Demonstrations of kprobe, the Linux ftrace version.


This traces the kernel do_sys_open() function, when it is called:

# ./kprobe p:do_sys_open
Tracing kprobe do_sys_open. Ctrl-C to end.
          kprobe-26042 [001] d... 6910441.001452: do_sys_open: (do_sys_open+0x0/0x220)
          kprobe-26042 [001] d... 6910441.001475: do_sys_open: (do_sys_open+0x0/0x220)
          kprobe-26042 [001] d... 6910441.001866: do_sys_open: (do_sys_open+0x0/0x220)
          kprobe-26042 [001] d... 6910441.001966: do_sys_open: (do_sys_open+0x0/0x220)
       supervise-1689  [000] d... 6910441.083302: do_sys_open: (do_sys_open+0x0/0x220)
       supervise-1693  [001] d... 6910441.083530: do_sys_open: (do_sys_open+0x0/0x220)
       supervise-1689  [000] d... 6910441.083759: do_sys_open: (do_sys_open+0x0/0x220)
       supervise-1693  [001] d... 6910441.083877: do_sys_open: (do_sys_open+0x0/0x220)
[...]

The "p:" is for creating a probe. Use "r:" to probe the return of the function:

# ./kprobe r:do_sys_open
Tracing kprobe do_sys_open. Ctrl-C to end.
          kprobe-29475 [001] d... 6910688.229777: do_sys_open: (SyS_open+0x1e/0x20 <- do_sys_open)
           <...>-29476 [001] d... 6910688.231101: do_sys_open: (SyS_open+0x1e/0x20 <- do_sys_open)
           <...>-29476 [001] d... 6910688.231123: do_sys_open: (SyS_open+0x1e/0x20 <- do_sys_open)
           <...>-29476 [001] d... 6910688.231530: do_sys_open: (SyS_open+0x1e/0x20 <- do_sys_open)
           <...>-29476 [001] d... 6910688.231624: do_sys_open: (SyS_open+0x1e/0x20 <- do_sys_open)
       supervise-1685  [001] d... 6910688.328776: do_sys_open: (SyS_open+0x1e/0x20 <- do_sys_open)
       supervise-1689  [000] d... 6910688.328780: do_sys_open: (SyS_open+0x1e/0x20 <- do_sys_open)
[...]

This output includes the function that the traced function is returning to.


The trace output can be a little different between kernel versions. Use -H to
print the header:

# ./kprobe -H p:do_sys_open
Tracing kprobe do_sys_open. Ctrl-C to end.
# tracer: nop
#
# entries-in-buffer/entries-written: 4/4   #P:2
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
          kprobe-27952 [001] d... 6910580.008086: do_sys_open: (do_sys_open+0x0/0x220)
          kprobe-27952 [001] d... 6910580.008109: do_sys_open: (do_sys_open+0x0/0x220)
          kprobe-27952 [001] d... 6910580.008483: do_sys_open: (do_sys_open+0x0/0x220)
[...]

These columns are explained in the kernel source under Documentation/trace/ftrace.txt.


This traces do_sys_open() returns, using a probe alias "myopen", and showing
the return value ($retval):

# ./kprobe 'r:myopen do_sys_open $retval'
Tracing kprobe myopen. Ctrl-C to end.
          kprobe-26386 [001] d... 6593278.858754: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x3
           <...>-26387 [001] d... 6593278.860043: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x3
           <...>-26387 [001] d... 6593278.860064: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x3
           <...>-26387 [001] d... 6593278.860433: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x3
           <...>-26387 [001] d... 6593278.860521: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x3
       supervise-1685  [001] d... 6593279.178806: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1689  [001] d... 6593279.228756: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1689  [001] d... 6593279.229106: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1688  [000] d... 6593279.229501: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1695  [000] d... 6593279.229944: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1685  [001] d... 6593279.230104: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1687  [001] d... 6593279.230293: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1699  [000] d... 6593279.230381: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1692  [000] d... 6593279.230825: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1698  [000] d... 6593279.230915: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1698  [000] d... 6593279.231277: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
       supervise-1690  [000] d... 6593279.231703: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) arg1=0x9
^C
Ending tracing...

The string specified, 'r:myopen do_sys_open $retval', is a kprobe definition,
and is the same as those documented in the Linux kernel source under
Documentation/trace/kprobetrace.txt, which can be written to the
/sys/kernel/debug/tracing/kprobe_events file.

Apart from probe name aliases, you can also provide arbitrary names for
arguments. Eg, instead of the "arg1" default, calling it "rval":

# ./kprobe 'r:myopen do_sys_open rval=$retval'
Tracing kprobe myopen. Ctrl-C to end.
          kprobe-27454 [001] d... 6593356.250019: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) rval=0x3
           <...>-27455 [001] d... 6593356.251280: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) rval=0x3
           <...>-27455 [001] d... 6593356.251301: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) rval=0x3
           <...>-27455 [001] d... 6593356.251672: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) rval=0x3
           <...>-27455 [001] d... 6593356.251769: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) rval=0x3
       supervise-1689  [000] d... 6593356.859758: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) rval=0x9
       supervise-1689  [000] d... 6593356.860143: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) rval=0x9
       supervise-1696  [000] d... 6593356.862682: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) rval=0x9
       supervise-1685  [001] d... 6593356.862684: myopen: (SyS_open+0x1e/0x20 <- do_sys_open) rval=0x9
[...]

That's a bit better.


Tracing the open() mode:

# ./kprobe 'p:myopen do_sys_open mode=%cx:u16'
Tracing kprobe myopen. Ctrl-C to end.
          kprobe-29572 [001] d... 6593503.353923: myopen: (do_sys_open+0x0/0x220) mode=0x1
          kprobe-29572 [001] d... 6593503.353945: myopen: (do_sys_open+0x0/0x220) mode=0x0
          kprobe-29572 [001] d... 6593503.354307: myopen: (do_sys_open+0x0/0x220) mode=0x5c00
          kprobe-29572 [001] d... 6593503.354401: myopen: (do_sys_open+0x0/0x220) mode=0x0
       supervise-1689  [000] d... 6593503.944125: myopen: (do_sys_open+0x0/0x220) mode=0x1a4
       supervise-1688  [001] d... 6593503.944125: myopen: (do_sys_open+0x0/0x220) mode=0x1a4
       supervise-1688  [001] d... 6593503.944606: myopen: (do_sys_open+0x0/0x220) mode=0x1a4
       supervise-1689  [000] d... 6593503.944606: myopen: (do_sys_open+0x0/0x220) mode=0x1a4
       supervise-1698  [000] d... 6593503.944728: myopen: (do_sys_open+0x0/0x220) mode=0x1a4
       supervise-1698  [000] d... 6593503.945077: myopen: (do_sys_open+0x0/0x220) mode=0x1a4
[...]

Here I guessed that the mode was in register %cx, and cast it as a 16-bit
unsigned integer (":u16"). Your platform and kernel may be different, and the
mode may be in a different register. If fiddling with such registers becomes too
painful or unreliable for you, consider installing kernel debuginfo and using
the named variables with perf_events "perf probe".


Tracing the open() filename:

# ./kprobe 'p:myopen do_sys_open filename=+0(%si):string'
Tracing kprobe myopen. Ctrl-C to end.
          kprobe-32369 [001] d... 6593706.999728: myopen: (do_sys_open+0x0/0x220) filename="/etc/ld.so.cache"
          kprobe-32369 [001] d... 6593706.999748: myopen: (do_sys_open+0x0/0x220) filename="/lib/x86_64-linux-gnu/libc.so.6"
          kprobe-32369 [001] d... 6593707.000092: myopen: (do_sys_open+0x0/0x220) filename="/usr/lib/locale/locale-archive"
          kprobe-32369 [001] d... 6593707.000176: myopen: (do_sys_open+0x0/0x220) filename="trace_pipe"
       supervise-1699  [000] d... 6593707.254970: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
       supervise-1689  [001] d... 6593707.254970: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
       supervise-1689  [001] d... 6593707.255432: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
       supervise-1699  [000] d... 6593707.255432: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
       supervise-1695  [001] d... 6593707.258805: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
[...]

As mentioned previously, the %si register may be different on your platform.
In this example, I cast it as a string.


Specifying a duration will buffer in-kernel (reducing overhead), and write at
the end. Here's tracing for 10 seconds, and writing to the "out" file:

# ./kprobe -d 10 'p:myopen do_sys_open filename=+0(%si):string' > out


You can match on a single PID only:

# ./kprobe -p 1696 'p:myopen do_sys_open filename=+0(%si):string' 
Tracing kprobe myopen. Ctrl-C to end.
       supervise-1696  [001] d... 6593773.677033: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
       supervise-1696  [001] d... 6593773.677332: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
       supervise-1696  [001] d... 6593774.697144: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
       supervise-1696  [001] d... 6593774.697675: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
       supervise-1696  [001] d... 6593775.717986: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
       supervise-1696  [001] d... 6593775.718499: myopen: (do_sys_open+0x0/0x220) filename="supervise/status.new"
^C
Ending tracing...

This will only show events when that PID is on-CPU.


The -v option will show you the available variables you can use in custom
filters:

# ./kprobe -v 'p:myopen do_sys_open filename=+0(%si):string' 
name: myopen
ID: 1443
format:
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
	field:int common_pid;	offset:4;	size:4;	signed:1;

	field:unsigned long __probe_ip;	offset:8;	size:8;	signed:0;
	field:__data_loc char[] filename;	offset:16;	size:4;	signed:1;

print fmt: "(%lx) filename=\"%s\"", REC->__probe_ip, __get_str(filename)


Tracing filenames that end in "stat", by adding a filter:

# ./kprobe 'p:myopen do_sys_open filename=+0(%si):string' 'filename ~ "*stat"'
Tracing kprobe myopen. Ctrl-C to end.
        postgres-1172  [000] d... 6594028.787166: myopen: (do_sys_open+0x0/0x220) filename="pg_stat_tmp/pgstat.stat"
        postgres-1172  [001] d... 6594028.797410: myopen: (do_sys_open+0x0/0x220) filename="pg_stat_tmp/pgstat.stat"
        postgres-1172  [001] d... 6594028.797467: myopen: (do_sys_open+0x0/0x220) filename="pg_stat_tmp/pgstat.stat"
        postgres-4443  [001] d... 6594028.800908: myopen: (do_sys_open+0x0/0x220) filename="pg_stat_tmp/pgstat.stat"
        postgres-4443  [000] d... 6594028.811237: myopen: (do_sys_open+0x0/0x220) filename="pg_stat_tmp/pgstat.stat"
        postgres-4443  [000] d... 6594028.811290: myopen: (do_sys_open+0x0/0x220) filename="pg_stat_tmp/pgstat.stat"
^C
Ending tracing...

This filtering is done in-kernel context.


As an example of tracing a deeper kernel function, lets trace bio_alloc() and
entry registers:

# ./kprobe 'p:myprobe bio_alloc %ax %bx %cx %dx'
Tracing kprobe myprobe. Ctrl-C to end.
       supervise-3055  [000] 2172148.728250: myprobe: (bio_alloc+0x0/0x30) arg1=ffff880064acc8d0 arg2=ffff8800e56a7990 arg3=0 arg4=ffff880064acc910
       supervise-3055  [000] 2172148.728527: myprobe: (bio_alloc+0x0/0x30) arg1=ffff880064acf948 arg2=ffff8800e56a7990 arg3=0 arg4=ffff880064acf988
    jbd2/xvda1-8-212   [000] 2172149.749474: myprobe: (bio_alloc+0x0/0x30) arg1=ffffffff arg2=ffff8800ad1f87b8 arg3=ffff8800ba22c06c arg4=8
    jbd2/xvda1-8-212   [000] 2172149.749485: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff880089d053a8 arg3=10f16c5bb arg4=0
    jbd2/xvda1-8-212   [000] 2172149.749487: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff880089d05958 arg3=5 arg4=0
    jbd2/xvda1-8-212   [000] 2172149.749488: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff880089d05b60 arg3=5 arg4=0
    jbd2/xvda1-8-212   [000] 2172149.749489: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff880089d05820 arg3=5 arg4=0
    jbd2/xvda1-8-212   [000] 2172149.749489: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff880089d055b0 arg3=5 arg4=0
    jbd2/xvda1-8-212   [000] 2172149.749490: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff88006ff22ea0 arg3=5 arg4=0
    jbd2/xvda1-8-212   [000] 2172149.749491: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff880089d1f000 arg3=5 arg4=0
    jbd2/xvda1-8-212   [000] 2172149.749492: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff880089d1f138 arg3=5 arg4=0
    jbd2/xvda1-8-212   [000] 2172149.749493: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff88005d267138 arg3=5 arg4=0
    jbd2/xvda1-8-212   [000] 2172149.749494: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff88005d267680 arg3=5 arg4=0
    jbd2/xvda1-8-212   [000] 2172149.749495: myprobe: (bio_alloc+0x0/0x30) arg1=0 arg2=ffff88005d2675b0 arg3=5 arg4=0
    jbd2/xvda1-8-212   [000] 2172149.751044: myprobe: (bio_alloc+0x0/0x30) arg1=ffffffff arg2=ffff8800cc241ea0 arg3=445f0300 arg4=ffff8800effba000
       supervise-3055  [000] 2172149.751095: myprobe: (bio_alloc+0x0/0x30) arg1=ffff880064acf948 arg2=ffff8800e56a7990 arg3=0 arg4=ffff880064acf988
       supervise-3055  [000] 2172149.751341: myprobe: (bio_alloc+0x0/0x30) arg1=ffff880064acc8d0 arg2=ffff8800e56a7990 arg3=0 arg4=ffff880064acc910
       supervise-3055  [000] 2172150.772033: myprobe: (bio_alloc+0x0/0x30) arg1=ffff880064acc8d0 arg2=ffff8800e56a7990 arg3=0 arg4=ffff880064acc910
       supervise-3055  [000] 2172150.772305: myprobe: (bio_alloc+0x0/0x30) arg1=ffff880064acf948 arg2=ffff8800e56a7990 arg3=0 arg4=ffff880064acf988
     flush-202:1-409   [000] 2172151.087815: myprobe: (bio_alloc+0x0/0x30) arg1=ffffffff arg2=ffff8800da51d6e8 arg3=16afd arg4=1
     flush-202:1-409   [000] 2172151.087829: myprobe: (bio_alloc+0x0/0x30) arg1=ffffffff arg2=ffff8800e7537f08 arg3=16afd arg4=2
     flush-202:1-409   [000] 2172151.087844: myprobe: (bio_alloc+0x0/0x30) arg1=ffffffff arg2=ffff8800e7519af8 arg3=16afd arg4=3
     flush-202:1-409   [000] 2172151.087846: myprobe: (bio_alloc+0x0/0x30) arg1=ffffffff arg2=ffff8800e7511478 arg3=16afd arg4=4
     flush-202:1-409   [000] 2172151.087849: myprobe: (bio_alloc+0x0/0x30) arg1=ffffffff arg2=ffff8800e75e6a90 arg3=16afd arg4=5
     flush-202:1-409   [000] 2172151.087851: myprobe: (bio_alloc+0x0/0x30) arg1=ffffffff arg2=ffff8800e7512bc8 arg3=16afd arg4=6
     flush-202:1-409   [000] 2172151.087853: myprobe: (bio_alloc+0x0/0x30) arg1=ffffffff arg2=ffff8800eb3bf410 arg3=16afd arg4=7
^C

The output includes who is on-CPU, high resolution timestamps, and the arguments
we requested (registers %ax to %dx). These registers are platform dependent,
and are mapped by the compiler to the entry arguments of the function.

How are these useful? If you are debugging this kernel function, you'll know. :)


Note that you can add qualifiers, eg, if I knew %ax was a uint32:

# ./kprobe 'p:myprobe bio_alloc %ax:u32'
Tracing kprobe myprobe. Ctrl-C to end.
       supervise-3055  [000] 2172389.734606: myprobe: (bio_alloc+0x0/0x30) arg1=64acf948
       supervise-3055  [000] 2172389.734865: myprobe: (bio_alloc+0x0/0x30) arg1=64acc8d0
       supervise-3055  [000] 2172390.772391: myprobe: (bio_alloc+0x0/0x30) arg1=64acf948
       supervise-3055  [000] 2172390.772676: myprobe: (bio_alloc+0x0/0x30) arg1=64acc8d0
^C
Ending tracing...

You can give them aliases too, instead of the default arg1..N:

# ./kprobe 'p:myprobe bio_alloc ax=%ax'
Tracing kprobe myprobe. Ctrl-C to end.
       supervise-3055  [000] 2172420.451663: myprobe: (bio_alloc+0x0/0x30) ax=ffff880064acc8d0
       supervise-3055  [000] 2172420.451938: myprobe: (bio_alloc+0x0/0x30) ax=ffff880064acf948
     flush-202:1-409   [000] 2172421.163462: myprobe: (bio_alloc+0x0/0x30) ax=ffff880064acc8d0
       supervise-3055  [000] 2172421.500994: myprobe: (bio_alloc+0x0/0x30) ax=ffff880064acc8d0
       supervise-3055  [000] 2172421.501307: myprobe: (bio_alloc+0x0/0x30) ax=ffff880064acf948
^C
Ending tracing...


Now for the return of bio_alloc():

# ./kprobe 'r:myprobe bio_alloc $retval'
Tracing kprobe myprobe. Ctrl-C to end.
       supervise-3055  [000] 2172164.145533: myprobe: (io_submit_init.isra.6+0x74/0x100 <- bio_alloc) arg1=ffff8800e55843c0
       supervise-3055  [000] 2172164.145829: myprobe: (io_submit_init.isra.6+0x74/0x100 <- bio_alloc) arg1=ffff8800e5584840
    jbd2/xvda1-8-212   [000] 2172165.166453: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e57596c0
    jbd2/xvda1-8-212   [000] 2172165.166493: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759c00
    jbd2/xvda1-8-212   [000] 2172165.166496: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759600
    jbd2/xvda1-8-212   [000] 2172165.166497: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759e40
    jbd2/xvda1-8-212   [000] 2172165.166498: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e57590c0
    jbd2/xvda1-8-212   [000] 2172165.166500: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e57599c0
    jbd2/xvda1-8-212   [000] 2172165.166500: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759a80
    jbd2/xvda1-8-212   [000] 2172165.166502: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759f00
    jbd2/xvda1-8-212   [000] 2172165.166503: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759540
    jbd2/xvda1-8-212   [000] 2172165.166504: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759180
    jbd2/xvda1-8-212   [000] 2172165.166504: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759900
    jbd2/xvda1-8-212   [000] 2172165.166505: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759000
    jbd2/xvda1-8-212   [000] 2172165.166506: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759480
           <...>-212   [000] 2172165.176261: myprobe: (submit_bh+0x76/0x120 <- bio_alloc) arg1=ffff8800e5759480
       supervise-3055  [000] 2172165.176317: myprobe: (io_submit_init.isra.6+0x74/0x100 <- bio_alloc) arg1=ffff8800e57596c0
       supervise-3055  [000] 2172165.176586: myprobe: (io_submit_init.isra.6+0x74/0x100 <- bio_alloc) arg1=ffff8800e5759900
^C
Ending tracing...

Great. This output includes the function we are returning to, in most cases,
submit_bh().

Note that this mode (without a duration) prints events as they happen,
so the overheads can be high for frequent events. You could try the -d mode,
which buffers in-kernel.


The -s option will print the kernel stack trace after the event:

# ./kprobe -s 'p:mytcp tcp_init_cwnd'
Tracing kprobe mytcp. Ctrl-C to end.
            sshd-5121  [000] d... 6897275.911301: mytcp: (tcp_init_cwnd+0x0/0x40)
            sshd-5121  [000] d... 6897275.911309: <stack trace>
 => tcp_write_xmit
 => __tcp_push_pending_frames
 => tcp_push
 => tcp_sendmsg
 => inet_sendmsg
 => sock_aio_write
 => do_sync_write
 => vfs_write
 => SyS_write
 => system_call_fastpath
            sshd-32219 [000] d... 6897275.911467: mytcp: (tcp_init_cwnd+0x0/0x40)
            sshd-32219 [000] d... 6897275.911471: <stack trace>
 => tcp_write_xmit
 => __tcp_push_pending_frames
 => tcp_push
 => tcp_sendmsg
 => inet_sendmsg
 => sock_aio_write
 => do_sync_write
 => vfs_write
 => SyS_write
 => system_call_fastpath
            sshd-5121  [000] d... 6897277.878794: mytcp: (tcp_init_cwnd+0x0/0x40)
            sshd-5121  [000] d... 6897277.878801: <stack trace>
 => tcp_write_xmit
 => __tcp_push_pending_frames
 => tcp_push
 => tcp_sendmsg
 => inet_sendmsg
 => sock_aio_write
 => do_sync_write
 => vfs_write
 => SyS_write
 => system_call_fastpath

This makes use of the kernel options/stacktrace feature.


Use -h to print the USAGE message:

# ./kprobe -h
USAGE: kprobe [-FhHsv] [-d secs] [-p PID] kprobe_definition [filter]
                 -F              # force. trace despite warnings.
                 -d seconds      # trace duration, and use buffers
                 -p PID          # PID to match on I/O issue
                 -v              # view format file (don't trace)
                 -H              # include column headers
                 -s              # show kernel stack traces
                 -h              # this usage message

Note that these examples may need modification to match your kernel
version's function names and platform's register usage.
   eg,
       kprobe p:do_sys_open
                                 # trace open() entry
       kprobe r:do_sys_open
                                 # trace open() return
       kprobe 'r:do_sys_open $retval'
                                 # trace open() return value
       kprobe 'r:myopen do_sys_open $retval'
                                 # use a custom probe name
       kprobe 'p:myopen do_sys_open mode=%cx:u16'
                                 # trace open() file mode
       kprobe 'p:myopen do_sys_open filename=+0(%si):string'
                                 # trace open() with filename
       kprobe -s 'p:myprobe tcp_retransmit_skb'
                                 # show kernel stacks

See the man page and example file for more info.
