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KSE(2)

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NAME    [Toc]    [Back]

     kse -- kernel support for user threads

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <sys/types.h>
     #include <sys/kse.h>

     int
     kse_create(struct kse_mailbox *mbx, int newgroup);

     int
     kse_exit(void);

     int
     kse_release(struct timespec *timeout);

     int
     kse_wakeup(struct kse_mailbox *mbx);

     int
     kse_thr_interrupt(struct kse_thr_mailbox *tmbx);

DESCRIPTION    [Toc]    [Back]

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

   Overview    [Toc]    [Back]
     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:

     +o	 The user process can run multiple threads simultaneously on multiprocessor
 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    [Toc]    [Back]
     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.

     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:

     +o	 The thread invokes a system call that blocks.

     +o	 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.

     +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, and the current thread is returning 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 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.

     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 currently
 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    [Toc]    [Back]
     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 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 immediately.

     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.  (This is not yet implemented.)

     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_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''.

     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    [Toc]    [Back]
     When a process has at least one KSE with an associated mailbox, then signals
 might no longer be delivered on the process stack.  Instead, signals
     may be delivered via upcalls.  Multiple signals may be delivered with one
     upcall.  (This feature is not yet coded.)

     If there are multiple KSE groups in the process, which KSE group is chosen
 to deliver the signal is indeterminate.

   KSE Mailboxes    [Toc]    [Back]
     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 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, reenabling
 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:

	     (No flags are defined yet.)

   Thread Mailboxes    [Toc]    [Back]
     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 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:

	     (No flags are defined yet.)

RETURN VALUES    [Toc]    [Back]

     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    [Toc]    [Back]

     The kse_create() system call 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 execution
 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 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 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).

     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    [Toc]    [Back]

      
      
     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    [Toc]    [Back]

     The KSE system calls first appeared in FreeBSD 5.0.

AUTHORS    [Toc]    [Back]

     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    [Toc]    [Back]

     The KSE code is currently under development.


FreeBSD 5.2.1		      September 10, 2002		 FreeBSD 5.2.1
[ Back ]
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