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
17.3.1.3. Preemptable Softirqs
17.3.1.3. Preemptable Softirqs
CONFIG_PREEMPT_SOFTIRQ reduces latency by running softirqs within the context of the kernel's softirq daemon (ksoftirqd). ksoftirqd is a proper Linux task (process). As such, it can be prioritized and scheduled along with other tasks. If your kernel is configured for real time, and CONFIG_PREEMPT_SOFTIRQ is enabled, the ksoftirqd kernel task is elevated to real-time priority to handle the softirq processing.[119] Listing 17-3 shows the code responsible for this from a recent Linux kernel, found in .../kernel/softirq.c.
Listing 17-3. Promoting ksoftirq to Real-Time Status
static int ksoftirqd(void * __bind_cpu) {
struct sched_param param = { .sched_priority = 24 };
printk("ksoftirqd started up.n");
#ifdef CONFIG_PREEMPT_SOFTIRQS
printk("softirq RT prio: %d.n", param.sched_priority);
sys_sched_setscheduler(current->pid, SCHED_FIFO, ¶m);
#else
set_user_nice(current, -10);
#endif
...
Here we see that if CONFIG_PREEMPT_SOFTIRQS is enabled in the kernel configuration, the ksoftirqd kernel task is promoted to a real-time task (SCHED_FIFO) at a real-time priority of 24 using the sys_sched_setscheduler() kernel function.
SoftIRQ threading can be disabled at runtime through the /proc file system, as well as through the kernel command line at boot time. When enabled in the configuration, unless you specify otherwise, SoftIRQ threading is enabled by default. To disable SoftIRQ threading at runtime, issue the following command as root:
# echo '0' >/proc/sys/kernel/softirq_preemption
To verify the setting, display it as follows:
# cat /proc/sys/kernel/softirq_preemption
1
To disable SoftIRQ threading at boot time, add the following parameter to the kernel command line:
softirq-preempt=0
