Kernel Function Instrumentation

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Alert.gif - NOTE: KFI has been renamed to KFT (Kernel Function Trace) starting with the patches for kernel version 2.6.12. If you are using a recent kernel, version 2.6.12 or later, please see the Kernel Function Trace page instead of this one.


Kernel Function Instrumentation (KFI) 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 KFI system provides for capturing these callouts and generating a trace of events, with timing details. KFI 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 KFI is to use the system with a dynamic trace configuration. That is, you can set a trace configuration after kernel startup, using the /proc/kfi interface, and retrieve trace data immediately. However, another (special) mode of operation is available, called STATIC_RUN mode, where the configuration for a KFI 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 KFI 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.) KFI trace data is retrieved by reading from /proc/kfi_trace 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 KFI trace.

Basic Use

Documentation for KFI is available (as of 2.6.11) in Documentation/kfi.txt, after applying the kfi-2.patch.

Here's a presentation about KFI usage:

For prior releases of KFI, see KFI Docs



  • Patch for CELF kernel (based on linux-2.4.20): attachment:kfi-24-test4.patch
  • Patch for Linux 2.6.7 (for x86 only): attachment:kfi-26-test1.patch
  • Patch for Linux see the PatchArchive page
  • Patch for Linux 2.6.11: see the PatchArchive page (or just download attachment:kfi-2.patch)

KFI utilities

  • User-space programs: attachment:kfi-0.8.tar.gz (for KFI version 1)

For KFI version 2 and above, the only user-space programs are scripts, which are now located in the kernel scripts directory:

  • addr2sym - script to convert function addresses to symbols in the trace data
  • kd - kfi dump - does filtering, sorting, and analysis of KFI trace logs

See Documentation/kfi.txt for instructions on using these programs.

How To Use

  • download both the patch
  • apply the patch in the kernel top-level directory:
  • patch -p1 <kfi-2.patch
  • read the rest of the instructions in the Documentation/kfi.txt file. (my apologies for being lazy!)

Adding platform support for the kfi clock source

The current patch (from Sep 2004), uses sched_clock() as the clock source for kfi_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 KFI 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 kfi_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 kfi_readclock() routines in the current patch (one for x86 using the TSC, and one for PPC using the TBU.


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
  • would like a tutorial on the configuration language for defining a tracing run
  • documentation needs lots of work
  • should especially document how to do a dynamic trace


Mitsubishi measured the overhead of KFI. 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 KFI. 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. Todd Poynor from MontaVista answered: kfistatic.conf

    trigger start entry start_kernel
    trigger stop entry to_userspace
    filter mintime 1
    filter maxtime 0
 #   filter noints

The above filters out only those routines that take less than 1 microsecond. We usually are not interested in routines that execute so quickly, and instead use something like "filter mintime 500" to filter out routines taking less than 500 microseconds.

I didn't track down the original log file being discussed, but if the "quiet" command line parameter wasn't used then even a 500us filter may include a lot of calls for serial console printks.

The filters don't affect memory usage so far as I understand. You can set the amount of memory used for a static run by specifying "logsize <n>", where <n> is the number of entries, in kfistatic.conf.

Q. Is it possible to specifically omit certain routines with a filter.

A. I believe there's a filter for including only certain routines (rather than excluding certain routines). It shouldn't normally be necessary, but if there's a routine that matches the time filtering suggested above and is called so often as to be a problem, then adding attribute "__noinstrument" to the function definition and recompiling will exclude it; see drivers/char/kfi.c for an example.

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().

kfi 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+0xa6

The log is attached here: attachment:kfiboot-9.lst

A Delta value of 0 usually means the exit from the routine was not seen.

kfi 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/kfi/kfi/kd -n 30 kfiboot-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