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Table Of Contents
Contents
Introduction
Kernel Function Trace (KFT) is a kernel function tracing system, which uses the "-finstrument-functions" capability of the gcc compiler to add instrumentation callouts to every function entry and exit. The KFT system provides for capturing these callouts and generating a trace of events, with timing details. KFT is excellent at providing a good timing overview of kernel procedures, allowing you to see where time is spent in functions and sub-routines in the kernel.
The main mode of operation with KFT is to use the system with a dynamic trace configuration. That is, you can set a trace configuration after kernel startup, using the /proc/kft interface, and retrieve trace data immediately. However, another (special) mode of operation is available, called STATIC_RUN mode, where the configuration for a KFT run is configured and compiled statically into the kernel. This mode is useful for getting a trace of kernel operation during system bootup (before user space is running).
The KFT configuration lets you specify how to automatically start and stop a trace, whether to include interrupts as part of the trace, and whether to filter the trace data by various criteria (for minimum function duration, only certain listed functions, etc.) KFT trace data is retrieved by reading from /proc/kft_data after the trace is complete.
Tools are supplied to convert numeric trace data to kernel symbols, and to process and analyze the data in a KFT trace.
Basic Use
Documentation for KFT is available (as of 2.6.12) in Documentation/kft.txt, after applying the kft-all-in-one.patch.
Also, you can see this online guide: UsingKernelFunctionTrace
Here's a presentation about KFT usage:
"Learning the Kernel and Finding Performance Problems with KFI"
omap-serial_init.trace.txt - Sample trace used with presentation
KFT used to be called KFI (for Kernel Function Instrumentation). For prior releases of KFT, see KernelFunctionInstrumentation
Download
Patches
Patch for CELF kernel (based on linux-2.4.20): kfi-24-test4.patch
Patch for Linux 2.6.7 (for x86 only): kfi-26-test1.patch
Patch for Linux 2.6.8.1: see the PatchArchive page
Patch for Linux 2.6.11: see the PatchArchive page (or just download kfi-2.patch)
Patch for Linux 2.6.12: see the PatchArchive page (available as an all-in-one patch or a tar archive of broken-out patches)
KFT utilities
KFT includes several helper scripts which are located in the kernel scripts directory:
- addr2sym - convert function addresses to symbols in the trace data
- kd - KFT dump - does filtering, sorting, analysis and trace formatting of KFT trace logs
- mkkftrun.pl - used during building the kernel to convert a configuration file into a C file to be compiled into the kernel
- sym2addr - convert function names to addresses in a KFT configuration file (for a dynamic trace)
See Documentation/kft.txt, in the kernel source tree after applying the patch, for instructions on using these programs.
How To Use
- download both the patch
- apply the patch in the kernel top-level directory:
patch -p1 <kft.patch
- read the rest of the instructions in the Documentation/kft.txt file. (my apologies for being lazy!)
see the online documentation at: UsingKernelFunctionTrace
Adding platform support for the kft clock source
The current patch (from Sep 2005), uses sched_clock() as the clock source for 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:
- improve support for sched_clock() in your board port, or
- write a custom kft_readclock() routine.
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 patch for different architectures.
Issues
Here is a list of things that need more work:
- may need to add noinstrument attributes for some time-critical code (need to check this)
- maybe can check "Function Trace in KDB" patch for help with this
Overhead
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%
Similar technologies
There are other technologies for doing call traces or kernel profiling that are similar to KFT. Some of these are mentioned on the KernelInstrumentation page.
One that is very similar is a kernel trace mechanism for use with KDB. A patch was posted to LKML in January of 2002. See the message: http://www.uwsg.iu.edu/hypermail/linux/kernel/0201.3/0888.html
Filter Q&A
Tim asked the question:
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.
Sample results
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().
kft log output (excerpt)
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+0xa6The log is attached here: kfiboot-9.lst A Delta value of 0 usually means the exit from the routine was not seen.
kft log analysis with 'kd'
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
kft nested call trace with 'kd -c'
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: