206 lines
5.6 KiB
Groff
206 lines
5.6 KiB
Groff
.\" $NetBSD: kcont.9,v 1.6 2004/03/24 06:03:26 snj Exp $
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.\"
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.\" The author of this man page is Jonathan Stone (jonathan@dsg.stanford.edu)
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.\"
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.\" Copyright (c) 2003, 2004 Jonathan STone
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.\"
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.\" Permission to use, copy, and modify this software with or without fee
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.\" is hereby granted, provided that this entire notice is included in
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.\" all source code copies of any software which is or includes a copy or
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.\" modification of this software.
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.\"
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.\" THIS SOFTWARE IS BEING PROVIDED "AS IS", WITHOUT ANY EXPRESS OR
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.\" IMPLIED WARRANTY. IN PARTICULAR, NONE OF THE AUTHORS MAKES ANY
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.\" REPRESENTATION OR WARRANTY OF ANY KIND CONCERNING THE
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.\" MERCHANTABILITY OF THIS SOFTWARE OR ITS FITNESS FOR ANY PARTICULAR
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.\" PURPOSE.
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.\"
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.\"
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.\"
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.Dd March 23, 2004
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.Dt KCONT 9
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.Os
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.Sh NAME
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.Nm kcont
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.Nd continuation-passing framework for deferring execution and for
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notification of asynchronous events
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.Sh SYNOPSIS
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.In sys/kcont.h
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.\"
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.Ft struct kc *
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.Fo kcont
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.Fa "struct kc *"
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.Fa "void (*fn) (void *kernel_obj, void *env_arg, int status)"
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.Fa "void *env_arg"
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.Fa "int desired_ipl"
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.Fc
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.\"
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.Ft struct kc *
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.Fo kcont_malloc
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.Fa "int mallocflags"
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.Fa "void (*fn) (void *kernel_obj, void *env_arg, int status)"
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.Fa "void *env_arg"
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.Fa "int desired_ipl"
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.Fc
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.\"
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.Ft void
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.Fo kcont_defer
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.Fa "struct kc *kc"
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.Fa "void (*fn) (void *kernel_obj, void *env_arg, int status)"
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.Fa "void *kernel_obj"
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.Fa "void *env_arg"
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.Fa "int status"
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.Fa "int desired_ipl"
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.Fc
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.Ft void
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.Fo kcont_defer_malloc
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.Fa "int mallocflags"
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.Fa "void (*fn) (void *kernel_obj, void *env_arg, int status)"
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.Fa "void *kernel_obj"
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.Fa "void *env_arg"
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.Fa "int status"
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.Fa "int desired_ipl"
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.Fc
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.\"
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.Ft void
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.Fn kcont_enqueue "struct kcq *kcont_queue" "struct kc* kcont"
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.\" "void (*fn) (void *kernel_obj, void *env_arg, int status)" "void *env_arg" "int desired_ipl"
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.Ft void
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.Fn kcont_run "kcq_t *kcq" "void *obj" "int status" "int curipl"
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.Sh DESCRIPTION
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.Nm
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provides a method of asynchronous notification of kernel events
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(such as I/O completion events) loosely modelled on the
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continuation-passing model associated with functional languages such as
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Scheme.
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In the context of C and Unix kernels, a continuation can
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be thought of as a combination of three items:
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.Pp
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.Bl -bullet -compact
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.It
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a function pointer, or
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.Dq callback
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function;
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.It
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an argument (or arguments) to that function; and
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.It
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other
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.Dq environment
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state which is passed into the function.
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.El
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.Pp
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The continuations supported by
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.Nm
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can be used to defer execution from a high-priority context
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(such as a hardware-priority interrupt handler) to a lower-priority
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context (such as a software-interrupt callout queue specifically for
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executing
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.Nm
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functions).
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The
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.Nm
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mechanism can also be used to deliver notification of asynchronous
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events, such as I/O completion.
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.Sh TRADITIONAL I/O COMPLETION NOTIFICATION
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The traditional Unix methods for asynchronous notification are
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.Xr ltsleep 9 ,
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.Xr tsleep 9 ,
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and
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.Xr wakeup 9 ,
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which assume the operation or I/O request are being issued from process
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context.
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Once the I/O operation is actually issued, the requesting process
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is suspended by issuing a
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.Xr tsleep 9
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on the address of the I/O buffer.
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When the I/O operation completes, the I/O subsystem issues a
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.Xr wakeup 9
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call on the address of the I/O buffer.
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The process is then awakened, rescheduled, and continues execution
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from the
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.Fn tsleep
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function call, eventually returning to the function which requested I/O.
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.Sh CONTINUATIONS
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In contrast,
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.Nm
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constructs an object called a continuation -- a function representing
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.Dq the rest of the program to be performed .
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Continuations are constructed via the
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.Fn kcont
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and
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.Fn kcont_malloc
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functions, which use caller-supplied or dynamically-allocated space,
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respectively.
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In addition to the continuation-function, argument-object,
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and environment, the constructor of each continuation specifies an
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target interrupt priority (IPL) above which the continuation should not be run.
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.Pp
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Once constructed, the continuation can be destined for execution
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in one of two ways.
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First,
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.Fn kcont_defer
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or
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.Fn kcont_defer_malloc
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schedule a continuation for execution as soon as interrupt priority
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falls to or below the target IPL of the continuation.
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Alternatively, a
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.Nm
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can be passed by its creator across kernel interfaces which accept
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.Nm
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objects as methods for notifying the caller (i.e., the
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.Nm
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creator) on completion of an asynchronous operation.
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.Sh DATA TYPES
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.Ss Deferral Priorities
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Arguments shown above with the name
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.Ar desired_ipl
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are drawn from the following namespace:
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.Bl -tag -offset indent -width KC_IPL_DEFER_SOFTSERIALXX
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.It Dv KCL_IPL_IMMEDIATE
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Execute the continuation as soon as possible, without regard to current IPL.
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.It Dv KCL_IPL_DEFER_SOFTSERIAL
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Execute the continuation once IPL falls to
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.Dv IPL_SOFTSERIAL
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or below.
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.It Dv KCL_IPL_DEFER_SOFTCLOCK
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Execute the continuation once IPL falls to
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.Dv IPL_SOFTCLOCK
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or below.
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.It Dv KCL_IPL_DEFER_SOFTNET
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Execute the continuation once IPL falls to
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.Dv IPL_SOFTNET
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or below.
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.It Dv KCL_IPL_DEFER_PROCESS
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Defer execution of the continuation to process context.
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.El
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.Sh NOTES
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The
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.Ar desired_ipl
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argument should be interpreted as an upper-bound:
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the continuation will not be executed before interrupt priority drops
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to, or below, the requested level.
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In particular, the current implementation uses
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.Nx
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generic software callouts.
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On
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.Nx
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ports which lack generic software callouts, all
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.Nm
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continuations will always be deferred all the way to the context
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of a
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.Dq helper
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kernel thread.
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.Sh SEE ALSO
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.Xr kthread 9 ,
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.Xr spl 9 ,
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.Xr tsleep 9 ,
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.Xr wakeup 9
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.Sh AUTHORS
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The
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.Nm
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interface was designed and implemented by
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.An Jonathan Stone .
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Additional input on the
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.Nm
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design was provided by Jason Thorpe and Nathan Williams.
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