Re: Can't set statement probe in self-built linux-4.4

[fche at redhat dot com (Frank Ch. Eigler)]

Hi -

chris wrote:

> [...]
> I can't seem to set statement probes in a self-built linux-4.4. What can
> be going wrong? [...]

It sounds like you're setting up everything all right, good job.


> I can set a kernel function probe, e.g.:
>
> $ stap -p2 -e 'probe kernel.function("register_shrinker") { printf("foo\n"); }' 
> [...]
> But I can't set a kernel statement probe using either a function offset or
> explicit line number [...]
> $ stap -v -p2 -e 'probe kernel.statement("register_shinker@mm/vmscan.c+3") { printf("foo\n"); }'
> [...]

Have you tried:

    $ stap -L 'kernel.statement("register_shrinker@*:*")'

to get the list of line numbers that systemtap recognizes?


> The full -vvvvvv output from above is available at:
> https://www.dropbox.com/s/dfepzebixd7yb36/stap.out.gz?dl=0

Will look at that shortly.


> ----------------------------------------------------------------------
> $ readelf --debug-dump=line $SYSTEMTAP_RELEASE/vmlinux | head -45
> Raw dump of debug contents of section .debug_line:
> [...]
>   [0x0000006e]  Copy
>   [0x0000006f]  Special opcode 81: advance Address by 5 to 0xffffffff81000005 and Line by 6 to 75
>   [0x00000070]  Special opcode 104: advance Address by 7 to 0xffffffff8100000c and Line by 1 to 76
>   [0x00000071]  Special opcode 106: advance Address by 7 to 0xffffffff81000013 and Line by 3 to 79
>   [0x00000072]  Special opcode 48: advance Address by 3 to 0xffffffff81000016 and Line by 1 to 80
>   [0x00000073]  Special opcode 76: advance Address by 5 to 0xffffffff8100001b and Line by 1 to 81
>   [0x00000074]  Special opcode 34: advance Address by 2 to 0xffffffff8100001d and Line by 1 to 82
>   [0x00000075]  Special opcode 94: advance Address by 6 to 0xffffffff81000023 and Line by 5 to 87
> ----------------------------------------------------------------------

This too looks OK.  Keep in mind though that systemtap does filter out
line numbers that are not uniquely mappable to a single PC address.
Highly optimized code can smear statements from a single line to
multiple locations, and stap doesn't want to guess wrong from ambiguous
alternatives.

- FChE