CONTENTS

LIBRARY

SYNOPSIS

DESCRIPTION

Overview

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.
• Multiple thread schedulers within the same process are possible, and they may operate independently of each other.

Definitions

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.

A "KSE group" is a collection of KSEs that are scheduled uniformly and which share access to the same pool of threads, which are associated with the KSE group. A KSE group is the smallest entity to which a kernel scheduling priority may be assigned. For the purposes of process scheduling and accounting, each KSE group counts similarly to a traditional unthreaded process. Individual KSEs within a KSE group are effectively indistinguishable, and any KSE in a KSE group may be assigned by the kernel to any runnable (in the kernel) thread associated with that KSE group. 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 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 KSE group’s list of completed threads. This list is presented to the user code at the next upcall as a linked list of thread mailboxes.

Each process initially has a single KSE in a single KSE group 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. 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 newgroup 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 KSE group 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 KSE group, there must be no remaining threads associated with the KSE group blocked in the kernel. This system call does not return unless there is an error.

As a special case, if the last remaining KSE in the last remaining KSE group invokes this system call, then the KSE is not destroyed; instead, the KSE just looses the association with its mailbox and kse_exit returns normally. This returns the process to its original, unthreaded state.

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 KSE group"".

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.

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 KSE group 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:

Thread Mailboxes

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:

RETURN VALUES

The kse_exit system call will fail if:

The kse_release system call will fail if:

The kse_wakeup system call will fail if:

The kse_thr_interrupt system call will fail if:

SEE ALSO

HISTORY

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