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timeout: add TIMEOUT_MPSAFE flag

Add a TIMEOUT_MPSAFE flag to signal that a timeout is safe to run
without the kernel lock.  Currently, TIMEOUT_MPSAFE requires
TIMEOUT_PROC.  When the softclock() is unlocked in the future this
dependency will be removed.

On MULTIPROCESSOR kernels, softclock() now shunts TIMEOUT_MPSAFE
timeouts to a dedicated "timeout_proc_mp" bucket for processing by the
dedicated softclock_thread_mp() kthread.  Unlike softclock_thread(),
softclock_thread_mp() is not pinned to any CPU and runs run at IPL_NONE.

Prompted by bluhm@.  Lots of input from bluhm@.  Joint work with mvs@.

Prompt: https://marc.info/?l=openbsd-tech&m=169646019109736&w=2
Thread: https://marc.info/?l=openbsd-tech&m=169652212131109&w=2

ok mvs@

.\"	$OpenBSD: timeout.9,v 1.57 2023/10/12 15:32:38 cheloha Exp $
.\"
.\" Copyright (c) 2000 Artur Grabowski <art@openbsd.org>
.\" Copyright (c) 2021, 2022 Scott Cheloha <cheloha@openbsd.org>
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.Dd $Mdocdate: October 12 2023 $
.Dt TIMEOUT_SET 9
.Os
.Sh NAME
.Nm timeout_set ,
.Nm timeout_set_flags ,
.Nm timeout_set_proc ,
.Nm timeout_add ,
.Nm timeout_add_sec ,
.Nm timeout_add_msec ,
.Nm timeout_add_usec ,
.Nm timeout_add_nsec ,
.Nm timeout_add_tv ,
.Nm timeout_abs_ts ,
.Nm timeout_del ,
.Nm timeout_del_barrier ,
.Nm timeout_barrier ,
.Nm timeout_pending ,
.Nm timeout_initialized ,
.Nm timeout_triggered ,
.Nm TIMEOUT_INITIALIZER ,
.Nm TIMEOUT_INITIALIZER_FLAGS
.Nd execute a function in the future
.Sh SYNOPSIS
.In sys/types.h
.In sys/timeout.h
.Ft void
.Fo timeout_set
.Fa "struct timeout *to"
.Fa "void (*fn)(void *)"
.Fa "void *arg"
.Fc
.Ft void
.Fo timeout_set_flags
.Fa "struct timeout *to"
.Fa "void (*fn)(void *)"
.Fa "void *arg"
.Fa "int kclock"
.Fa "int flags"
.Fc
.Ft void
.Fo timeout_set_proc
.Fa "struct timeout *to"
.Fa "void (*fn)(void *)"
.Fa "void *arg"
.Fc
.Ft int
.Fo timeout_add
.Fa "struct timeout *to"
.Fa "int nticks"
.Fc
.Ft int
.Fo timeout_add_sec
.Fa "struct timeout *to"
.Fa "int secs"
.Fc
.Ft int
.Fo timeout_add_msec
.Fa "struct timeout *to"
.Fa "int msecs"
.Fc
.Ft int
.Fo timeout_add_usec
.Fa "struct timeout *to"
.Fa "int usecs"
.Fc
.Ft int
.Fo timeout_add_nsec
.Fa "struct timeout *to"
.Fa "int nsecs"
.Fc
.Ft int
.Fo timeout_add_tv
.Fa "struct timeout *to"
.Fa "struct timeval *tv"
.Fc
.Ft int
.Fo timeout_abs_ts
.Fa "struct timeout *to"
.Fa "const struct timespec *abs"
.Fc
.Ft int
.Fo timeout_del
.Fa "struct timeout *to"
.Fc
.Ft int
.Fo timeout_del_barrier
.Fa "struct timeout *to"
.Fc
.Ft void
.Fo timeout_barrier
.Fa "struct timeout *to"
.Fc
.Ft int
.Fo timeout_pending
.Fa "struct timeout *to"
.Fc
.Ft int
.Fo timeout_initialized
.Fa "struct timeout *to"
.Fc
.Ft int
.Fo timeout_triggered
.Fa "struct timeout *to"
.Fc
.Fo TIMEOUT_INITIALIZER
.Fa "void (*fn)(void *)"
.Fa "void *arg"
.Fc
.Fo TIMEOUT_INITIALIZER_FLAGS
.Fa "void (*fn)(void *)"
.Fa "void *arg"
.Fa "int kclock"
.Fa "int flags"
.Fc
.Sh DESCRIPTION
The
.Nm timeout
subsystem schedules functions for asynchronous execution in the future.
.Pp
All state is encapsulated in a
.Vt struct timeout
allocated by the caller.
A timeout must be initialized before it may be used as input to other
functions in the API.
Once initialized,
a timeout does not need to be reinitialized unless its function or argument
must change.
.Pp
The
.Fn timeout_set
function initializes the timeout
.Fa to .
When the timeout is executed,
the function
.Fa fn
will be called with
.Fa arg
as its sole parameter.
The timeout is implicitly scheduled against the
.Dv KCLOCK_NONE
clock and is not configured with any additional flags.
.Pp
The
.Fn timeout_set_flags
function is similar to
.Fn timeout_set ,
except that it takes two additional parameters:
.Bl -tag -width kclock
.It Fa kclock
The timeout is scheduled against the given
.Fa kclock ,
which must be one of the following:
.Bl -tag -width KCLOCK_UPTIME
.It Dv KCLOCK_NONE
Low resolution tick-based clock.
The granularity of this clock is limited by the
.Xr hardclock 9 ,
which executes roughly
.Xr hz 9
times per second.
.It Dv KCLOCK_UPTIME
The uptime clock.
Counts the time elapsed since the system booted.
.El
.It Fa flags
The timeout's behavior may be configured with the bitwise OR of
zero or more of the following
.Fa flags :
.Bl -tag -width TIMEOUT_MPSAFE
.It Dv TIMEOUT_PROC
Execute the timeout in a process context instead of the default
.Dv IPL_SOFTCLOCK
interrupt context.
.It Dv TIMEOUT_MPSAFE
Execute the timeout without the kernel lock.
Requires the
.Dv TIMEOUT_PROC
flag.
.El
.El
.Pp
The
.Fn timeout_set_proc
function is similar to
.Fn timeout_set ,
except that the given timeout is configured with the
.Dv TIMEOUT_PROC
flag.
.Pp
A timeout may also be initialized statically.
The
.Fn TIMEOUT_INITIALIZER
macro is equivalent to the
.Fn timeout_set
function and the
.Fn TIMEOUT_INITIALIZER_FLAGS
macro is equivalent to the
.Fn timeout_set_flags
function.
.Pp
The interfaces available for scheduling a timeout vary with the
.Fa kclock
set during initialization.
.Pp
.Dv KCLOCK_NONE
timeouts may be scheduled with the function
.Fn timeout_add ,
which schedules the given timeout to execute after at least
.Fa nticks
.Xr hardclock 9
ticks have elapsed.
In practice,
.Fa nticks
ticks will usually elapse in slightly less than
.Pq Fa nticks Cm / Dv hz
seconds.
Negative values of
.Fa nticks
are illegal.
If
.Fa nticks
is zero it will be silently rounded up to one.
.Pp
For convenience,
.Dv KCLOCK_NONE
timeouts may also be scheduled with
.Fn timeout_add_sec ,
.Fn timeout_add_msec ,
.Fn timeout_add_usec ,
.Fn timeout_add_nsec ,
or
.Fn timeout_add_tv .
These wrapper functions convert their input durations to a count of
.Xr hardclock 9
ticks before calling
.Fn timeout_add
to schedule the given timeout.
.Pp
Timeouts for any other
.Fa kclock
may be scheduled with
.Fn timeout_abs_ts ,
which schedules the given timeout to execute at or after the absolute time
.Fa abs
has elapsed on the timeout's
.Fa kclock .
.Pp
Once scheduled,
a timeout is said to be
.Qq pending .
A pending timeout may not be reinitialized with
.Fn timeout_set ,
.Fn timeout_set_flags ,
or
.Fn timeout_set_proc
until it has been executed or it has been cancelled with
.Fn timeout_del
or
.Fn timeout_del_barrier .
A pending timeout may be rescheduled without first cancelling it with
.Fn timeout_del
or
.Fn timeout_del_barrier :
the new expiration time will quietly supersede the original.
.Pp
The function
.Fn timeout_del
cancels any pending execution of the given timeout.
.Pp
The
.Fn timeout_del_barrier
function is similar to
.Fn timeout_del ,
except that it also blocks until any current execution of the given timeout
has completed.
.Pp
The
.Fn timeout_barrier
function blocks until any current execution of the given timeout
has completed.
.Pp
Callers of
.Fn timeout_barrier
and
.Fn timeout_del_barrier
must not hold locks that can block processing in the timeout's context.
Otherwise, the system will deadlock.
.Pp
The
.Fn timeout_pending
macro indicates whether the given timeout is scheduled for execution.
A timeout's pending status is cleared when it executes or is cancelled.
.Pp
The
.Fn timeout_initialized
macro indicates whether the given timeout has been initialized with
.Fn timeout_set
or
.Fn timeout_set_flags .
This macro must not be used unless the memory pointed to by
.Fa to
has been zeroed,
or its return value is meaningless.
.Pp
The
.Fn timeout_triggered
macro indicates whether the given timeout is executing or has finished
executing.
Rescheduling or cancelling a timeout clears its triggered status.
.Sh CONTEXT
.Fn timeout_set ,
.Fn timeout_set_flags ,
.Fn timeout_set_proc ,
.Fn timeout_add ,
.Fn timeout_add_sec ,
.Fn timeout_add_msec ,
.Fn timeout_add_usec ,
.Fn timeout_add_nsec ,
.Fn timeout_add_tv ,
.Fn timeout_abs_ts ,
.Fn timeout_del ,
.Fn timeout_pending ,
.Fn timeout_initialized ,
and
.Fn timeout_triggered
may be called during autoconf,
from process context,
or from any interrupt context.
.Pp
.Fn timeout_barrier
and
.Fn timeout_del_barrier
may only be called from process context.
.Pp
When a timeout is executed,
the function
.Fa fn
set during initialization is called from the
.Dv IPL_SOFTCLOCK
interrupt context,
or a process context if the timeout was configured with the
.Dv TIMEOUT_PROC
flag.
The function
.Fa fn
must not block and must be safe to execute on any CPU in the system.
.Pp
Timeouts without the
.Dv TIMEOUT_MPSAFE
flag are executed under the kernel lock.
.Sh RETURN VALUES
.Fn timeout_add ,
.Fn timeout_add_sec ,
.Fn timeout_add_msec ,
.Fn timeout_add_usec ,
.Fn timeout_add_nsec ,
.Fn timeout_add_tv ,
and
.Fn timeout_abs_ts
return 1 if the timeout
.Fa to
is newly scheduled,
or zero if the timeout was already pending.
.Pp
.Fn timeout_del
and
.Fn timeout_del_barrier
return 1 if the timeout
.Fa to
was pending,
or zero otherwise.
.Pp
.Fn timeout_pending ,
.Fn timeout_initialized ,
and
.Fn timeout_triggered
return non-zero if the corresponding condition is true,
or zero otherwise.
.Sh CODE REFERENCES
.Pa sys/kern/kern_timeout.c
.Sh SEE ALSO
.Xr hardclock 9 ,
.Xr hz 9 ,
.Xr microtime 9 ,
.Xr splclock 9 ,
.Xr task_add 9 ,
.Xr tsleep 9 ,
.Xr tvtohz 9
.Rs
.%A George Varghese
.%A Anthony Lauck
.%B Hashed and hierarchical timing wheels: efficient data structures for \
implementing a timer facility
.%O especially Schemes 6 and 7
.%I IEEE/ACM
.%J Transactions on Networking
.%V vol. 5
.%N no. 6
.%P pp. 824\(en834
.%D December 1997
.Re