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KSE(2) FreeBSD System Calls Manual KSE(2) NAME kse -- kernel support for user threads LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <sys/types.h> #include <sys/kse.h> int kse_create(struct kse_mailbox *mbx, int newgroup); int kse_exit(void); int kse_release(void); int kse_wakeup(struct kse_mailbox *mbx); int kse_thr_interrupt(struct kse_thr_mailbox *tmbx); DESCRIPTION These functions 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 tak- ing 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. Arbitrarily many 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: +o 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. +o All operations that block in the kernel become asynchronous, allowing the user process to schedule another thread when any thread blocks. +o Multiple thread schedulers within the same process are possible, and they may operate independently of each other. Definitions KSE allows a user process to have multiple threads of execution in exis- tence 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. The KSE becomes unassigned, and the associated thread is suspended, when the KSE has an associated mailbox (see below) and any of the following occurs: +o The thread invokes a blocking system call. +o The thread makes any other demand of the kernel that cannot be imme- diately satisfied, e.g., touches a page of memory that needs to be fetched from disk, causing a page fault. +o 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. +o 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 sched- uling and accounting, each KSE group counts the same as 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 ker- nel to any runnable 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. Each KSE has a unique KSE mailbox supplied by the user process. A mail- box 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 deliv- ered 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 exe- cuting. 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. There is a kernel-imposed limit on the number of threads in a KSE group that may be simultaneously blocked in the kernel (this number is not cur- rently visible to the user). When this limit is reached, upcalls are blocked and no work is performed for the KSE group until one of the threads completes (or a signal is received). Managing KSEs To become multi-threaded, a process must first invoke kse_create(). kse_create() 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 newgroup is non-zero, a new KSE group is also created containing the KSE. Otherwise, the new KSE is added to the current KSE group. Newly created KSEs are initially unassigned; therefore, they will upcall imme- diately. 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 immedi- ate upcall results. However, the upcall will be invoked the next time the thread blocks. The kernel does not allow more KSEs to exist in a KSE group than the num- ber 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. kse_exit() 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 function does not return. As a special case, if the last remaining KSE in the last remaining KSE group invokes this function, 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. kse_release() is used to ``park'' the KSE assigned to the currently run- ning thread when it is not needed, e.g., when there are more available KSEs than runnable user threads. The KSE remains unassigned but does not upcall until there is a new reason to do so, e.g., a previously blocked thread becomes runnable. If successful, kse_release() does not return. kse_wakeup() is the opposite of kse_release(). It causes the KSE associ- ated 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 func- tion has no effect. The mbx may be NULL to specify ``any KSE in the current KSE group''. kse_thr_interrupt() 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 When a process has at least one KSE with an associated mailbox, then sig- nals are no longer delivered on the process stack. Instead, signals are delivered via upcalls. Multiple signals may be delivered with one upcall. If there are multiple KSE groups in the process, which KSE group is cho- sen to deliver the signal is indeterminate. However, once a signal has been delivered to a specific KSE group, that KSE group then takes owner- ship of signal delivery and all subsequent signals are delivered via that KSE group. When this KSE group is destroyed, a new KSE group is chosen as needed. KSE Mailboxes Each KSE has a unique mailbox for user-kernel communication: /* Upcall function type */ typedef void kse_func_t(struct kse_mailbox *); /* KSE mailbox */ struct kse_mailbox { int km_version; /* Mailbox version */ struct kse_thr_mailbox *km_curthread; /* Current thread */ struct kse_thr_mailbox *km_completed; /* Completed threads */ sigset_t km_sigscaught; /* Caught signals */ unsigned int km_flags; /* KSE flags */ kse_func_t *km_func; /* UTS function */ stack_t km_stack; /* UTS context */ void *km_udata; /* For use by the UTS */ struct timespec km_timeofday; /* Time of upcall */ }; 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 mail- box 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 to avoid the race condition where the kernel saves a partially modified value. 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 sched- uler should put these threads back into its own runnable queue. Each thread in a KSE group that completes is guaranteed to be linked into exactly one KSE's km_completed list; which KSE in the group, however, is indeterminate. Furthermore, the thread will appear 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. km_timeofday is set by the kernel to the current system time before per- forming each upcall. km_flags may contain any of the following bits OR'ed together: (No flags are defined yet.) Thread Mailboxes Each user thread must have associated with it a unique struct kse_thr_mailbox: /* Thread mailbox */ struct kse_thr_mailbox { ucontext_t tm_context; /* User thread context */ unsigned int tm_flags; /* Thread flags */ struct kse_thr_mailbox *tm_next; /* Next thread in list */ void *tm_udata; /* For use by the UTS */ unsigned int tm_uticks; /* User time counter */ unsigned int tm_sticks; /* Kernel time counter */ }; 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 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 ker- nel 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: (No flags are defined yet.) RETURN VALUES kse_create(), kse_wakeup, and kse_thr_interrupt() return zero if success- ful. kse_exit() and kse_release() do not return if successful. All of these functions return a non-zero error code in case of an error. Note: error codes are returned directly rather than via errno. ERRORS kse_create() will fail if: [ENXIO] There are already as many KSEs in the KSE group as hardware processors. [EAGAIN] The system-imposed limit on the total number of KSE groups under execution would be exceeded. The limit is given by the sysctl(3) MIB variable KERN_MAXPROC. (The limit is actually ten less than this except for the super user.) [EAGAIN] The user is not the super user, and the system-imposed limit on the total number of KSE groups under execu- tion by a single user would be exceeded. The limit is given by the sysctl(3) MIB variable KERN_MAXPROCPERUID. [EAGAIN] The user is not the super user, and the soft resource limit corresponding to the resource parameter RLIMIT_NPROC would be exceeded (see getrlimit(2)). [EFAULT] mbx points to an address which is not a valid part of the process address space. kse_exit() will fail if: [EDEADLK] The current KSE is the last in its KSE group and there are still one or more threads associated with the KSE group 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). kse_release() will fail if: [ESRCH] The current KSE has no associated mailbox, i.e., the process is operating is traditional, unthreaded mode. kse_wakeup() will fail if: [ESRCH] mbx is not NULL and the mailbox pointed to by mbx is not associated with any KSE in the process. [ESRCH] mbx is NULL and the current KSE has no associated mailbox, i.e., the process is operating in tradi- tional, unthreaded mode. kse_thr_interrupt() 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, and Henry M. Levy, "Scheduler activations: effective kernel support for the user-level management of parallelism", ACM Press, ACM Transactions on Computer Systems, Issue 1, Volume 10, pp. 53-79, February 1992. HISTORY The KSE function calls first appeared in FreeBSD 5.0. AUTHORS KSE was originally implemented by 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 currently under development. FreeBSD 11.1 September 10, 2002 FreeBSD 11.1
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY | AUTHORS | BUGS
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