the trace buffer. It would be inappropriate to use an instrumented kernel for production use.
Local-time: Be careful when using the 'local time' numbers provided by 'kd'. These are calculated using the entry and exit times for the functions, and then subtracting the duration of other functions called during the top function's lifetime. However, due to filtering, interrupt handling, or context-switching, these numbers can be way off.
Mitsubishi measured the overhead of KFI (the predecessor to KFT) The period is from start_kernel() to smp_init().
Platform was: SH7751R 240MHz (Memory Clock 80MHz)
With KFI : 922.419 msec
Without KFI : 666.982 msec
Overhead : 27.69%
Because every function in the kernel is traced, with certain trace configuration settings it is possible to VERY rapidly fill up the trace buffer. Kernel functions are executed several thousand times a second, even when the machine appears to be doing nothing.
The trace buffer is not circular. As soon as the buffer fills up with data, the trace capture automatically stops. For this reason, it is common to have the trace buffer exhausted during a trace.
How to fix:
* increase the trace buffer size
* use filters
* filter only by certain functions
* increase the minimum function duration to save in the trace
Q. Is there a way to adjust the trigger or filters to reduce the memory usage?
A. The memory usage is determined by the size of the log, which is
specified by logentries in the KFT configuration. If logentries is
not specified, it defaults to a rather large number (20,000 in the
current code). To use a smaller trace log, specify a smaller number of
logentries in the KFT configuration.
The use of triggers and filters can help you fit more data (or more pertinent data) into the log, so you can more readily see the information you are interested in.
By setting start and stop triggers with a narrower "range" of operation, then the amount of data put into the log will be more limited. For example, the default configuration for a static trace uses
trigger start entry start_kernel
trigger stop entry to_userspace
This will trace EVERYTHING that the kernel does between those two routines. However, you can limit tracing to a much smaller time area of kernel initialization using better triggers. Here is an example showing a triggers for just watching mem_init():
trigger start entry mem_init
trigger stop exit mem_init
Filters are also vital to reduce the number of entries the trace log.
With no time filters in place, KFT will log every single function
executed by the kernel. This will quickly overrun the log (no matter
what size you have reserved with logentries.
When using KFT to find long-duration functions in the kernel, we usually are not interested in routines that execute quickly, and instead use something like "filter mintime 500" to filter out routines taking less than 500 microseconds.
On many platforms, the clock used for performing trace timings is not available immediately when the kernel begins execution. Often, the clock is initialized sometime during the time_init() function of kernel startup. In this case, the function entry times and durations may be incorrect, for functions which begin before the clock is set up. Also time-related filters will not operate correctly on these functions. Usually, this is not a problem, since the times come back as zero, and any minimum time filters in place will remove the events from the trace buffer.
The result of all this is that, on machines where the clock is not immediately available at kernel start, there will be a "blind spot" during initialization, which is effectively not traceable by KFT. You can get event data for this "blind" period, by turning off the time filter for events, but this will result in a very large set of events (all without valid timing information) at the beginning of the event log.
By default, KFT uses sched_clock() as the clock source for event timings. This is called from the routine kft_readclock().
sched_clock() is new in the 2.6 kernel, and returns a 64-bit value containing nanoseconds (not necessarily relative to any particular time base, but assumed to be monotonically increasing, and relatively frequency-stable.)
If your platform has good support for sched_clock(), then KFT should work for you unmodified. If not, you may wish to do one of two things:
A "good" sched_clock() routine will provide at least microsecond
resolution on return values. Some architectures have sched_clock()
returning values based on the jiffy variable, which on many embedded
platforms only has resolution to 10 milliseconds.
There are some sample custom kft_readclock() routines in the current code for different architectures. These alternate routines are not active, via pre-processor conditionals. However, you can use them for samples of how to write your own custom KFT clock routine.
| Date | Version | Description | |
|---|---|---|---|
| 2006-10-13 | 0.1.0 | First draft of document | |
| 2007-04-26 | 0.1.1 | Fixed "run run" typo, and added some material on filters and triggers |