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Linux Processes: Structure, Hangs and Core Dumps

Efficient and effective resolution practices

If an offending process is consuming system resources at an extraordinary rate and starving production applications, killing the offending process is justified if the process can be killed. However, sometimes a process cannot be killed. When a process has exhausted its timeslice, it is put to sleep() with a given priority. When the priority of the process falls below PZERO, it is in an uninterruptible state and cannot be signaled; however, signals can be queued, and for some operations, this is normal. For others, where the program has hung and never returns, the cause is usually located in the driver or hardware. If the process has a state of D (blocked on I/O), it is uninterruptible and cannot be killed. For example, a process accessing a file over a failed hard NFS mount would be in a state of D while attempting to stat() a file or directory.

Uninterruptible processes usually take place when entering I/O calls, at which point the process has called into the kernel, which is in driver code, during which the process cannot receive signals from user space. In this state, a command cannot be signaled even by a SIGKILL (kill -9). It is important to note that signals are queued if not ignored by a sigmask and executed after the code returns from kernel space. Some signals cannot be masked; see the signal man page for more details.
Here is an excerpt from the signal man page:

...
Using a signal handler function for a signal is called "catching the signal".
The signals SIGKILL and SIGSTOP cannot be caught or ignored.
...

A zombie process is another process that a user cannot kill. These processes, however, should not be consuming any CPU cycles or memory resources other than the overhead of having the task structure in the kernel's Virtual Address Space (VAS). The main goal of troubleshooting a zombie process is determining why the parent died without reaping its children. In short, you should focus on why and how the parent dies.
Listed next are process state codes pulled right out of the source code.

./linux/fs/proc/array.c
/*
* The task state array is a strange "bitmap" of
* reasons to sleep. Thus "running" is zero, and
* you can test for combinations of others with
* simple bit tests.
*/
static const char *task_state_array[] = {
      "R (running)", /* 0 */
      "S (sleeping)", /* 1 */
      "D (disk sleep)", /* 2 */
      "Z (zombie)", /* 4 */
      "T (stopped)", /* 8 */
      "W (paging)" /* 16 */
};
...

In Scenario 1, we demonstrate an instance in which a process cannot be killed.

Scenario 1: Troubleshooting a Process That Does Not Respond to kill
A user begins rewinding a tape but realizes that the wrong tape is in the drive. The user tries to kill the job but must wait for the process to finish.

Why?

The mt command has made an ioctl call to the SCSI tape driver (st) and must wait for the driver to release the process back to user space so that use signals will be handled.

# mt -f /dev/st0 rewind
# ps -emo state,pid,ppid,pri,size,stime,time,comm,wchan | grep mt
D 9225 8916 24 112 20:46 00:00:00 mt wait_for_completion

[[email protected] root]# kill -9 9225
[[email protected] root]# echo $? # This produces the return code for the previous command. 0 = success
0
[[email protected] root]# ps -elf | grep 9225
0 D root 9225 8916 0 24 0 - 112 wait_f 20:46 pts/1 00:00:00 mt -f /dev/st0

The mt command has entered a wait channel, and after the code returns from the driver, the signal will be processed.

Let's check the pending signals:

cat ../9225/status
Name: mt
State: D (disk sleep)
Tgid: 9225
Pid: 9225
PPid: 8916
TracerPid: 0
Uid: 0 0 0 0
Gid: 0 0 0 0
FDSize: 256
Groups: 0 1 2 3 4 6 10
VmSize: 2800 kB
VmLck: 0 kB
VmRSS: 640 kB
VmData: 96 kB
VmStk: 16 kB
VmExe: 32 kB
VmLib: 2560 kB
SigPnd: 0000000000000100 <-- SigPnd is a bit mask which indicates the value of the pending signal. Each byte
accounts for 4 bits. In this case, the pending signal has a value of 9, so the first bit on the 3rd byte is set. This
algorithm is detailed in linux/fs/proc/array.c under the render_sigset_t() function. The following table
illustrates this function.

Signal : 1 2 3 4 . 5 6 7 8 . 9 10 11 12 . 13 14 15 16
bit value : 1 2 4 8 . 1 2 4 8 . 1 2 4 8 . 1 2 4 8

kill -3 yields bit mask 0000000000000004
kill -9 yields bit mask 0000000000000100

ShdPnd: 0000000000000100
SigBlk: 0000000000000000
SigIgn: 0000000000000000
SigCgt: 0000000000000000
CapInh: 0000000000000000
CapPrm: 00000000fffffeff
CapEff: 00000000fffffeff


More Stories By James Kirkland

James Kirkland is the advocate for Red Hat's initiatives and solutions for the Internet of Things(IoT) and is the architect of its three-tier strategy for IoT deployments. For the past five years, James has been focused on IoT solutions for the transportation and energy sectors. A frequent public speaker and writer on a wide range of technical topics, James is also the co-author of Linux Troubleshooting for System Administrators and Power Users (ISBN: 0131855158) published by Prentice Hall PTR. He has been working with UNIX and Linux variants over the course of 20 years in his positions at Red Hat, and in previous roles at Racemi and Hewlett-Packard.

More Stories By David Carmichael

David Carmichael works for Hewlett-Packard as a technical problem manager in Alpharetta, Georgia. He earned a bachelors degree in computer science from West Virginia University in 1987 and has been helping customers resolve their IT problems ever since. David has written articles for HP's IT Resource Center (http://itrc.hp.com) and presented at HP World 2003.

More Stories By Greg Tinker

Greg Tinker began his career while at Bellsouth in Atlanta, Georgia. Greg joined Hewlett-Packard in 1999. Greg's primary role is as a storage business recovery specialist and has participated in HP World, taught several classes in Unix/Linux and Disk Array technology, and obtained various certifications including certifications in Advanced Clusters, SAN, and Linux.

More Stories By Chris Tinker

Chris Tinker began his career in computers while working as a Unix System Administrator for Lockheed Martin in Marietta, Georgia. Chris joined Hewlett-Packard in 1999. Chris's primary role at HP is as a senior software business recovery specialist and has participated in HP World, taught several classes in Unix/Linux and Disk Array technology, and obtained various certifications including certifications in Advanced Clusters, SAN, and Linux.

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