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KSE(2)			  FreeBSD System Calls Manual			KSE(2)

     kse -- kernel support for user threads

     Standard C	Library	(libc, -lc)

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

     kse_create(struct kse_mailbox *mbx, int newgroup);


     kse_release(struct	timespec *timeout);

     kse_switchin(mcontext_t *mcp, long	val, long *loc);

     kse_thr_interrupt(struct kse_thr_mailbox *tmbx);

     kse_wakeup(struct kse_mailbox *mbx);

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

     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.  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 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.

     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) the	thread has an associ-
     ated 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	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, and the current thread is returning to user

     +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 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 ker-
     nel 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	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

     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().  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 cre-
     ated 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 immedi-
     ate 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 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.

     The kse_exit() system call	causes the KSE assigned	to the currently run-
     ning 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

     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''.

     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	inter-
     rupted.  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.

     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 deliv-
     ered 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.  Addi-
     tional 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	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 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 sched-
     uler 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

     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 per-
     forming each upcall.

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

	     Block upcalls from	happening.  The	thread is in some critical

	     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.

	     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

     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 com-
     pleted 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:

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

     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

     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 execu-
			tion by	a single user would be exceeded.  The limit is
			given by the sysctl(3) MIB variable

     [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

     [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

     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.

     The KSE system calls first	appeared in FreeBSD 5.0.

     KSE was originally	implemented by Julian Elischer <>,
     with additional contributions by Jonathan Mini <>,	Daniel
     Eischen <>, and David Xu <>.

     This manual page was written by Archie Cobbs <>.

     The KSE code is currently under development.

FreeBSD	6.1		      September	10, 2002		   FreeBSD 6.1


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