|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:
|The slides to this talk are available from:
|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
| - 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:
|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
| - 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
|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
|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
|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
|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 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() 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/stats and can be made available for enhanced power
|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