Process Descriptor and the Task Structure

The kernel stores the list of processes in a circular doubly linked list called the task list^[3]. Each element in the task list is a process descriptor of the type struct task_struct, which is defined in <linux/sched.h>. The process descriptor contains all the information about a specific process.

^[3] Some texts on operating system design call this list the task array. Because the Linux implementation is a linked list and not a static array, it is called the task list.

The task_struct is a relatively large data structure, at around 1.7 kilobytes on a 32-bit machine. This size, however, is quite small considering that the structure contains all the information that the kernel has and needs about a process. The process descriptor contains the data that describes the executing programopen files, the process's address space, pending signals, the process's state, and much more (see Figure 3.1).

Allocating the Process Descriptor

The task_struct structure is allocated via the slab allocator to provide object reuse and cache coloring (see Chapter 11, "Memory Management"). Prior to the 2.6 kernel series, struct task_struct was stored at the end of the kernel stack of each process. This allowed architectures with few registers, such as x86, to calculate the location of the process descriptor via the stack pointer without using an extra register to store the location. With the process descriptor now dynamically created via the slab allocator, a new structure, struct thread_info, was created that again lives at the bottom of the stack (for stacks that grow down) and at the top of the stack (for stacks that grow up)^[4]. See Figure 3.2. The new structure also makes it rather easy to calculate offsets of its values for use in assembly code.

^[4] Register-impaired architectures were not the only reason for creating struct thread_info.

The thread_info structure is defined on x86 in <asm/thread_info.h> as

struct thread_info {
        struct task_struct    *task;
        struct exec_domain    *exec_domain;
        unsigned long         flags;
        unsigned long         status;
        __u32                 cpu;
        __s32                 preempt_count;
        mm_segment_t          addr_limit;
        struct restart_block  restart_block;
        unsigned long         previous_esp;
        __u8                  supervisor_stack[0];
};

Each task's tHRead_info structure is allocated at the end of its stack. The task element of the structure is a pointer to the task's actual task_struct.

Storing the Process Descriptor

The system identifies processes by a unique process identification value or PID. The PID is a numerical value that is represented by the opaque type^[5] pid_t, which is typically an int. Because of backward compatibility with earlier Unix and Linux versions, however, the default maximum value is only 32,768 (that of a short int), although the value can optionally be increased to the full range afforded the type. The kernel stores this value as pid inside each process descriptor.

^[5] An opaque type is a data type whose physical representation is unknown or irrelevant.

This maximum value is important because it is essentially the maximum number of processes that may exist concurrently on the system. Although 32,768 might be sufficient for a desktop system, large servers may require many more processes. The lower the value, the sooner the values will wrap around, destroying the useful notion that higher values indicate later run processes than lower values. If the system is willing to break compatibility with old applications, the administrator may increase the maximum value via /proc/sys/kernel/pid_max.

Inside the kernel, tasks are typically referenced directly by a pointer to their task_struct structure. In fact, most kernel code that deals with processes works directly with struct task_struct. Consequently, it is very useful to be able to quickly look up the process descriptor of the currently executing task, which is done via the current macro. This macro must be separately implemented by each architecture. Some architectures save a pointer to the task_struct structure of the currently running process in a register, allowing for efficient access. Other architectures, such as x86 (which has few registers to waste), make use of the fact that struct thread_info is stored on the kernel stack to calculate the location of thread_info and subsequently the task_struct.

On x86, current is calculated by masking out the 13 least significant bits of the stack pointer to obtain the thread_info structure. This is done by the current_thread_info() function. The assembly is shown here:

movl $-8192, %eax
andl %esp, %eax

This assumes that the stack size is 8KB. When 4KB stacks are enabled, 4096 is used in lieu of 8192.

Finally, current dereferences the task member of thread_info to return the task_struct:

current_thread_info()->task;

Contrast this approach with that taken by PowerPC (IBM's modern RISC-based microprocessor), which stores the current task_struct in a register. Thus, current on PPC merely returns the value stored in the register r2. PPC can take this approach because, unlike x86, it has plenty of registers. Because accessing the process descriptor is a common and important job, the PPC kernel developers deem using a register worthy for the task.

Process State

The state field of the process descriptor describes the current condition of the process (see Figure 3.3). Each process on the system is in exactly one of five different states. This value is represented by one of five flags:

• TASK_RUNNING The process is runnable; it is either currently running or on a runqueue waiting to run (runqueues are discussed in Chapter 4, "Scheduling"). This is the only possible state for a process executing in user-space; it can also apply to a process in kernel-space that is actively running.

• TASK_INTERRUPTIBLE The process is sleeping (that is, it is blocked), waiting for some condition to exist. When this condition exists, the kernel sets the process's state to TASK_RUNNING. The process also awakes prematurely and becomes runnable if it receives a signal.

• TASK_UNINTERRUPTIBLE This state is identical to TASK_INTERRUPTIBLE except that it does not wake up and become runnable if it receives a signal. This is used in situations where the process must wait without interruption or when the event is expected to occur quite quickly. Because the task does not respond to signals in this state, TASK_UNINTERRUPTIBLE is less often used than TASK_INTERRUPTIBLE^[6].

  ^[6] This is why you have those dreaded unkillable processes with state D in ps(1). Because the task will not respond to signals, you cannot send it a SIGKILL signal. Further, even if you could terminate the task, it would not be wise as the task is supposedly in the middle of an important operation and may hold a semaphore.

• TASK_ZOMBIE The task has terminated, but its parent has not yet issued a wait4() system call. The task's process descriptor must remain in case the parent wants to access it. If the parent calls wait4(), the process descriptor is deallocated.

• TASK_STOPPED Process execution has stopped; the task is not running nor is it eligible to run. This occurs if the task receives the SIGSTOP, SIGTSTP, SIGTTIN, or SIGTTOU signal or if it receives any signal while it is being debugged.

Manipulating the Current Process State

Kernel code often needs to change a process's state. The preferred mechanism is using

set_task_state(task, state);        /* set task 'task' to state 'state' */

This function sets the given task to the given state. If applicable, it also provides a memory barrier to force ordering on other processors (this is only needed on SMP systems). Otherwise, it is equivalent to

task->state = state;

The method set_current_state(state) is synonymous to set_task_state(current, state).

Process Context

One of the most important parts of a process is the executing program code. This code is read in from an executable file and executed within the program's address space. Normal program execution occurs in user-space. When a program executes a system call (see Chapter 5, "System Calls") or triggers an exception, it enters kernel-space. At this point, the kernel is said to be "executing on behalf of the process" and is in process context. When in process context, the current macro is valid^[7]. Upon exiting the kernel, the process resumes execution in user-space, unless a higher-priority process has become runnable in the interim, in which case the scheduler is invoked to select the higher priority process.

^[7] Other than process context there is interrupt context, which we discuss in Chapter 6, "Interrupts and Interrupt Handlers." In interrupt context, the system is not running on behalf of a process, but is executing an interrupt handler. There is no process tied to interrupt handlers and consequently no process context.

System calls and exception handlers are well-defined interfaces into the kernel. A process can begin executing in kernel-space only through one of these interfacesall access to the kernel is through these interfaces.

The Process Family Tree

A distinct hierarchy exists between processes in Unix systems, and Linux is no exception. All processes are descendents of the init process, whose PID is one. The kernel starts init in the last step of the boot process. The init process, in turn, reads the system initscripts and executes more programs, eventually completing the boot process.

Every process on the system has exactly one parent. Likewise, every process has zero or more children. Processes that are all direct children of the same parent are called siblings. The relationship between processes is stored in the process descriptor. Each task_struct has a pointer to the parent's task_struct, named parent, and a list of children, named children. Consequently, given the current process, it is possible to obtain the process descriptor of its parent with the following code:

struct task_struct *my_parent = current->parent;

Similarly, it is possible to iterate over a process's children with

struct task_struct *task;
struct list_head *list;

list_for_each(list, &current->children) {
        task = list_entry(list, struct task_struct, sibling);
        /* task now points to one of current's children */
}

The init task's process descriptor is statically allocated as init_task. A good example of the relationship between all processes is the fact that this code will always succeed:

struct task_struct *task;

for (task = current; task != &init_task; task = task->parent)
        ;
/* task now points to init */

In fact, you can follow the process hierarchy from any one process in the system to any other. Oftentimes, however, it is desirable simply to iterate over all processes in the system. This is easy because the task list is a circular doubly linked list. To obtain the next task in the list, given any valid task, use:

list_entry(task->tasks.next, struct task_struct, tasks)

Obtaining the previous works the same way:

list_entry(task->tasks.prev, struct task_struct, tasks)

These two routines are provided by the macros next_task(task) and prev_task(task), respectively. Finally, the macro for_each_process(task) is provided, which iterates over the entire task list. On each iteration, task points to the next task in the list:

struct task_struct *task;

for_each_process(task) {
        /* this pointlessly prints the name and PID of each task */
        printk("%s[%d]\n", task->comm, task->pid);
}

Note: It can be expensive to iterate over every task in a system with many processes; code should have good reason (and no alternative) before doing so.