Comprehensive Real-World Guidance for Every Embedded Developer and Engineer
This book brings together indispensable knowledge for building efficient, high-value, Linux-based embedded products: information that has never been assembled in one place before. Drawing on years of experience as an embedded Linux consultant and field application engineer, Christopher Hallinan offers solutions for the specific technical issues you're most likely to face, demonstrates how to build an effective embedded Linux environment, and shows how to use it as productively as possible.
Hallinan begins by touring a typical Linux-based embedded system, introducing key concepts and components, and calling attention to differences between Linux and traditional embedded environments. Writing from the embedded developer's viewpoint, he thoroughly addresses issues ranging from kernel building and initialization to bootloaders, device drivers to file systems.
Hallinan thoroughly covers the increasingly popular BusyBox utilities; presents a step-by-step walkthrough of porting Linux to custom boards; and introduces real-time configuration via CONFIG_RT--one of today's most exciting developments in embedded Linux. You'll find especially detailed coverage of using development tools to analyze and debug embedded systems--including the art of kernel debugging.
• Compare leading embedded Linux processors
• Understand the details of the Linux kernel initialization process
• Learn about the special role of bootloaders in embedded Linux systems, with specific emphasis on U-Boot
• Use embedded Linux file systems, including JFFS2--with detailed guidelines for building Flash-resident file system images
• Understand the Memory Technology Devices subsystem for flash (and other) memory devices
• Master gdb, KGDB, and hardware JTAG debugging
• Learn many tips and techniques for debugging within the Linux kernel
• Maximize your productivity in cross-development environments
• Prepare your entire development environment, including TFTP, DHCP, and NFS target servers
• Configure, build, and initialize BusyBox to support your unique requirements
17.2.3. SMP Kernel
17.2.3. SMP Kernel
It is interesting to note that much of the work involved in creating an efficient multiprocessor architecture also benefits real time. The SMP challenge is more complex than the uniprocessor challenge because there is an additional element of concurrency to protect against. In the uniprocessor model, only a single task can be executing in the kernel at a time. Protection from concurrency involves only protection from interrupt or exception processing. In the SMP model, multiple threads of execution in the kernel are possible in addition to the threat from interrupt and exception processing.
SMP has been supported from early Linux 2. x kernels. A Big Kernel Lock (BKL) was used to protect against concurrency in the transition from uniprocessor to SMP operation. The BKL is a global spinlock, which prevents any other tasks from executing in the kernel. In his excellent book Linux Kernel Development (Novell Press, 2005), Robert Love characterized the BKL as the "redheaded stepchild of the kernel." In describing the characteristics of the BKL, Robert jokingly added "evil" to its list of attributes!
Early implementations of the SMP kernel based on the BKL led to significant inefficiencies in scheduling. It was found that one of the CPUs could be kept idle for long periods of time. Much of the work that led to an efficient SMP kernel also directly benefited real-time applicationsprimarily lowered latency. Replacing the BKL with smaller-grained locking surrounding only the actual shared data to be protected led to significantly reduced preemption latency.
