libbpf-tools/drsnoop_example.txt

Demonstrations of drsnoop, the Linux BPF CO-RE version.


drsnoop traces the direct reclaim system-wide, and prints various details.
Example output:

# drsnoop

Tracing direct reclaim events... Hit Ctrl-C to end.
TIME     COMM             TID     LAT(ms) PAGES
14:56:43 in:imklog        268     106.637    39
14:56:43 systemd-udevd    232     110.708    53
14:56:43 systemd-journal  19531   106.083    62
^C

While tracing, the processes alloc pages due to insufficient memory available
in the system, direct reclaim events happened, which will increase the waiting
delay of the processes.

drsnoop can be useful for discovering when allocstall(/proc/vmstat) continues
to increase, whether it is caused by some critical processes or not.

The -p option can be used to filter on a PID, which is filtered in-kernel.

# drsnoop -p 17491

Tracing direct reclaim events... Hit Ctrl-C to end.
TIME     COMM             TID     LAT(ms) PAGES
14:59:56 summond          17491      0.24    50
14:59:56 summond          17491      0.26    38
14:59:56 summond          17491      0.36    72
^C

This shows the summond process allocs pages, and direct reclaim events happening,
and the delays are not affected much.

A maximum tracing duration can be set with the -d option. For example, to trace
for 2 seconds:

# drsnoop -d 2

Tracing direct reclaim events for 2 secs.
TIME     COMM             TID     LAT(ms) PAGES
15:02:16 head             21715      0.15   195

USAGE message:

# drsnoop --help

Usage: drsnoop [OPTION...]
Trace direct reclaim latency.

USAGE: drsnoop [--help] [-p PID] [-t TID] [-d DURATION] [-e]

EXAMPLES:
    drsnoop         # trace all direct reclaim events
    drsnoop -p 123  # trace pid 123
    drsnoop -t 123  # trace tid 123 (use for threads only)
    drsnoop -d 10   # trace for 10 seconds only
    drsnoop -e      # trace all direct reclaim events with extended faileds

  -d, --duration=DURATION    Total duration of trace in seconds
  -e, --extended             Extended fields output
  -p, --pid=PID              Process PID to trace
  -t, --tid=TID              Thread TID to trace
  -v, --verbose              Verbose debug output
  -?, --help                 Give this help list
      --usage                Give a short usage message
  -V, --version              Print program version

Report bugs to <bpf@vger.kernel.org>.