KTHREAD(9)

NAME
kthread_create, kthread_exit, kthread_join, kthread_fpu_enter, kthread_fpu_exit - kernel threads

SYNOPSIS
#include <sys/kthread.h>
int
kthread_create(pri_t pri, int flags, struct cpu_info *ci, void (*func)(void *), void *arg, lwp_t **newlp, const char *fmt, ...);
void
kthread_exit(int ecode);
int
kthread_join(lwp_t *l);
int
kthread_fpu_enter();
void
kthread_fpu_exit(int s);

DESCRIPTION
Kernel threads are light-weight processes which execute entirely within the kernel.

Any process can request the creation of a new kernel thread. Kernel threads are not swapped out during memory congestion. The VM space and limits are shared with proc0 (usually swapper).

If the machine has any per-CPU floating-point units or SIMD vector units that are normally available to user threads, they can be used by kthreads between kthread_fpu_enter() and kthread_fpu_exit().

FUNCTIONS

kthread_create(pri, flags, ci, func, arg, newlp, fmt, ...)
    Create a kernel thread. The arguments are as follows.

    pri Priority level for the thread. If no priority level is desired specify PRI_NONE, causing kthread_create() to select the default priority level.

    flags
        Flags that can be logically ORed together to alter the thread's behaviour.

    ci  If non-NULL, the thread will be created bound to the CPU specified by ci, meaning that it will only ever execute on that CPU. By default, the threads are free to execute on any CPU in the system.

    func
        A function to be called when the thread begins executing. This function must not return. If the thread runs to completion, it must call kthread_exit() to properly terminate itself.

    arg An argument to be passed to func(). May be NULL if not required.

    newlp
        A pointer to receive the new LWP structure for the kernel thread. May be NULL, unless KTHREAD_MUSTJOIN is specified in flags.

    fmt A string containing format information used to display the kernel thread name. Must not be NULL.

    The following flags are defined.

    KTHREAD_IDLE
        Causes the thread to be created in the LSIDL (idle) state. By default, the threads are created in the LSRUN (runnable) state, meaning they will begin execution shortly after creation.

    KTHREAD_MPSAFE
        Specifies that the thread does its own locking and so is multiprocessor safe. If not specified, the global kernel lock will be held whenever the thread is running (unless explicitly dropped by the thread).

    KTHREAD_INTR
        Specifies that the thread services device interrupts. This flag is intended for kernel internal use and should not normally be specified.

    KTHREAD_TS
        Causes the kthread to be created in the SCHED_OTHER class (timeshared). The thread's priority will be dynamically adjusted by the scheduler. Increased activity by the kthread will cause its priority to fall; decreased activity will cause its priority to rise. By default, kthreads are created in the SCHED_RR class, with a fixed priority specified by pri. Threads in the SCHED_RR class do not have their priority dynamically adjusted by the scheduler.

    KTHREAD_MUSTJOIN
        Indicates that created kthread must be joined. In such case kthread_exit() will wait until kthread_join() will be called.

kthread_exit(ecode)
    Exit from a kernel thread. Must only be called by a kernel thread.

kthread_join(l)
    Suspend execution of calling thread until the target kthread terminates. Conceptually the function can be compared to the user space pthread_join(3), however it must be called only once for kernel thread which was created using the KTHREAD_MUSTJOIN flag and would wait on kthread_exit.

kthread_fpu_enter()
    Allow the current kthread to use any machine-dependent per-CPU floating-point units or SIMD vector units normally available to user threads. Returns a cookie that must be passed to kthread_fpu_exit() when done.

    Matching pairs of kthread_fpu_enter() and kthread_fpu_exit() may be nested.

kthread_fpu_exit(s)
    Restore the current kthread's access to machine-dependent per-CPU floating-point units or SIMD vector units to what it was before the call to kthread_fpu_enter() that returned s.

    On the last kthread_fpu_exit(), zero all the units' registers to avoid leaking secrets — such units are often used for cryptography.

RETURN VALUES
Upon successful completion, kthread_create() returns 0. Otherwise, the following error values are returned:

[EAGAIN]
    The limit on the total number of system processes would be exceeded; or the limit RLIMIT_NPROC on the total number of processes under execution by this user id would be exceeded.

CODE REFERENCES
The kthread framework itself is implemented within the file sys/kern/kern_kthread.c. Data structures and function prototypes for the framework are located in sys/sys/kthread.h.

SEE ALSO
condvar(9), driver(9), softint(9), workqueue(9)

HISTORY
The kthread framework appeared in NetBSD 1.4.