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-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:
-official http://tglx.de/projects/hrtimers/ols2006-hrtimers.pdf
-[[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 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:
-https://www.kernel.org/doc/ols/2005/ols2005v1-pages-227-240.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/stats 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

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