1. How & When The Kernel Runs
David Ferry, Chris Gill
Department of Computer Science and Engineering
Washington University, St. Louis MO

2. Traditional View of Process Execution
Ready
Scheduled
Running
Preempted
Creation
Terminate
Unblocked
Blocked
Waiting
However, the kernel is not a traditional process!
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3. Kernel Execution: Boot
System Boot
Bootloader loads kernel
Initialize System
Initial kernel
never returns
Idle Task (pid 0)
Power On
Creates Init, which
creates all other threads
The kernel only runs deterministically at boot time. Otherwise, the kernel is entirely event driven.
ksoftirq
Init (pid 1)
migration
Like process threads, kernel threads are also scheduled
Kernel entry point:
start_kernel() in /init/main.c
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4. Kernel Execution: init Thread
Q: Why does the kernel create an init thread at boot instead of using the original execution context? A: The start_kernel execution context in main.c eventually becomes the main processor idle loop, a.k.a. the swapper or sched process. (Follow the start_kernel function through to cpu_idle_loop() )
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5. Kernel Execution: Threads
Kernel threads perform background operations, e.g.
[ksoftirq] does delayed interrupt handling
[migrate] does inter-processor load balancing
[kworker] handles misc. tasks
Kernel threads are similar to user threads:
are scheduled
can be preempted
However, there are differences:
run in kernel context
have no process memory space
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6. Kernel Execution: Event Driven
Software/hardware
Interrupt arrives
Interrupt Handler
Hardware interrupt
Runs in interrupt
context
Returns control
Software interrupt
Kernel
preemption
Runs in syscall context
Returns control
Interrupt context is not preemptive
Syscall context is preemptive (though preemption can be disabled or enabled)
Return path may return control to kernel or user space, or may call schedule()
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7. CSE 422S –Operating Systems Organization
System Calls
The syscall interface relies on an integer
User library loads syscall number into register
May load syscall arguments into other registers
Executes syscall trap (software exception)
Trap is caught by the kernel
Puts arguments on kernel stack (asmlinkage)
Kernel looks up syscall number in the interrupt vector
Jumps to syscall routine specified in interrupt table
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8. CSE 422S – Operating Systems Organization
System Calls on ARM
The ARM-specific details are as follows
User library loads syscall number into register R7
May load syscall arguments into registers R0-R6
Executes SWI instruction(software exception)
Trap is caught by the kernel
Control jumps to function vector_swi() in arch/arm/kernel/entry-common.S
Control eventually jumps to a C function inside the kernel
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9. Linux System Programming in C
User space programs may call kernel functions
Indirectly via library calls (more portable)
printf ("getuid returned: %u\n", getuid());
Directly via the syscall interface
printf ("syscall to getuid returned: %u\n", syscall(__NR_getuid));
Look at the man pages, for files to #include
userspace_programs]$ man getuid
userspace_programs]$ man syscall
Use manifest constants (portability, good style)
__NR_getuid
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10. Extending the Kernel in C
Linux is open source: you can modify a kernel
Adding your own system call prototypes
asmlinkage long sys_noargs(void);
Adding your own system call definitions
SYSCALL_DEFINE0( noargs ){ … }
Connecting them to the system call dispatch table
Adding the new definition files to the Makefile
Today’s studio will give you experience invoking existing system calls, and adding new ones
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11. CSE 422S –Operating Systems Organization
Rebuilding the Kernel
Q: Do we really need to issue make clean when rebuilding the kernel? A: Not always, but probably a good idea to do so anyway
unistd.h
#define NR_SYSCALLS 292
baz.c
foo.c
foo.o
NR_SYSCALLS = 388
baz.o
NR_SYSCALLS = 292
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