Welcome!

Open Source Cloud Authors: Zakia Bouachraoui, Elizabeth White, Liz McMillan, Yeshim Deniz, 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

Signals
Process core dumps are initiated by the process receiving a signal. Signals are similar to hardware interrupts. As with interrupts, a signal causes a task to branch from its normal execution, handling a routine and returning to the point of interruption. Normal executing threads encounter signals throughout their life cycles. However, there are a finite number of signal types that result in a core dump, whereas other signal types result in process termination. A process can receive a signal from three sources: the user, the process, or the kernel.

From the User
A user can send a signal in two ways: either using an external command such as kill or within a controlling tty, typing Ctrl+c to send a sigint as defined by stty -a. (Note that by definition, daemons do not have a controlling tty and therefore cannot be signaled in this manner.)

# stty -a
speed 9600 baud; rows 41; columns 110; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
eol2 = <undef>; start = ^Q; stop = ^S;

From the Program
From a program, you can perform the raise() or alarm() system call, allowing a program to signal itself. Consider this example: a ten-second sleep without using the sleep call.

main()
{
alarm(10);
pause()
}

From the Kernel
The kernel can send a signal, such as SIGSEGV, to a process when it attempts an illegal action, such as accessing memory that it does not own or that is outside of its address range.

Linux supports two types of signals: standard and real-time. A complete overview of signals is outside the scope of this chapter; however, there are a few key differences to note. Standard signals have predefined meanings, whereas real-time signals are defined by the programmer. Additionally, only one standard signal of each type can be queued per process, whereas real-time signals can build up. An example of this was shown earlier in this chapter when a process was blocked on I/O. A kill -9 sigkill was sent to the process and placed in SigPnd.

SigPnd: 0000000000000100 <- A signal waiting to be processed, in this case sigkill

In troubleshooting a process, a user might want to force a process to dump core. As stated, this is accomplished by sending the appropriate signal to the process. Sometimes after this step is taken, the dump does not follow because the process has not returned from an interrupt due to some other issue. The result is a pending signal that needs to be processed. Because the signals that result in a core are standard signals, sending the same signal multiple times does not work because subsequent signals are ignored until the pending signal has been processed. The pending signals are processed after the program returns from the interrupt but before proceeding to user space. This fact is illustrated in the entry.S source file, as shown in the following:

arch/i386/kernel/entry.S
...
ret_from_intr()
...
_reschedule()
...
_signal_return()
...
     jsr do_signal ; arch/cris/kernel/signal.c
...

It is also possible to have difficulty achieving the dump because signals are being blocked (masked), caught, or ignored. An application might have signal handlers that catch the signal and perform their own actions. Signal blocking prevents the delivery of the signal to the process. Ignoring a signal just means that the process throws it away upon delivery. Additionally, the signal structure of a process is like any other structure in that the child inherits the parent's configuration. That being stated, if a signal is blocked for the parent, the child of that process has the same signals blocked or masked. However, some signals cannot be masked or ignored, as detailed in the man page on signal. Two such signals are sigkill and sigstop.

The user can obtain a list of signals from the kill command. This yields a list of signals that the user can send to a process. Possible signals include the following (note that this is not a complete list):

$ kill -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL
5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE
9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2
13) SIGPIPE 14) SIGALRM 15) SIGTERM 17) SIGCHLD
...

As mentioned earlier and illustrated next, the man page on signal details the signals that produce a core file.
$ man 7 signal

...
Signal Value Action Comment
-------------------------------------------------------------------------
SIGHUP 1 Term Hangup detected on controlling terminal
or death of controlling process
SIGINT 2 Term Interrupt from keyboard
SIGQUIT 3 Core Quit from keyboard
SIGILL 4 Core Illegal Instruction
SIGABRT 6 Core Abort signal from abort(3)
SIGFPE 8 Core Floating point exception
...
The source code on signal also provides this list as illustrated next:
linux/kernel/signal.c
...
#define SIG_KERNEL_COREDUMP_MASK (\
M(SIGQUIT) | M(SIGILL) | M(SIGTRAP) | M(SIGABRT) | \
M(SIGFPE) | M(SIGSEGV) | M(SIGBUS) | M(SIGSYS) | \
M(SIGXCPU) | M(SIGXFSZ) | M_SIGEMT )
...


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.

IoT & Smart Cities Stories
The challenges of aggregating data from consumer-oriented devices, such as wearable technologies and smart thermostats, are fairly well-understood. However, there are a new set of challenges for IoT devices that generate megabytes or gigabytes of data per second. Certainly, the infrastructure will have to change, as those volumes of data will likely overwhelm the available bandwidth for aggregating the data into a central repository. Ochandarena discusses a whole new way to think about your next...
CloudEXPO | DevOpsSUMMIT | DXWorldEXPO are the world's most influential, independent events where Cloud Computing was coined and where technology buyers and vendors meet to experience and discuss the big picture of Digital Transformation and all of the strategies, tactics, and tools they need to realize their goals. Sponsors of DXWorldEXPO | CloudEXPO benefit from unmatched branding, profile building and lead generation opportunities.
DXWorldEXPO LLC announced today that Big Data Federation to Exhibit at the 22nd International CloudEXPO, colocated with DevOpsSUMMIT and DXWorldEXPO, November 12-13, 2018 in New York City. Big Data Federation, Inc. develops and applies artificial intelligence to predict financial and economic events that matter. The company uncovers patterns and precise drivers of performance and outcomes with the aid of machine-learning algorithms, big data, and fundamental analysis. Their products are deployed...
All in Mobile is a place where we continually maximize their impact by fostering understanding, empathy, insights, creativity and joy. They believe that a truly useful and desirable mobile app doesn't need the brightest idea or the most advanced technology. A great product begins with understanding people. It's easy to think that customers will love your app, but can you justify it? They make sure your final app is something that users truly want and need. The only way to do this is by ...
Digital Transformation and Disruption, Amazon Style - What You Can Learn. Chris Kocher is a co-founder of Grey Heron, a management and strategic marketing consulting firm. He has 25+ years in both strategic and hands-on operating experience helping executives and investors build revenues and shareholder value. He has consulted with over 130 companies on innovating with new business models, product strategies and monetization. Chris has held management positions at HP and Symantec in addition to ...
Dynatrace is an application performance management software company with products for the information technology departments and digital business owners of medium and large businesses. Building the Future of Monitoring with Artificial Intelligence. Today we can collect lots and lots of performance data. We build beautiful dashboards and even have fancy query languages to access and transform the data. Still performance data is a secret language only a couple of people understand. The more busine...
Cell networks have the advantage of long-range communications, reaching an estimated 90% of the world. But cell networks such as 2G, 3G and LTE consume lots of power and were designed for connecting people. They are not optimized for low- or battery-powered devices or for IoT applications with infrequently transmitted data. Cell IoT modules that support narrow-band IoT and 4G cell networks will enable cell connectivity, device management, and app enablement for low-power wide-area network IoT. B...
The hierarchical architecture that distributes "compute" within the network specially at the edge can enable new services by harnessing emerging technologies. But Edge-Compute comes at increased cost that needs to be managed and potentially augmented by creative architecture solutions as there will always a catching-up with the capacity demands. Processing power in smartphones has enhanced YoY and there is increasingly spare compute capacity that can be potentially pooled. Uber has successfully ...
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...
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, its about the data consumed and generated. The devices are tools, mechanisms, conduits. This paper discusses the considerations when dealing with the...