mirror of https://gitee.com/openkylin/linux.git
250 lines
12 KiB
Plaintext
250 lines
12 KiB
Plaintext
High resolution timers and dynamic ticks design notes
|
|
-----------------------------------------------------
|
|
|
|
Further information can be found in the paper of the OLS 2006 talk "hrtimers
|
|
and beyond". The paper is part of the OLS 2006 Proceedings Volume 1, which can
|
|
be found on the OLS website:
|
|
https://www.kernel.org/doc/ols/2006/ols2006v1-pages-333-346.pdf
|
|
|
|
The slides to this talk are available from:
|
|
http://www.cs.columbia.edu/~nahum/w6998/papers/ols2006-hrtimers-slides.pdf
|
|
|
|
The slides contain five figures (pages 2, 15, 18, 20, 22), which illustrate the
|
|
changes in the time(r) related Linux subsystems. Figure #1 (p. 2) shows the
|
|
design of the Linux time(r) system before hrtimers and other building blocks
|
|
got merged into mainline.
|
|
|
|
Note: the paper and the slides are talking about "clock event source", while we
|
|
switched to the name "clock event devices" in meantime.
|
|
|
|
The design contains the following basic building blocks:
|
|
|
|
- hrtimer base infrastructure
|
|
- timeofday and clock source management
|
|
- clock event management
|
|
- high resolution timer functionality
|
|
- dynamic ticks
|
|
|
|
|
|
hrtimer base infrastructure
|
|
---------------------------
|
|
|
|
The hrtimer base infrastructure was merged into the 2.6.16 kernel. Details of
|
|
the base implementation are covered in Documentation/timers/hrtimers.txt. See
|
|
also figure #2 (OLS slides p. 15)
|
|
|
|
The main differences to the timer wheel, which holds the armed timer_list type
|
|
timers are:
|
|
- time ordered enqueueing into a rb-tree
|
|
- independent of ticks (the processing is based on nanoseconds)
|
|
|
|
|
|
timeofday and clock source management
|
|
-------------------------------------
|
|
|
|
John Stultz's Generic Time Of Day (GTOD) framework moves a large portion of
|
|
code out of the architecture-specific areas into a generic management
|
|
framework, as illustrated in figure #3 (OLS slides p. 18). The architecture
|
|
specific portion is reduced to the low level hardware details of the clock
|
|
sources, which are registered in the framework and selected on a quality based
|
|
decision. The low level code provides hardware setup and readout routines and
|
|
initializes data structures, which are used by the generic time keeping code to
|
|
convert the clock ticks to nanosecond based time values. All other time keeping
|
|
related functionality is moved into the generic code. The GTOD base patch got
|
|
merged into the 2.6.18 kernel.
|
|
|
|
Further information about the Generic Time Of Day framework is available in the
|
|
OLS 2005 Proceedings Volume 1:
|
|
http://www.linuxsymposium.org/2005/linuxsymposium_procv1.pdf
|
|
|
|
The paper "We Are Not Getting Any Younger: A New Approach to Time and
|
|
Timers" was written by J. Stultz, D.V. Hart, & N. Aravamudan.
|
|
|
|
Figure #3 (OLS slides p.18) illustrates the transformation.
|
|
|
|
|
|
clock event management
|
|
----------------------
|
|
|
|
While clock sources provide read access to the monotonically increasing time
|
|
value, clock event devices are used to schedule the next event
|
|
interrupt(s). The next event is currently defined to be periodic, with its
|
|
period defined at compile time. The setup and selection of the event device
|
|
for various event driven functionalities is hardwired into the architecture
|
|
dependent code. This results in duplicated code across all architectures and
|
|
makes it extremely difficult to change the configuration of the system to use
|
|
event interrupt devices other than those already built into the
|
|
architecture. Another implication of the current design is that it is necessary
|
|
to touch all the architecture-specific implementations in order to provide new
|
|
functionality like high resolution timers or dynamic ticks.
|
|
|
|
The clock events subsystem tries to address this problem by providing a generic
|
|
solution to manage clock event devices and their usage for the various clock
|
|
event driven kernel functionalities. The goal of the clock event subsystem is
|
|
to minimize the clock event related architecture dependent code to the pure
|
|
hardware related handling and to allow easy addition and utilization of new
|
|
clock event devices. It also minimizes the duplicated code across the
|
|
architectures as it provides generic functionality down to the interrupt
|
|
service handler, which is almost inherently hardware dependent.
|
|
|
|
Clock event devices are registered either by the architecture dependent boot
|
|
code or at module insertion time. Each clock event device fills a data
|
|
structure with clock-specific property parameters and callback functions. The
|
|
clock event management decides, by using the specified property parameters, the
|
|
set of system functions a clock event device will be used to support. This
|
|
includes the distinction of per-CPU and per-system global event devices.
|
|
|
|
System-level global event devices are used for the Linux periodic tick. Per-CPU
|
|
event devices are used to provide local CPU functionality such as process
|
|
accounting, profiling, and high resolution timers.
|
|
|
|
The management layer assigns one or more of the following functions to a clock
|
|
event device:
|
|
- system global periodic tick (jiffies update)
|
|
- cpu local update_process_times
|
|
- cpu local profiling
|
|
- cpu local next event interrupt (non periodic mode)
|
|
|
|
The clock event device delegates the selection of those timer interrupt related
|
|
functions completely to the management layer. The clock management layer stores
|
|
a function pointer in the device description structure, which has to be called
|
|
from the hardware level handler. This removes a lot of duplicated code from the
|
|
architecture specific timer interrupt handlers and hands the control over the
|
|
clock event devices and the assignment of timer interrupt related functionality
|
|
to the core code.
|
|
|
|
The clock event layer API is rather small. Aside from the clock event device
|
|
registration interface it provides functions to schedule the next event
|
|
interrupt, clock event device notification service and support for suspend and
|
|
resume.
|
|
|
|
The framework adds about 700 lines of code which results in a 2KB increase of
|
|
the kernel binary size. The conversion of i386 removes about 100 lines of
|
|
code. The binary size decrease is in the range of 400 byte. We believe that the
|
|
increase of flexibility and the avoidance of duplicated code across
|
|
architectures justifies the slight increase of the binary size.
|
|
|
|
The conversion of an architecture has no functional impact, but allows to
|
|
utilize the high resolution and dynamic tick functionalities without any change
|
|
to the clock event device and timer interrupt code. After the conversion the
|
|
enabling of high resolution timers and dynamic ticks is simply provided by
|
|
adding the kernel/time/Kconfig file to the architecture specific Kconfig and
|
|
adding the dynamic tick specific calls to the idle routine (a total of 3 lines
|
|
added to the idle function and the Kconfig file)
|
|
|
|
Figure #4 (OLS slides p.20) illustrates the transformation.
|
|
|
|
|
|
high resolution timer functionality
|
|
-----------------------------------
|
|
|
|
During system boot it is not possible to use the high resolution timer
|
|
functionality, while making it possible would be difficult and would serve no
|
|
useful function. The initialization of the clock event device framework, the
|
|
clock source framework (GTOD) and hrtimers itself has to be done and
|
|
appropriate clock sources and clock event devices have to be registered before
|
|
the high resolution functionality can work. Up to the point where hrtimers are
|
|
initialized, the system works in the usual low resolution periodic mode. The
|
|
clock source and the clock event device layers provide notification functions
|
|
which inform hrtimers about availability of new hardware. hrtimers validates
|
|
the usability of the registered clock sources and clock event devices before
|
|
switching to high resolution mode. This ensures also that a kernel which is
|
|
configured for high resolution timers can run on a system which lacks the
|
|
necessary hardware support.
|
|
|
|
The high resolution timer code does not support SMP machines which have only
|
|
global clock event devices. The support of such hardware would involve IPI
|
|
calls when an interrupt happens. The overhead would be much larger than the
|
|
benefit. This is the reason why we currently disable high resolution and
|
|
dynamic ticks on i386 SMP systems which stop the local APIC in C3 power
|
|
state. A workaround is available as an idea, but the problem has not been
|
|
tackled yet.
|
|
|
|
The time ordered insertion of timers provides all the infrastructure to decide
|
|
whether the event device has to be reprogrammed when a timer is added. The
|
|
decision is made per timer base and synchronized across per-cpu timer bases in
|
|
a support function. The design allows the system to utilize separate per-CPU
|
|
clock event devices for the per-CPU timer bases, but currently only one
|
|
reprogrammable clock event device per-CPU is utilized.
|
|
|
|
When the timer interrupt happens, the next event interrupt handler is called
|
|
from the clock event distribution code and moves expired timers from the
|
|
red-black tree to a separate double linked list and invokes the softirq
|
|
handler. An additional mode field in the hrtimer structure allows the system to
|
|
execute callback functions directly from the next event interrupt handler. This
|
|
is restricted to code which can safely be executed in the hard interrupt
|
|
context. This applies, for example, to the common case of a wakeup function as
|
|
used by nanosleep. The advantage of executing the handler in the interrupt
|
|
context is the avoidance of up to two context switches - from the interrupted
|
|
context to the softirq and to the task which is woken up by the expired
|
|
timer.
|
|
|
|
Once a system has switched to high resolution mode, the periodic tick is
|
|
switched off. This disables the per system global periodic clock event device -
|
|
e.g. the PIT on i386 SMP systems.
|
|
|
|
The periodic tick functionality is provided by an per-cpu hrtimer. The callback
|
|
function is executed in the next event interrupt context and updates jiffies
|
|
and calls update_process_times and profiling. The implementation of the hrtimer
|
|
based periodic tick is designed to be extended with dynamic tick functionality.
|
|
This allows to use a single clock event device to schedule high resolution
|
|
timer and periodic events (jiffies tick, profiling, process accounting) on UP
|
|
systems. This has been proved to work with the PIT on i386 and the Incrementer
|
|
on PPC.
|
|
|
|
The softirq for running the hrtimer queues and executing the callbacks has been
|
|
separated from the tick bound timer softirq to allow accurate delivery of high
|
|
resolution timer signals which are used by itimer and POSIX interval
|
|
timers. The execution of this softirq can still be delayed by other softirqs,
|
|
but the overall latencies have been significantly improved by this separation.
|
|
|
|
Figure #5 (OLS slides p.22) illustrates the transformation.
|
|
|
|
|
|
dynamic ticks
|
|
-------------
|
|
|
|
Dynamic ticks are the logical consequence of the hrtimer based periodic tick
|
|
replacement (sched_tick). The functionality of the sched_tick hrtimer is
|
|
extended by three functions:
|
|
|
|
- hrtimer_stop_sched_tick
|
|
- hrtimer_restart_sched_tick
|
|
- hrtimer_update_jiffies
|
|
|
|
hrtimer_stop_sched_tick() is called when a CPU goes into idle state. The code
|
|
evaluates the next scheduled timer event (from both hrtimers and the timer
|
|
wheel) and in case that the next event is further away than the next tick it
|
|
reprograms the sched_tick to this future event, to allow longer idle sleeps
|
|
without worthless interruption by the periodic tick. The function is also
|
|
called when an interrupt happens during the idle period, which does not cause a
|
|
reschedule. The call is necessary as the interrupt handler might have armed a
|
|
new timer whose expiry time is before the time which was identified as the
|
|
nearest event in the previous call to hrtimer_stop_sched_tick.
|
|
|
|
hrtimer_restart_sched_tick() is called when the CPU leaves the idle state before
|
|
it calls schedule(). hrtimer_restart_sched_tick() resumes the periodic tick,
|
|
which is kept active until the next call to hrtimer_stop_sched_tick().
|
|
|
|
hrtimer_update_jiffies() is called from irq_enter() when an interrupt happens
|
|
in the idle period to make sure that jiffies are up to date and the interrupt
|
|
handler has not to deal with an eventually stale jiffy value.
|
|
|
|
The dynamic tick feature provides statistical values which are exported to
|
|
userspace via /proc/stat and can be made available for enhanced power
|
|
management control.
|
|
|
|
The implementation leaves room for further development like full tickless
|
|
systems, where the time slice is controlled by the scheduler, variable
|
|
frequency profiling, and a complete removal of jiffies in the future.
|
|
|
|
|
|
Aside the current initial submission of i386 support, the patchset has been
|
|
extended to x86_64 and ARM already. Initial (work in progress) support is also
|
|
available for MIPS and PowerPC.
|
|
|
|
Thomas, Ingo
|
|
|
|
|
|
|