162 lines
5.7 KiB
Groff
162 lines
5.7 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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.It Va kern.eventtimer.activetick
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makes each CPU to receive all kinds of timer interrupts when they are busy.
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Disabling it allows to skip some
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.Fn hardclock
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calls in some cases.
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By default this options is enabled.
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If chosen timer is per-CPU, this option has no effect - all interrupts are
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always generating, as timer reprogramming is too expensive for that case.
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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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