man9: Add man page for kern_yield() and friends
The page is fairly simple and will be referenced by mi_switch(9). Provide some usage notes so that the broader implications of how/when to use these functions are understood. Reviewed by: kib, markj MFC after: 1 week Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D38182
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Mitchell Horne
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@ -196,6 +196,7 @@ MAN= accept_filter.9 \
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KASSERT.9 \
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kern_reboot.9 \
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kern_testfrwk.9 \
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kern_yield.9 \
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kernacc.9 \
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kernel_mount.9 \
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khelp.9 \
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@ -1332,6 +1333,8 @@ MLINKS+=intr_event.9 intr_event_add_handler.9 \
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intr_event.9 intr_event_handle.9 \
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intr_event.9 intr_event_remove_handler.9 \
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intr_event.9 intr_priority.9
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MLINKS+=kern_yield.9 maybe_yield.9 \
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kern_yield.9 should_yield.9
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MLINKS+=kernacc.9 useracc.9
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MLINKS+=kernel_mount.9 free_mntarg.9 \
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kernel_mount.9 mount_arg.9 \
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134
share/man/man9/kern_yield.9
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134
share/man/man9/kern_yield.9
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@ -0,0 +1,134 @@
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.\"
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.\" Copyright (c) 2023 The FreeBSD Foundation
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.\"
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.\" This documentation was written by Mitchell Horne <mhorne@FreeBSD.org> under
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.\" sponsorship from the FreeBSD Foundation.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.Dd January 30, 2023
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.Dt KERN_YIELD 9
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.Os
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.Sh NAME
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.Nm kern_yield ,
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.Nm maybe_yield ,
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.Nm should_yield
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.Nd "functions for yielding execution of the current thread"
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.Sh SYNOPSIS
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.Ft void
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.Fn kern_yield "int prio"
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.Ft void
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.Fn maybe_yield
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.Ft bool
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.Fn should_yield
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.Sh DESCRIPTION
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The
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.Fn kern_yield
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function causes the currently running thread to voluntarily, but
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unconditionally, surrender its execution to the scheduler.
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The
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.Va prio
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argument specifies the scheduling priority to be assigned before the context
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switch, which has an influence on when execution will resume.
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Note that the requested priority will take effect until the thread returns to
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usermode, after which its base user priority will be restored.
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Valid values for
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.Va prio
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are any of the
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.Dv PRI_*
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values defined in
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.In sys/priority.h ,
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as well as the following special values:
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.Bl -tag -width "PRI_UNCHANGED"
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.It Dv PRI_USER
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Schedule the thread with its base user priority; the value corresponding to
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.Xr setpriority 2 /
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.Xr nice 3 .
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.It Dv PRI_UNCHANGED
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Yield the thread without changing its priority.
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.El
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.Pp
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The
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.Fn should_yield
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function checks if sufficient time has passed since the thread's last voluntary
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context switch that yielding would be a useful service to other threads.
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It returns
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.Va true
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when this is the case.
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See
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.Sx USAGE NOTES
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for an elaboration of what this means.
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.Pp
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The
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.Fn maybe_yield
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function is a helper function for the common task of optionally yielding the
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processor.
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Internally,
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.Fn kern_yield "PRI_USER"
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will be called if
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.Fn should_yield
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returns
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.Va true .
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.Sh USAGE NOTES
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Although the kernel supports preemption, this is usually reserved for
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high-priority realtime or interrupt threads.
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Kernel worker threads and timesharing threads are not guaranteed to preempt
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each another.
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Thus, threads executing in the kernel are expected to behave cooperatively
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with respect to other threads in the system.
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The yield functions are mostly intended to be used by threads which perform a
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lot of work inside the kernel.
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For example:
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.Fn maybe_yield
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is called by the
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.Dv vlnru
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process each time it reclaims a vnode.
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.Pp
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The scheduler aims to identify threads which monopolize the CPU, and will
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schedule them with decreased priority.
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Threads which regularly yield the processor will be given the chance to run
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more often.
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The possibly surprising effect of this is that, depending on the disposition of
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other threads on the CPU's runqueue, a call to
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.Fn kern_yield
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does not guarantee that the yielding thread will be taken off the CPU.
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.Pp
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With the above considerations in mind, it is advised that code written using
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.Fn kern_yield
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be measured to confirm that its use has a positive effect on relevant
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performance or responsiveness metrics.
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Switching thread contexts has a non-zero cost, and thus yielding the processor
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too eagerly could have a negative impact on performance.
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.Pp
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Additionally, since yielding is a cooperative action, it is advised that the
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yielding thread release any locks that it may be holding, when possible.
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Otherwise, threads which have been given the chance to run could end up waiting
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on the yielding thread to release the lock, largely defeating the purpose of
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the yield.
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.Sh SEE ALSO
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.Xr setpriority 2 ,
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.Xr nice 3 ,
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.Xr mi_switch 9
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.Sh AUTHORS
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.An -nosplit
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This manual page was written by
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.An Mitchell Horne Aq Mt mhorne@FreeBSD.org .
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