2c8bc5c70c
Dcoument the new functions added to condvar(9), sleep(9), sleepqueue(9) KPIs. Also document recent changes in timeout(9) and eventtimers(4).
154 lines
5.3 KiB
Groff
154 lines
5.3 KiB
Groff
.\" Copyright (c) 2010 Alexander Motin <mav@FreeBSD.org>
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.\" All rights reserved.
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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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.\" $FreeBSD$
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.\"
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.Dd March 13, 2012
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.Dt EVENTTIMERS 4
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.Os
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.Sh NAME
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.Nm eventtimers
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.Nd kernel event timers subsystem
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.Sh SYNOPSIS
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Kernel uses several types of time-related devices, such as: real time clocks,
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time counters and event timers.
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Real time clocks responsible for tracking real world time, mostly when system
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is down.
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Time counters are responsible for generation of monotonically increasing
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timestamps for precise uptime tracking purposes, when system is running.
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Event timers are responsible for generating interrupts at specified time or
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periodically, to run different time-based events.
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This page is about the last.
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.Sh DESCRIPTION
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Kernel uses time-based events for many different purposes: scheduling,
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statistics, time keeping, profiling and many other things, based on
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.Xr callout 9
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mechanism.
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These purposes now grouped into three main callbacks:
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.Bl -tag -width ".Fn hardclock"
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.It Fn hardclock
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.Xr callout 9
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and timekeeping events entry.
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Called with frequency defined by
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.Va hz
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variable,
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usually 1000Hz.
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.It Fn statclock
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statistics and scheduler events entry.
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Called with frequency about 128Hz.
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.It Fn profclock
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profiler events entry.
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When enabled, called with frequency about 8KHz.
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.El
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.Pp
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Different platforms provide different kinds of timer hardware.
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The goal of the event timers subsystem is to provide unified way to control
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that hardware, and to use it, supplying kernel with all required time-based
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events.
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.Pp
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Each driver implementing event timers, registers them at the subsystem.
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It is possible to see the list of present event timers, like this, via
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.Va kern.eventtimer
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sysctl:
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.Bd -literal
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kern.eventtimer.choice: HPET(550) LAPIC(400) i8254(100) RTC(0)
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kern.eventtimer.et.LAPIC.flags: 15
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kern.eventtimer.et.LAPIC.frequency: 0
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kern.eventtimer.et.LAPIC.quality: 400
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kern.eventtimer.et.i8254.flags: 1
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kern.eventtimer.et.i8254.frequency: 1193182
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kern.eventtimer.et.i8254.quality: 100
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kern.eventtimer.et.RTC.flags: 17
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kern.eventtimer.et.RTC.frequency: 32768
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kern.eventtimer.et.RTC.quality: 0
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kern.eventtimer.et.HPET.flags: 7
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kern.eventtimer.et.HPET.frequency: 14318180
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kern.eventtimer.et.HPET.quality: 550
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.Ed
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.Pp
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where:
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.Bl -inset
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.It Va kern.eventtimer.et. Ns Ar X Ns Va .flags
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is a
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bitmask, defining event timer capabilities:
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.Bl -tag -offset indent -width indent -compact
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.It 1
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periodic mode supported,
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.It 2
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one-shot mode supported,
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.It 4
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timer is per-CPU,
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.It 8
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timer may stop when CPU goes to sleep state,
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.It 16
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timer supports only power-of-2 divisors.
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.El
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.It Va kern.eventtimer.et. Ns Ar X Ns Va .frequency
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is a
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timer base frequency,
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.It Va kern.eventtimer.et. Ns Ar X Ns Va .quality
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is an
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integral value, defining how good is this timer, comparing to others.
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.El
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.Pp
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Timers management code of the kernel chooses one timer from that list.
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Current choice can be read and affected via
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.Va kern.eventtimer.timer
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tunable/sysctl.
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Several other tunables/sysctls are affecting how exactly this timer is used:
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.Bl -inset
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.It Va kern.eventtimer.periodic
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allows to choose periodic and one-shot operation mode.
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In periodic mode, periodic interrupts from timer hardware are taken as the
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only source of time for time events.
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One-shot mode instead uses currently selected time counter to precisely
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schedule all needed events and programs event timer to generate interrupt
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exactly in specified time.
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Default value depends of chosen timer capabilities, but one-shot mode is
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preferred, until other is forced by user or hardware.
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.It Va kern.eventtimer.singlemul
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in periodic mode specifies how much times higher timer frequency should be,
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to not strictly alias
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.Fn hardclock
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and
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.Fn statclock
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events.
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Default values are
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1, 2 or 4, depending on configured HZ value.
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.It Va kern.eventtimer.idletick
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makes each CPU to receive every timer interrupt independently of whether they
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busy or not.
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By default this options is disabled.
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If chosen timer is per-CPU
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and runs in periodic mode, this option has no effect - all interrupts are
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always generating.
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.El
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.Sh SEE ALSO
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.Xr apic 4 ,
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.Xr atrtc 4 ,
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.Xr attimer 4 ,
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.Xr hpet 4 ,
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.Xr timecounters 4 ,
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.Xr eventtimers 9
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