Chapter 4. Useful SystemTap Scripts
This chapter enumerates several SystemTap scripts you can use to monitor and investigate different subsystems. All of these scripts are available at /usr/share/systemtap/testsuite/systemtap.examples/ once you install the systemtap-testsuite RPM.
The following sections showcase scripts that trace network-related functions and build a profile of network activity.
This section describes how to profile network activity. nettop.stp provides a glimpse into how much network traffic each process is generating on a machine.
#! /usr/bin/env stap
global ifxmit, ifrecv
probe netdev.transmit
{
ifxmit[pid(), dev_name, execname(), uid()] <<< length
}
probe netdev.receive
{
ifrecv[pid(), dev_name, execname(), uid()] <<< length
}
function print_activity()
{
printf("%5s %5s %-7s %7s %7s %7s %7s %-15s\n",
"PID", "UID", "DEV", "XMIT_PK", "RECV_PK",
"XMIT_KB", "RECV_KB", "COMMAND")
foreach ([pid, dev, exec, uid] in ifrecv-) {
n_xmit = @count(ifxmit[pid, dev, exec, uid])
n_recv = @count(ifrecv[pid, dev, exec, uid])
printf("%5d %5d %-7s %7d %7d %7d %7d %-15s\n",
pid, uid, dev, n_xmit, n_recv,
n_xmit ? @sum(ifxmit[pid, dev, exec, uid])/1024 : 0,
n_recv ? @sum(ifrecv[pid, dev, exec, uid])/1024 : 0,
exec)
}
print("\n")
delete ifxmit
delete ifrecv
}
probe timer.ms(5000), end, error
{
print_activity()
}
Note that function print_activity() uses the following expressions:
n_xmit ? @sum(ifxmit[pid, dev, exec, uid])/1024 : 0 n_recv ? @sum(ifrecv[pid, dev, exec, uid])/1024 : 0
These expressions are if/else conditionals. The first statement is simply a more concise way of writing the following psuedo code:
if n_recv != 0 then @sum(ifrecv[pid, dev, exec, uid])/1024 else 0
nettop.stp tracks which processes are generating network traffic on the system, and provides the following information about each process:
PID — the ID of the listed process.
UID — user ID. A user ID of 0 refers to the root user.
DEV — which ethernet device the process used to send / receive data (e.g. eth0, eth1)
XMIT_PK — number of packets transmitted by the process
RECV_PK — number of packets received by the process
XMIT_KB — amount of data sent by the process, in kilobytes
RECV_KB — amount of data received by the service, in kilobytes
nettop.stp provides network profile sampling every 5 seconds. You can change this setting by editing probe timer.ms(5000) accordingly. Example 4.1, “nettop.stp Sample Output” contains an excerpt of the output from nettop.stp over a 20-second period:
[...]
PID UID DEV XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND
0 0 eth0 0 5 0 0 swapper
11178 0 eth0 2 0 0 0 synergyc
PID UID DEV XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND
2886 4 eth0 79 0 5 0 cups-polld
11362 0 eth0 0 61 0 5 firefox
0 0 eth0 3 32 0 3 swapper
2886 4 lo 4 4 0 0 cups-polld
11178 0 eth0 3 0 0 0 synergyc
PID UID DEV XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND
0 0 eth0 0 6 0 0 swapper
2886 4 lo 2 2 0 0 cups-polld
11178 0 eth0 3 0 0 0 synergyc
3611 0 eth0 0 1 0 0 Xorg
PID UID DEV XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND
0 0 eth0 3 42 0 2 swapper
11178 0 eth0 43 1 3 0 synergyc
11362 0 eth0 0 7 0 0 firefox
3897 0 eth0 0 1 0 0 multiload-apple
[...]
This section describes how to trace functions called from the kernel's net/socket.c file. This task helps you identify, in finer detail, how each process interacts with the network at the kernel level.
#! /usr/bin/env stap
probe kernel.function("*@net/socket.c").call {
printf ("%s -> %s\n", thread_indent(1), probefunc())
}
probe kernel.function("*@net/socket.c").return {
printf ("%s <- %s\n", thread_indent(-1), probefunc())
}
socket-trace.stp is identical to Example 3.6, “thread_indent.stp”, which was earlier used in SystemTap Functions to illustrate how thread_indent() works.
[...] 0 Xorg(3611): -> sock_poll 3 Xorg(3611): <- sock_poll 0 Xorg(3611): -> sock_poll 3 Xorg(3611): <- sock_poll 0 gnome-terminal(11106): -> sock_poll 5 gnome-terminal(11106): <- sock_poll 0 scim-bridge(3883): -> sock_poll 3 scim-bridge(3883): <- sock_poll 0 scim-bridge(3883): -> sys_socketcall 4 scim-bridge(3883): -> sys_recv 8 scim-bridge(3883): -> sys_recvfrom 12 scim-bridge(3883):-> sock_from_file 16 scim-bridge(3883):<- sock_from_file 20 scim-bridge(3883):-> sock_recvmsg 24 scim-bridge(3883):<- sock_recvmsg 28 scim-bridge(3883): <- sys_recvfrom 31 scim-bridge(3883): <- sys_recv 35 scim-bridge(3883): <- sys_socketcall [...]
Example 4.2, “socket-trace.stp Sample Output” contains a 3-second excerpt of the output for socket-trace.stp. For more information about the output of this script as provided by thread_indent(), refer to SystemTap Functions Example 3.6, “thread_indent.stp”.