Process Control DaemonFor Embedded Linux Platforms

Speaker: Hai Shalom

rt-embedded.com/pcd

Background review: What were the reasons that led to the development of PCD.

PCD project review: Features and high level overview of the project.

Live demonstration. Q & A.

Agenda

Does your product have a process controller? Does your product automatically recover after a

crash? Do you think your product’s boot time is fast

enough? Are you using methods other than printf to debug

a crashed application? Are you familiar with all the processes which are

running in your product and their dependencies?

Some questions

Most of you probably answered “No” to at least one question.

People who answered “Yes” to all questions are probably using PCD already!

Let’s review some facts about Embedded Linux based products…

What were your answers?

Done by scripts (rcS, rc.*). These are great, but might be: Not optimal for embedded / not deterministic:

Limited ways to synchronize depended processes (delay). Limited ways to verify successful start of a process No error checking (usually). No formal way to define dependencies. Difficult to start processes in parallel.

Not trivial to understand, maintain and extend: Require additional shell scripting expertise. Tend to be long and unreadable. Plenty of commented code, old remarks, different code styles.

System start up

Done by scripts (rcS, rc.*). These are great, but might be: Not optimal for embedded / not deterministic:

Limited ways to synchronize depended processes (delay). Limited ways to verify successful start of a process No error checking (usually). No formal way to define dependencies. Difficult to start processes in parallel.

Not trivial to understand, maintain and extend: Require additional shell scripting expertise. Tend to be long and unreadable. Plenty of commented code, old remarks, different code styles.

System start up

Looks familiar?

A crashed program just terminates, usually after printing “Segmentation Fault”. Now what? Where is the debug information? Kernel crashes are assumed to be handled by the system’s

watchdog. Signal Handlers not always implemented correctly.

Unsafe to use printf, and many other functions. The system remains unstable and unusable.

End user must power-cycle (again?).

Crash handling and recovery

A crashed program just terminates, usually after printing “Segmentation Fault”. Now what? Where is the debug information? Kernel crashes are assumed to be handled by the system’s

watchdog. Signal Handlers not always implemented correctly.

Unsafe to use printf, and many other functions. The system remains unstable and unusable.

End user must power-cycle (again?).

Crash handling and recovery

No central management entity. init is the parent of all processes.

Must know process’ pid in order to signal or kill. Each process must manage his own children.

Child process inherits his father’s priority. Parents must retrieve child’s

exit status, or else we end upwith Zombies…

Process management

A customer reports a crash in the field or in his lab tests: There is no standard method for generating and collecting

remote debug information. When a process abnormally terminates, all its

information goes away and no log is saved. You might be on the next flight to the customer’s lab.

Field/Remote debugging

What is PCD?

A great (and free) solution: PCD

PCD – Process Control Daemon, is an open source, light-weight system level process manager for Embedded-Linux based products (consumer electronics, network devices, etc).

The PCD provides a complementary service for any Embedded Linux driven product.

Designed and implemented by Hai Shalom during employment at Texas Instruments for Next-Gen Puma5 Cable chipset.

Released to open source as part of his M.Sc. Degree research. PCD is a proven solution that already drives millions of devices

in the world.

What is PCD?

System startup: PCD starts up the system in an efficient, synchronized and deterministic manner.

Process management: a centralized entity that controls and monitors all processes, and provides API to manage them.

System recovery: Configurable per process recovery action is taken in case of a crash.

Debug information: PCD provides a detailed crash log in case of a program error.

PCD Features in high-level

What are the advantages of products with PCD?

How does it work?

Rule blocks replace/extend traditional shell scripts. Each rule defines a single process. Rule inter-dependency is well defined.

PCD Scripts: Rule blocks

Process 1

Process 2

Process 3

Rule 1

Rule 2

Rule 3

PCDScriptFile

Very simple and readable syntax. Easy to extend and maintain. Each Rule block is based on the same template and

contains the following details: What is the process name and parameters? When to start it (depends on event…)? What is the required priority? What is the completion event? How much time to wait for it to complete? What to do in case of a crash?

PCD Scripts: Rule blocks

Very simple and readable syntax. Easy to extend and maintain. Each Rule block is based on the same template and

contains the following details: What is the process name and parameters? When to start it (depends on event…)? What is the required priority? What is the completion event? How much time to wait for it to complete? What to do in case of a crash?

PCD Scripts: Rule blocks

Once all rules are parsed, the PCD builds a dependency graph database.

PCD starts each rule in the “right” time.

PCD continuously monitors the system.

Event Driven System Startup

PCD Rule

Rule

RuleRuleRule

RuleRule

Rule

Rule

Rule

Last

Right time when a Start event occurred: Another rule or set of rules have completed successfully, or; A resource has been created (Network device, file).

Completion event when the attached process: Has exited with the correct status, or; Sent a “Process ready” event to the PCD, or; Created a resource, or; Was running for a specified amount of time, or; Was created.

A Completion event of one rule could be the Start event of another rule.

Event Driven System Startup

Dependencies between processes are well defined. Rules are started as soon as their start event comes. No need for non-deterministic delays between

starting processes. Rules without inter-dependency are started in

parallel. Improve user experience and product reputation

(Fast product!)

Reduced startup time

Enhanced stability and robustness

Crash

Rule PCD

SignalProcess

Restart

Reboot

Recover

Ignore Rule

Enhanced monitoring on processes and recovery in case of failure.

Each Rule defines what to do in case its process crashes: Restart the process: Usually for non-critical services such as

a web server, or processes that can recover by restarting themselves.

Reboot the system: In case of a fatal, non-recoverable error. Initiate a recovery rule. Ignore: Similar behavior without PCD.

Enhanced stability and robustness

Enhanced debugging capabilities

Crash

Rule PCD

PCDAPI

Signal

Prepare and send

exception info

Process

Detailed CrashLog

Log in NVRAM

The PCD exception handlers will catch and handle any fault exception (Signals).

The PCD will provide useful debug information. The information speeds up the error fixing cycle and

improves product robustness. Error logs are saved in non-volatile memory

Can be used for offline analysis after a validation cycle in the lab.

Can be used for post-mortem analysis of units from the field.

Enhanced debugging capabilities

Snapshot taken from an ARM platform.

Contains: Signal info Registers Map file

Registers pc and lr/ra can be used to trace the bug using addr2line or objdump.

Crash log with PCD

Process management

Rule 1

PCD

Request to restart Process 2

Process 1

Rule 4

New Configuration

Rule 2

Restart Process 2

Rule 3

Process 3 Process 4

User input: Disable

something

Process 2

Request to terminateProcess 4

TerminateProcess 4

Process 2

Process management with PCD API: Start or terminate a process. Send a “process ready” event. Signal a process. Register to exception handlers. Reboot the system (with logged a reason).

The PCD API is available by linking with the PCD library.

Process management

What is the order that the processes are started?

What is each process dependency? PCD can generate dependency graphs

for visual representation of all the rules and their dependencies.

Visibility provides an excellent means to examine and understand the dependencies between each rule in the system, and fix them in case of mistakes.

Dependency graph generation

PCD is architecture agnostic, except for the crash log code that displays register details.

Up to date, the following platforms are supported: ARM (primary development target). MIPS (secondary development target). x86 x64

For other platforms, the crash log will not include register details.

Last two architectures allow running a PCD driven system in any development PC running Linux.

Supported architectures

PCD is a light-weight process controller for embedded platforms.

Here are its modest memory requirements: PCD Code: 28KB PCD Data section: 4KB PCD Heap: 36KB (Typical). PCD Stack (Watermark): 84KB (Typical).

Memory Requirements

The PCD Project is an Open-Source project. The PCD project is licensed under the GNU Lesser

General Public License version 2.1, as published by the Free Software Foundation.

Its license allows linking proprietary software without any license contamination.

To view a copy of this license, visit http://www.gnu.org/licenses/lgpl-2.1.html#SEC1

Licensing

PCD improved the Puma5 products in various aspects: Startup time: The system boots much more quickly

comparing to scripts (15 seconds faster). Robustness, availability: Due to the recovery actions, the

system is more available and user experience is better. Quality: Detailed crash logs pointed out bugs, reduced

fix time, enabled remote and offline analysis. Customers found it very useful:

Added new rule blocks with their own modifications.

PCD contribution to product success

PCD Home page (Hai’s Real-Time Embedded blog): http://www.rt-embedded.com/pcd

Project management and source code at SourceForge: http://sourceforge.net/projects/pcd/

PCD Documentation and user guides (Yes! There is some): http://www.rt-embedded.com/blog/pcd-process-control-daemon/pcd-documentation/

PCD support forum: http://sourceforge.net/projects/pcd/support

New software engineers are welcomed to join the project and contribute.

PCD Resources

Questions and Answers

Hai, fixing a bug

Wrap Up

System startup: PCD starts up the system in an efficient, synchronized and deterministic manner.

Process management: a centralized entity that controls and monitors all processes, and provides API to manage them.

System recovery: Configurable per process recovery action is taken in case of a crash.

Debug information: PCD provides a detailed crash log in case of a program error.

PCD can make your product a better product!

PCD Features in high-level

Thank you!

Hai Shalom

Backup slides

PCD High level technical info

The PCD API provides an easy interface to request various services from the PCD:◦ Start or terminate a process.◦ Send a “process ready” event.◦ Signal a process.◦ Register to PCD default exception handlers.◦ Reboot the system (with logged a reason).

The PCD API is available by linking with the PCD library.

Standard API for PCD services

Every program can register to PCD’s exception handlers.

The PCD performs as a “crash daemon” which listens on a dedicated socket.

The exception handler collects debug information and sends it to the PCD using only “Safe functions”.

The PCD formats the data, displays it on the console and logs it in the non-volatile storage.

PCD Exception handler

The PCD design features various loosely coupled software modules:◦ Main: Performs the initializations and the main loop.◦ Rule Parser: Reads and parses the textual rules.◦ Rules DB: Stores all the rules as binary records.◦ Process: Starts, stops and monitors the processes◦ Timer: Provides the ticks for the pcd.◦ Condition check: Checks if a condition is satisfied.◦ Failure action: Performs failure/recovery actions.◦ Exception: Implements the detailed exception handlers.◦ API: The PCD API interface (As a separate library).

PCD Software modules

PCD Software modules block diagramPARSER

MAIN

RULESDB

Textual configuration file

with rules

Activate Rules

Parse Rules File

Add RuleRule Info

Activate /Stop

TIMER

FAILUREACTION

PROCESS CONTRL

CONDITIONCHECK

Activate failure action

Activate Rule

Tick

CheckCondition

OK / NOK Enqueue Process

EnqueueRule

Iterate

OK/Fail

OK/Fail

Process

Spawn / Signal /Monitor

Stopped / Signaled / Exited

PCD API

IPC

Check Messages

Enqueue /Dequeue

Rule

Application

EXCEPTION HANDLER

Crashed

Activate failure action

A textual file, similar to shell script syntax. Contains a list of “Rule Blocks”. A Rule block is defined per process. Scripts can be extended by including other scripts.

◦ dividing dedicated scripts per each logical or functional sub-system in the system.

PCD Rules Script

Rules and Processes block diagram

Rule

Rule

Rule

Process

Process

Process

Associated

Associated

Associated

RULESDB

Depends

Depends

PROCESS CONTROL

Started, Stopped, Monitored

Started, Stopped, Monitored

Started, Stopped, Monitored

PCD Script

RuleRuleRule…Rule

PARSER

ReadAdd Rule

PCD Rules Script Syntax

The PCD provides a parser which provides an easy way to verify that your PCD scripts do not contain syntax errors, similarly to compilation process.

The parser allows to fix the configuration files on the host, without the need to run them on the target, and rebuilding an image in case of an error.

Syntax Checking

No purchase costs or royalty fees. Source code is fully available. High quality code due to extensive exposure. LGPL allows linking proprietary code with PCD. Continuous development and bug fixes. Need a new feature?

◦ Either request it in the project tracker system◦ Or join the PCD community and develop it, so other could

also enjoy your productivity.

PCD - Open Source Benefits

Support more platforms. Watchdog/Keep alive mechanism. Kernel monitoring agent/module. Rule enhancements:

◦Affinity◦Resource limitation (CPU, Heap, Stack, Fork Bombs..)◦Current working directory◦Others…

PCD – Wish list (Future Features)