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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:

KFT used to be called KFI (for Kernel Function Instrumentation). For prior releases of KFT, see KernelFunctionInstrumentation



KFT utilities

KFT includes several helper scripts which are located in the kernel scripts directory:

See Documentation/kft.txt, in the kernel source tree after applying the patch, for instructions on using these programs.

How To Use

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:

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.


Here is a list of things that need more work:


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:

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

        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: 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:

KernelFunctionTrace (last edited 2008-06-12 21:04:51 by 160)