Operating System Project / Lecture 1 Tasks and scheduling. Bon Keun Seo

Transcription

1 Operating System Project / Lecture 1 Tasks and scheduling Bon Keun Seo

2 Program: executable code Program and process Process: a running instance of a program /bin/bash Program (bash) Process 1 (bash) Process 2 (bash) Process 3 Runtime resources (in user s view) CPU: a set of registers, including IP (instruction pointer) register Memory: code, data, heap, stack, File: open files, sockets, device handle,

3 An execution sequence inside a process Shares resources: code, memory, files Resource allocation unit Process and threads Scheduling unit (registers, stack)

4 Modes of tasks A process / thread can run either in user or kernel mode System call or interrupt User mode write( ) Kernel mode sys_enter sys_write( ) 3 Application Kernel sys_exit

5 Thread implementation model User-level thread vs. kernel-level thread Process User-thread Kernel-thread Ring 3 Ring 0 User-level thread library Linux Solaris

6 Tasks in Linux kernel Both process and thread struct task_struct <include/linux/sched.h> Everything the kernel has to know 391 lines, about 2KB state mm parent Task list The list of tasks in kernel Doubly linked list tty thread fs files signal

7 Key resources in task_struct Variable pid state mm parent tty thread fs files signal / sighand pending Meaning Process identifier Task s state (running, stopped, interruptible, ) Virtual memory mapping Parent task I/O console CPU-specific state Current directory Open file descriptors Signal handler Pending signals

8 Kernel stack Kernel stack and task_struct An execution stack for kernel use After sysenter instruction, CPU jumps to a specific system call handler Sets stack frame pointer to a kernel stack Provides protection to kernel data structures Per task, 8KB by default task_struct was at the top of kernel stack < v2.6 Too large to fit in kernel stack anymore (>2KB) struct thread_info Light-weight process descriptor in the kernel stack

9 current_thread_info() = %ESP & 0xffffe000 Acquiring process descriptor Calculated from the kernel stack frame address 0x015fbfff current macro Current thread s task_struct current_thread_info()->task 0x015fb000 Kernel Stack task_struct current 0x015fa000 current_thread_info() thread_info

10 States of tasks State TASK_RUNNING TASK_INTERRUPTIBLE TASK_UNINTERRUPTIBLE TASK_ZOMBIE TASK_STOPPED Description Currently running or on a runqueue waiting to run. Sleeping, waiting for some condition to exist. Identical to TASK_INTERRUPTIBLE, except that it doesn t wake up and become runnable if it receives a signal. Terminated, but its parent has not yet issued a wait(). Process execution has stopped. (by signal)

11 Lifetime of a task

12 Process context Kernel mode contexts System call Kernel is executing on behalf of the process The current macro is valid Interrupt context Interrupt handler No valid task for the interrupt The current macro is invalid

13 fork() In POSIX API Implemented via clone() system call Copy on write method do_fork() <kernel/fork.c> Allocate a new PID copy_process() Process creation dup_task_struct() to create a new kernel stack, thread_info, task_struct Copy resources: copy_files(), copy_fs(), copy_mm(), copy_thread() to initialize the kernel stack sched_fork() to ready for the new task to schedule wake_up_new_task() Schedule the child task (run the child first)

14 No explicit thread in Linux kernel Thread management Implemented also via clone(clone_vm CLONE_FS CLONE_FILES CLONE_SIGHAND, 0); POSIX compatibility NPTL (Native POSIX Thread Library) from RedHat User-level library + kernel support Thread group PID-related system calls should be handled correctly getpid(), kill(), _exit() Thread group ID as a PID of a process: task_struct.tgid The PID of the thread group leader

15 Kernel thread Standard process that exist solely in kernel space No address space Schedulable entity: can sleep and wake up periodically Creating a kernel thread int kernel_thread(int (*fn)(void *), void *arg, unsigned long flags); struct task_struct * kthread_run(int (*fn)(void *), void *arg, char *name); Managing status kthread_stop() a thread forces the other thread to stop kthread_should_stop() check whether to stop or not

16 PID hash table To find a task struct from PID struct pid *find_vpid_ns(int nr) Process management struct task_struct pid_task(struct pid *pid, enum pid_type) Process family tree Root: /sbin/init (PID 1) P 0 P 1 P 2 P 3 parent children.prev/next sibling.prev/next P 4