KSE(2)

NAME
kse - kernel support for user threads

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/kse.h>
int
kse_create(struct kse_mailbox *mbx, int sys-scope);
int
kse_exit(void);
int
kse_release(struct timespec *timeout);
int
kse_switchin(struct kse_thr_mailbox *tmbx, int flags);
int
kse_thr_interrupt(struct kse_thr_mailbox *tmbx, int cmd, long data);
int
kse_wakeup(struct kse_mailbox *mbx);

DESCRIPTION
These system calls implement kernel support for multi-threaded processes.

  Overview
Traditionally, user threading has been implemented in one of two ways: either all threads are managed in user space and the kernel is unaware of any threading (also known as “N to 1”), or else separate processes sharing a common memory space are created for each thread (also known as “N to N”). These approaches have advantages and disadvantages:
User threading | Kernel threading
+ Lightweight | - Heavyweight
+ User controls scheduling | - Kernel controls scheduling
- Syscalls must be wrapped | + No syscall wrapping required
- Cannot utilize multiple CPUs | + Can utilize multiple CPUs

The KSE system is a hybrid approach that achieves the advantages of both the user and kernel threading approaches. The underlying philosophy of the KSE system is to give kernel support for user threading without taking away any of the user threading library's ability to make scheduling decisions. A kernel-to-user upcall mechanism is used to pass control to the user threading library whenever a scheduling decision needs to be made. An arbitrarily number of user threads are multiplexed onto a fixed number of virtual CPUs supplied by the kernel. This can be thought of as an “N to M” threading scheme.

Some general implications of this approach include:

•   The user process can run multiple threads simultaneously on multi-processor machines. The kernel grants the process virtual CPUs to schedule as it wishes; these may run concurrently on real CPUs.

•   All operations that block in the kernel become asynchronous, allowing the user process to schedule another thread when any thread blocks.

  Definitions
KSE allows a user process to have multiple threads of execution in existence at the same time, some of which may be blocked in the kernel while others may be executing or blocked in user space. A kernel scheduling entity (KSE) is a “virtual CPU” granted to the process for the purpose of executing threads. A thread that is currently executing is always associated with exactly one KSE, whether executing in user space or in the kernel. The KSE is said to be assigned to the thread. KSEs (a user abstraction) are implemented on top of kernel threads using an 'upcall' entity.

The KSE becomes unassigned, and the associated thread is suspended, when the KSE has an associated mailbox, (see below) the thread has an associated thread mailbox, (also see below) and any of the following occurs:

•   The thread invokes a system call that blocks.

•   The thread makes any other demand of the kernel that cannot be immediately satisfied, e.g., touches a page of memory that needs to be fetched from disk, causing a page fault.

•   Another thread that was previously blocked in the kernel completes its work in the kernel (or is interrupted) and becomes ready to return to user space, and the current thread is returning to user space.

•   A signal is delivered to the process, and this KSE is chosen to deliver it.

In other words, as soon as there is a scheduling decision to be made, the KSE becomes unassigned, because the kernel does not presume to know how the process' other runnable threads should be scheduled. Unassigned KSEs always return to user space as soon as possible via the upcall mechanism (described below), allowing the user process to decide how that KSE should be utilized next. KSEs always complete as much work as possible in the kernel before becoming unassigned.

Individual KSEs within a process are effectively indistinguishable, and any KSE in a process may be assigned by the kernel to any runnable (in the kernel) thread associated with that process. In practice, the kernel attempts to preserve the affinity between threads and actual CPUs to optimize cache behavior, but this is invisible to the user process. (Affinity is not yet fully implemented.)

Each KSE has a unique KSE mailbox supplied by the user process. A mailbox consists of a control structure containing a pointer to an upcall function and a user stack. The KSE invokes this function whenever it becomes unassigned. The kernel updates this structure with information about threads that have become runnable and signals that have been delivered before each upcall. Upcalls may be temporarily blocked by the user thread scheduling code during critical sections.

Each user thread has a unique thread mailbox as well. Threads are referred to using pointers to these mailboxes when communicating between the kernel and the user thread scheduler. Each KSE's mailbox contains a pointer to the mailbox of the user thread that the KSE is currently executing. This pointer is saved when the thread blocks in the kernel.

Whenever a thread blocked in the kernel is ready to return to user space, it is added to the process's list of completed threads. This list is presented to the user code at the next upcall as a linked list of thread mailboxes.

There is a kernel-imposed limit on the number of threads in a process that may be simultaneously blocked in the kernel (this number is not currently visible to the user). When this limit is reached, upcalls are blocked and no work is performed for the process until one of the threads completes (or a signal is received).

  Managing KSEs
To become multi-threaded, a process must first invoke kse_create(). The kse_create() system call creates a new KSE (except for the very first invocation; see below). The KSE will be associated with the mailbox pointed to by mbx. If sys_scope is non-zero, then the new thread will be counted as a system scope thread. Other things must be done as well to make a system scope thread so this is not sufficient (yet). System scope variables are not covered in detail in this manual page yet, but briefly, they never perform upcalls and do not return to the user thread scheduler. Once launched they run autonomously. The pthreads library knows how to make system scope threads and users are encouraged to use the library interface.

Each process initially has a single KSE executing a single user thread. Since the KSE does not have an associated mailbox, it must remain assigned to the thread and does not perform any upcalls. (It is by definition a system scope thread). The result is the traditional, unthreaded mode of operation. Therefore, as a special case, the first call to kse_create() by this initial thread with sys_scope equal to zero does not create a new KSE; instead, it simply associates the current KSE with the supplied KSE mailbox, and no immediate upcall results. However, an upcall will be triggered the next time the thread blocks and the required conditions are met.

The kernel does not allow more KSEs to exist in a process than the number of physical CPUs in the system (this number is available as the sysctl(3) variable hw.ncpu). Having more KSEs than CPUs would not add any value to the user process, as the additional KSEs would just compete with each other for access to the real CPUs. Since the extra KSEs would always be side-lined, the result to the application would be exactly the same as having fewer KSEs. There may however be arbitrarily many user threads, and it is up to the user thread scheduler to handle mapping the application's user threads onto the available KSEs.

The kse_exit() system call causes the KSE assigned to the currently running thread to be destroyed. If this KSE is the last one in the process, there must be no remaining threads associated with that process blocked in the kernel. This system call does not return unless there is an error. Calling kse_exit() from the last thread is the same as calling exit().

The kse_release() system call is used to “park” the KSE assigned to the currently running thread when it is not needed, e.g., when there are more available KSEs than runnable user threads. The thread converts to an upcall but does not get scheduled until there is a new reason to do so, e.g., a previously blocked thread becomes runnable, or the timeout expires. If successful, kse_release() does not return to the caller.

The kse_switchin() system call can be used by the UTS, when it has selected a new thread, to switch to the context of that thread. The use of kse_switchin() is machine dependent. Some platforms do not need a system call to switch to a new context, while others require its use in particular cases.

The kse_wakeup() system call is the opposite of kse_release(). It causes the (parked) KSE associated with the mailbox pointed to by mbx to be woken up, causing it to upcall. If the KSE has already woken up for another reason, this system call has no effect. The mbx argument may be NULL to specify “any KSE in the current process”.

The kse_thr_interrupt() system call is used to interrupt a currently blocked thread. The thread must either be blocked in the kernel or assigned to a KSE (i.e., executing). The thread is then marked as interrupted. As soon as the thread invokes an interruptible system call (or immediately for threads already blocked in one), the thread will be made runnable again, even though the kernel operation may not have completed. The effect on the interrupted system call is the same as if it had been interrupted by a signal; typically this means an error is returned with errno set to EINTR.

  Signals
The current implementation creates a special signal thread. Kernel threads (KSEs) in a process mask all signals, and only the signal thread waits for signals to be delivered to the process, the signal thread is responsible for dispatching signals to user threads.

A downside of this is that if a multiplexed thread calls the execve() syscall, its signal mask and pending signals may not be available in the kernel. They are stored in userland and the kernel does not know where to get them, however POSIX requires them to be restored and passed them to new process. Just setting the mask for the thread before calling execve() is only a close approximation to the problem as it does not re-deliver back to the kernel any pending signals that the old process may have blocked, and it allows a window in which new signals may be delivered to the process between the setting of the mask and the execve().

For now this problem has been solved by adding a special combined kse_thr_interrupt()/execve() mode to the kse_thr_interrupt() syscall. The kse_thr_interrupt() syscall has a sub command KSE_INTR_EXECVE, that allows it to accept a kse_execv_args structure, and allowing it to adjust the signals and then atomically convert into an execve() call. Additional pending signals and the correct signal mask can be passed to the kernel in this way. The thread library overrides the execve() syscall and translates it into kse_intr_interrupt() call, allowing a multiplexed thread to restore pending signals and the correct signal mask before doing the exec(). This solution to the problem may change.

  KSE Mailboxes
Each KSE has a unique mailbox for user-kernel communication defined in <sys/kse.h>. Some of the fields there are:

km_version describes the version of this structure and must be equal to KSE_VER_0. km_udata is an opaque pointer ignored by the kernel.

km_func points to the KSE's upcall function; it will be invoked using km_stack, which must remain valid for the lifetime of the KSE.

km_curthread always points to the thread that is currently assigned to this KSE if any, or NULL otherwise. This field is modified by both the kernel and the user process as follows.

When km_curthread is not NULL, it is assumed to be pointing at the mailbox for the currently executing thread, and the KSE may be unassigned, e.g., if the thread blocks in the kernel. The kernel will then save the contents of km_curthread with the blocked thread, set km_curthread to NULL, and upcall to invoke km_func().

When km_curthread is NULL, the kernel will never perform any upcalls with this KSE; in other words, the KSE remains assigned to the thread even if it blocks. km_curthread must be NULL while the KSE is executing critical user thread scheduler code that would be disrupted by an intervening upcall; in particular, while km_func() itself is executing.

Before invoking km_func() in any upcall, the kernel always sets km_curthread to NULL. Once the user thread scheduler has chosen a new thread to run, it should point km_curthread at the thread's mailbox, re-enabling upcalls, and then resume the thread. Note: modification of km_curthread by the user thread scheduler must be atomic with the loading of the context of the new thread, to avoid the situation where the thread context area may be modified by a blocking async operation, while there is still valid information to be read out of it.

km_completed points to a linked list of user threads that have completed their work in the kernel since the last upcall. The user thread scheduler should put these threads back into its own runnable queue. Each thread in a process that completes a kernel operation (synchronous or asynchronous) that results in an upcall is guaranteed to be linked into exactly one KSE's km_completed list; which KSE in the group, however, is indeterminate. Furthermore, the completion will be reported in only one upcall.

km_sigscaught contains the list of signals caught by this process since the previous upcall to any KSE in the process. As long as there exists one or more KSEs with an associated mailbox in the user process, signals are delivered this way rather than the traditional way. (This has not been implemented and may change.)

km_timeofday is set by the kernel to the current system time before performing each upcall.

km_flags may contain any of the following bits OR'ed together:

KMF_NOUPCALL
    Block upcalls from happening. The thread is in some critical section.

KMF_NOCOMPLETED, KMF_DONE, KMF_BOUND
    This thread should be considered to be permanently bound to its KSE, and treated much like a non-threaded process would be. It is a “long term” version of KMF_NOUPCALL in some ways.

KMF_WAITSIGEVENT
    Implement characteristics needed for the signal delivery thread.

  Thread Mailboxes
Each user thread must have associated with it a unique struct kse_thr_mailbox as defined in <sys/kse.h>. It includes the following fields.

tm_udata is an opaque pointer ignored by the kernel.

tm_context stores the context for the thread when the thread is blocked in user space. This field is also updated by the kernel before a completed thread is returned to the user thread scheduler via km_completed.

tm_next links the km_completed threads together when returned by the kernel with an upcall. The end of the list is marked with a NULL pointer.

tm_uticks and tm_sticks are time counters for user mode and kernel mode execution, respectively. These counters count ticks of the statistics clock (see clocks(7)). While any thread is actively executing in the kernel, the corresponding tm_sticks counter is incremented. While any KSE is executing in user space and that KSE's km_curthread pointer is not equal to NULL, the corresponding tm_uticks counter is incremented.

tm_flags may contain any of the following bits OR'ed together:

TMF_NOUPCALL
    Similar to KMF_NOUPCALL. This flag inhibits upcalling for critical sections. Some architectures require this to be in one place and some in the other.

RETURN VALUES
The kse_create(), kse_wakeup(), and kse_thr_interrupt() system calls return zero if successful. The kse_exit() and kse_release() system calls do not return if successful.

All of these system calls return a non-zero error code in case of an error.

ERRORS
The kse_create() system call will fail if:

[ENXIO]
    There are already as many KSEs in the process as hardware processors.

[EAGAIN]
    The user is not the super user, and the soft resource limit corresponding to the resource argument RLIMIT_NPROC would be exceeded (see getrlimit(2)).

[EFAULT]
    The mbx argument points to an address which is not a valid part of the process address space.

The kse_exit() system call will fail if:

[EDEADLK]
    The current KSE is the last in its process and there are still one or more threads associated with the process blocked in the kernel.

[ESRCH]
    The current KSE has no associated mailbox, i.e., the process is operating in traditional, unthreaded mode (in this case use _exit(2) to exit the process).

The kse_release() system call will fail if:

[ESRCH]
    The current KSE has no associated mailbox, i.e., the process is operating is traditional, unthreaded mode.

The kse_wakeup() system call will fail if:

[ESRCH]
    The mbx argument is not NULL and the mailbox pointed to by mbx is not associated with any KSE in the process.

[ESRCH]
    The mbx argument is NULL and the current KSE has no associated mailbox, i.e., the process is operating in traditional, unthreaded mode.

The kse_thr_interrupt() system call will fail if:

[ESRCH]
    The thread corresponding to tmbx is neither currently assigned to any KSE in the process nor blocked in the kernel.

SEE ALSO
rfork(2), pthread(3), ucontext(3) Thomas E. Anderson, Brian N. Bershad, Edward D. Lazowska, Henry M. Levy, ACM Transactions on Computer Systems, Issue 1, Volume 10, February 1992, ACM Press, pp. 53-79, Scheduler activations: effective kernel support for the user-level management of parallelism.

HISTORY
The KSE system calls first appeared in FreeBSD 5.0.

AUTHORS
KSE was originally implemented by -nosplit Julian Elischer <julian@FreeBSD.org>, with additional contributions by Jonathan Mini <mini@FreeBSD.org>, Daniel Eischen <deischen@FreeBSD.org>, and David Xu <davidxu@FreeBSD.org>.

This manual page was written by Archie Cobbs <archie@FreeBSD.org>.

BUGS
The KSE code is