Re: [PATCH v3 2/2] perf/sdt: Directly record SDT events with 'perf record'

[Brendan Gregg <brendan.d.gregg@gmail.com>]

On Tue, Feb 28, 2017 at 2:31 PM, Brendan Gregg
<brendan.d.gregg@gmail.com> wrote:
> G'Day Ravi,
>
[...]
> Now retrying perf:
>
> # ./perf record -e sdt_node:http__server__request -a
> ^C[ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.446 MB perf.data (3 samples) ]
> # ./perf script
>             node  7646 [002]   361.012364:
> sdt_node:http__server__request: (dc2e69)
>             node  7646 [002]   361.204718:
> sdt_node:http__server__request: (dc2e69)
>             node  7646 [002]   361.363043:
> sdt_node:http__server__request: (dc2e69)
>
> Now perf works.
>
> If I restart the node process, it goes back to the broken state.
>

Oh sorry, I forgot about that these Node.js probes are behind an
is-enabled semaphore.

$ readelf -n `which node`
[...]
  stapsdt              0x00000089    NT_STAPSDT (SystemTap probe descriptors)
    Provider: node
    Name: http__server__request
    Location: 0x0000000000dc2e69, Base: 0x000000000112e064, Semaphore:
0x0000000001470954
    Arguments: 8@%r14 8@%rax 8@-4344(%rbp) -4@-4348(%rbp)
8@-4304(%rbp) 8@-4312(%rbp) -4@-4352(%rbp)
# dd if=/proc/31695/mem bs=1 count=1 skip=$(( 0x0000000001470954 ))
2>/dev/null | xxd
00000000: 00                                       .
 # printf "\x1" | dd of=/proc/31695/mem bs=1 count=1 seek=$((
0x0000000001470954 )) 2>/dev/null
# dd if=/proc/31695/mem bs=1 count=1 skip=$(( 0x0000000001470954 ))
2>/dev/null | xxd
00000000: 01                                       .
# ./perf record -e sdt_node:http__server__request -a
Matching event(s) from uprobe_events:
   sdt_node:http__server__request  0x9c2e69@/usr/local/bin/node
Use 'perf probe -d <event>' to delete event(s).
^C[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.280 MB perf.data (3 samples) ]
# ./perf script
            node 31695 [003] 24947.168761:
sdt_node:http__server__request: (dc2e69)
            node 31695 [003] 24947.476143:
sdt_node:http__server__request: (dc2e69)
            node 31695 [003] 24947.679090:
sdt_node:http__server__request: (dc2e69)

So setting that to 1 made the probe work from perf. I guess this is
not a problem with this patch set, but rather a feature request for
the next one: is-enabled SDT support.

Were probe arguments supposed to work? I don't notice them in the perf
script output.

PS, if it's helpful, here's the commands to build node with these SDT probes:

$ sudo apt-get install systemtap-sdt-dev       # adds "dtrace", used
by node build
$ wget https://nodejs.org/dist/v4.4.1/node-v4.4.1.tar.gz
$ tar xvf node-v4.4.1.tar.gz
$ cd node-v4.4.1
$ ./configure --with-dtrace
$ make -j 8

Brendan