Welcome!

Open Source Cloud Authors: Vaibhaw Pandey, Stackify Blog, John Walsh, Liz McMillan, Pat Romanski

Related Topics: Open Source Cloud, Linux Containers

Open Source Cloud: Article

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.

Comments (1) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Most Recent Comments
Linux News Desk 07/13/06 04:59:25 PM EDT

Troubleshooting a Linux process follows the same general methodology as that used with traditional UNIX systems. In both systems, for process hangs, we identify the system resources being used by the process and attempt to identify the cause for the process to stop responding. With application core dumps, we must identify the signal for which the process terminated and proceed with acquiring a stack trace to identify system calls made by the process at the time it died. There exists neither a 'golden' troubleshooting path nor a set of instructions that can be applied for all cases. Some conditions are much easier to solve than others, but with a good understanding of the fundamentals, a solution is not far from reach.

@ThingsExpo Stories
Large industrial manufacturing organizations are adopting the agile principles of cloud software companies. The industrial manufacturing development process has not scaled over time. Now that design CAD teams are geographically distributed, centralizing their work is key. With large multi-gigabyte projects, outdated tools have stifled industrial team agility, time-to-market milestones, and impacted P&L stakeholders.
"Cloud Academy is an enterprise training platform for the cloud, specifically public clouds. We offer guided learning experiences on AWS, Azure, Google Cloud and all the surrounding methodologies and technologies that you need to know and your teams need to know in order to leverage the full benefits of the cloud," explained Alex Brower, VP of Marketing at Cloud Academy, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clar...
Gemini is Yahoo’s native and search advertising platform. To ensure the quality of a complex distributed system that spans multiple products and components and across various desktop websites and mobile app and web experiences – both Yahoo owned and operated and third-party syndication (supply), with complex interaction with more than a billion users and numerous advertisers globally (demand) – it becomes imperative to automate a set of end-to-end tests 24x7 to detect bugs and regression. In th...
"Akvelon is a software development company and we also provide consultancy services to folks who are looking to scale or accelerate their engineering roadmaps," explained Jeremiah Mothersell, Marketing Manager at Akvelon, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
"MobiDev is a software development company and we do complex, custom software development for everybody from entrepreneurs to large enterprises," explained Alan Winters, U.S. Head of Business Development at MobiDev, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that CrowdReviews.com has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5–7, 2018, at the Javits Center in New York City, NY. CrowdReviews.com is a transparent online platform for determining which products and services are the best based on the opinion of the crowd. The crowd consists of Internet users that have experienced products and services first-hand and have an interest in letting other potential buye...
"IBM is really all in on blockchain. We take a look at sort of the history of blockchain ledger technologies. It started out with bitcoin, Ethereum, and IBM evaluated these particular blockchain technologies and found they were anonymous and permissionless and that many companies were looking for permissioned blockchain," stated René Bostic, Technical VP of the IBM Cloud Unit in North America, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Conventi...
SYS-CON Events announced today that Telecom Reseller has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5-7, 2018, at the Javits Center in New York, NY. Telecom Reseller reports on Unified Communications, UCaaS, BPaaS for enterprise and SMBs. They report extensively on both customer premises based solutions such as IP-PBX as well as cloud based and hosted platforms.
"Space Monkey by Vivent Smart Home is a product that is a distributed cloud-based edge storage network. Vivent Smart Home, our parent company, is a smart home provider that places a lot of hard drives across homes in North America," explained JT Olds, Director of Engineering, and Brandon Crowfeather, Product Manager, at Vivint Smart Home, in this SYS-CON.tv interview at @ThingsExpo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
Coca-Cola’s Google powered digital signage system lays the groundwork for a more valuable connection between Coke and its customers. Digital signs pair software with high-resolution displays so that a message can be changed instantly based on what the operator wants to communicate or sell. In their Day 3 Keynote at 21st Cloud Expo, Greg Chambers, Global Group Director, Digital Innovation, Coca-Cola, and Vidya Nagarajan, a Senior Product Manager at Google, discussed how from store operations and ...
In his session at 21st Cloud Expo, Carl J. Levine, Senior Technical Evangelist for NS1, will objectively discuss how DNS is used to solve Digital Transformation challenges in large SaaS applications, CDNs, AdTech platforms, and other demanding use cases. Carl J. Levine is the Senior Technical Evangelist for NS1. A veteran of the Internet Infrastructure space, he has over a decade of experience with startups, networking protocols and Internet infrastructure, combined with the unique ability to it...
It is of utmost importance for the future success of WebRTC to ensure that interoperability is operational between web browsers and any WebRTC-compliant client. To be guaranteed as operational and effective, interoperability must be tested extensively by establishing WebRTC data and media connections between different web browsers running on different devices and operating systems. In his session at WebRTC Summit at @ThingsExpo, Dr. Alex Gouaillard, CEO and Founder of CoSMo Software, presented ...
"There's plenty of bandwidth out there but it's never in the right place. So what Cedexis does is uses data to work out the best pathways to get data from the origin to the person who wants to get it," explained Simon Jones, Evangelist and Head of Marketing at Cedexis, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
WebRTC is great technology to build your own communication tools. It will be even more exciting experience it with advanced devices, such as a 360 Camera, 360 microphone, and a depth sensor camera. In his session at @ThingsExpo, Masashi Ganeko, a manager at INFOCOM Corporation, introduced two experimental projects from his team and what they learned from them. "Shotoku Tamago" uses the robot audition software HARK to track speakers in 360 video of a remote party. "Virtual Teleport" uses a multip...
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, whic...
SYS-CON Events announced today that Evatronix will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Evatronix SA offers comprehensive solutions in the design and implementation of electronic systems, in CAD / CAM deployment, and also is a designer and manufacturer of advanced 3D scanners for professional applications.
Leading companies, from the Global Fortune 500 to the smallest companies, are adopting hybrid cloud as the path to business advantage. Hybrid cloud depends on cloud services and on-premises infrastructure working in unison. Successful implementations require new levels of data mobility, enabled by an automated and seamless flow across on-premises and cloud resources. In his general session at 21st Cloud Expo, Greg Tevis, an IBM Storage Software Technical Strategist and Customer Solution Architec...
To get the most out of their data, successful companies are not focusing on queries and data lakes, they are actively integrating analytics into their operations with a data-first application development approach. Real-time adjustments to improve revenues, reduce costs, or mitigate risk rely on applications that minimize latency on a variety of data sources. In his session at @BigDataExpo, Jack Norris, Senior Vice President, Data and Applications at MapR Technologies, reviewed best practices to ...
An increasing number of companies are creating products that combine data with analytical capabilities. Running interactive queries on Big Data requires complex architectures to store and query data effectively, typically involving data streams, an choosing efficient file format/database and multiple independent systems that are tied together through custom-engineered pipelines. In his session at @BigDataExpo at @ThingsExpo, Tomer Levi, a senior software engineer at Intel’s Advanced Analytics gr...
When talking IoT we often focus on the devices, the sensors, the hardware itself. The new smart appliances, the new smart or self-driving cars (which are amalgamations of many ‘things’). When we are looking at the world of IoT, we should take a step back, look at the big picture. What value are these devices providing? IoT is not about the devices, it’s about the data consumed and generated. The devices are tools, mechanisms, conduits. In his session at Internet of Things at Cloud Expo | DXWor...