[patch 28/46] hrtimers: move and add documentation

[Thomas Gleixner <tglx@linutronix.de>]

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[-- Type: text/plain, Size: 32444 bytes --]

From: Thomas Gleixner <tglx@linutronix.de>

Move the initial hrtimers.txt document to the new directory
"Documentation/hrtimers"

Add design notes for the high resolution timer and dynamic tick
functionality.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 Documentation/hrtimers.txt          |  178 -------------------------
 Documentation/hrtimers/highres.txt  |  249 ++++++++++++++++++++++++++++++++++++
 Documentation/hrtimers/hrtimers.txt |  178 +++++++++++++++++++++++++
 3 files changed, 427 insertions(+), 178 deletions(-)

Index: linux-2.6.20-rc4-mm1-bo/Documentation/hrtimers.txt
===================================================================
--- linux-2.6.20-rc4-mm1-bo.orig/Documentation/hrtimers.txt
+++ /dev/null
@@ -1,178 +0,0 @@
-
-hrtimers - subsystem for high-resolution kernel timers
-----------------------------------------------------
-
-This patch introduces a new subsystem for high-resolution kernel timers.
-
-One might ask the question: we already have a timer subsystem
-(kernel/timers.c), why do we need two timer subsystems? After a lot of
-back and forth trying to integrate high-resolution and high-precision
-features into the existing timer framework, and after testing various
-such high-resolution timer implementations in practice, we came to the
-conclusion that the timer wheel code is fundamentally not suitable for
-such an approach. We initially didn't believe this ('there must be a way
-to solve this'), and spent a considerable effort trying to integrate
-things into the timer wheel, but we failed. In hindsight, there are
-several reasons why such integration is hard/impossible:
-
-- the forced handling of low-resolution and high-resolution timers in
-  the same way leads to a lot of compromises, macro magic and #ifdef
-  mess. The timers.c code is very "tightly coded" around jiffies and
-  32-bitness assumptions, and has been honed and micro-optimized for a
-  relatively narrow use case (jiffies in a relatively narrow HZ range)
-  for many years - and thus even small extensions to it easily break
-  the wheel concept, leading to even worse compromises. The timer wheel
-  code is very good and tight code, there's zero problems with it in its
-  current usage - but it is simply not suitable to be extended for
-  high-res timers.
-
-- the unpredictable [O(N)] overhead of cascading leads to delays which
-  necessitate a more complex handling of high resolution timers, which
-  in turn decreases robustness. Such a design still led to rather large
-  timing inaccuracies. Cascading is a fundamental property of the timer
-  wheel concept, it cannot be 'designed out' without unevitably
-  degrading other portions of the timers.c code in an unacceptable way.
-
-- the implementation of the current posix-timer subsystem on top of
-  the timer wheel has already introduced a quite complex handling of
-  the required readjusting of absolute CLOCK_REALTIME timers at
-  settimeofday or NTP time - further underlying our experience by
-  example: that the timer wheel data structure is too rigid for high-res
-  timers.
-
-- the timer wheel code is most optimal for use cases which can be
-  identified as "timeouts". Such timeouts are usually set up to cover
-  error conditions in various I/O paths, such as networking and block
-  I/O. The vast majority of those timers never expire and are rarely
-  recascaded because the expected correct event arrives in time so they
-  can be removed from the timer wheel before any further processing of
-  them becomes necessary. Thus the users of these timeouts can accept
-  the granularity and precision tradeoffs of the timer wheel, and
-  largely expect the timer subsystem to have near-zero overhead.
-  Accurate timing for them is not a core purpose - in fact most of the
-  timeout values used are ad-hoc. For them it is at most a necessary
-  evil to guarantee the processing of actual timeout completions
-  (because most of the timeouts are deleted before completion), which
-  should thus be as cheap and unintrusive as possible.
-
-The primary users of precision timers are user-space applications that
-utilize nanosleep, posix-timers and itimer interfaces. Also, in-kernel
-users like drivers and subsystems which require precise timed events
-(e.g. multimedia) can benefit from the availability of a separate
-high-resolution timer subsystem as well.
-
-While this subsystem does not offer high-resolution clock sources just
-yet, the hrtimer subsystem can be easily extended with high-resolution
-clock capabilities, and patches for that exist and are maturing quickly.
-The increasing demand for realtime and multimedia applications along
-with other potential users for precise timers gives another reason to
-separate the "timeout" and "precise timer" subsystems.
-
-Another potential benefit is that such a separation allows even more
-special-purpose optimization of the existing timer wheel for the low
-resolution and low precision use cases - once the precision-sensitive
-APIs are separated from the timer wheel and are migrated over to
-hrtimers. E.g. we could decrease the frequency of the timeout subsystem
-from 250 Hz to 100 HZ (or even smaller).
-
-hrtimer subsystem implementation details
-----------------------------------------
-
-the basic design considerations were:
-
-- simplicity
-
-- data structure not bound to jiffies or any other granularity. All the
-  kernel logic works at 64-bit nanoseconds resolution - no compromises.
-
-- simplification of existing, timing related kernel code
-
-another basic requirement was the immediate enqueueing and ordering of
-timers at activation time. After looking at several possible solutions
-such as radix trees and hashes, we chose the red black tree as the basic
-data structure. Rbtrees are available as a library in the kernel and are
-used in various performance-critical areas of e.g. memory management and
-file systems. The rbtree is solely used for time sorted ordering, while
-a separate list is used to give the expiry code fast access to the
-queued timers, without having to walk the rbtree.
-
-(This separate list is also useful for later when we'll introduce
-high-resolution clocks, where we need separate pending and expired
-queues while keeping the time-order intact.)
-
-Time-ordered enqueueing is not purely for the purposes of
-high-resolution clocks though, it also simplifies the handling of
-absolute timers based on a low-resolution CLOCK_REALTIME. The existing
-implementation needed to keep an extra list of all armed absolute
-CLOCK_REALTIME timers along with complex locking. In case of
-settimeofday and NTP, all the timers (!) had to be dequeued, the
-time-changing code had to fix them up one by one, and all of them had to
-be enqueued again. The time-ordered enqueueing and the storage of the
-expiry time in absolute time units removes all this complex and poorly
-scaling code from the posix-timer implementation - the clock can simply
-be set without having to touch the rbtree. This also makes the handling
-of posix-timers simpler in general.
-
-The locking and per-CPU behavior of hrtimers was mostly taken from the
-existing timer wheel code, as it is mature and well suited. Sharing code
-was not really a win, due to the different data structures. Also, the
-hrtimer functions now have clearer behavior and clearer names - such as
-hrtimer_try_to_cancel() and hrtimer_cancel() [which are roughly
-equivalent to del_timer() and del_timer_sync()] - so there's no direct
-1:1 mapping between them on the algorithmical level, and thus no real
-potential for code sharing either.
-
-Basic data types: every time value, absolute or relative, is in a
-special nanosecond-resolution type: ktime_t. The kernel-internal
-representation of ktime_t values and operations is implemented via
-macros and inline functions, and can be switched between a "hybrid
-union" type and a plain "scalar" 64bit nanoseconds representation (at
-compile time). The hybrid union type optimizes time conversions on 32bit
-CPUs. This build-time-selectable ktime_t storage format was implemented
-to avoid the performance impact of 64-bit multiplications and divisions
-on 32bit CPUs. Such operations are frequently necessary to convert
-between the storage formats provided by kernel and userspace interfaces
-and the internal time format. (See include/linux/ktime.h for further
-details.)
-
-hrtimers - rounding of timer values
------------------------------------
-
-the hrtimer code will round timer events to lower-resolution clocks
-because it has to. Otherwise it will do no artificial rounding at all.
-
-one question is, what resolution value should be returned to the user by
-the clock_getres() interface. This will return whatever real resolution
-a given clock has - be it low-res, high-res, or artificially-low-res.
-
-hrtimers - testing and verification
-----------------------------------
-
-We used the high-resolution clock subsystem ontop of hrtimers to verify
-the hrtimer implementation details in praxis, and we also ran the posix
-timer tests in order to ensure specification compliance. We also ran
-tests on low-resolution clocks.
-
-The hrtimer patch converts the following kernel functionality to use
-hrtimers:
-
- - nanosleep
- - itimers
- - posix-timers
-
-The conversion of nanosleep and posix-timers enabled the unification of
-nanosleep and clock_nanosleep.
-
-The code was successfully compiled for the following platforms:
-
- i386, x86_64, ARM, PPC, PPC64, IA64
-
-The code was run-tested on the following platforms:
-
- i386(UP/SMP), x86_64(UP/SMP), ARM, PPC
-
-hrtimers were also integrated into the -rt tree, along with a
-hrtimers-based high-resolution clock implementation, so the hrtimers
-code got a healthy amount of testing and use in practice.
-
-	Thomas Gleixner, Ingo Molnar
Index: linux-2.6.20-rc4-mm1-bo/Documentation/hrtimers/highres.txt
===================================================================
--- /dev/null
+++ linux-2.6.20-rc4-mm1-bo/Documentation/hrtimers/highres.txt
@@ -0,0 +1,249 @@
+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:
+http://www.linuxsymposium.org/2006/linuxsymposium_procv1.pdf
+
+The slides to this talk are available from:
+http://tglx.de/projects/hrtimers/ols2006-hrtimers.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/hrtimers/hrtimer.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 assignes one or more of the folliwing 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 functionalites 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
+
+
+
Index: linux-2.6.20-rc4-mm1-bo/Documentation/hrtimers/hrtimers.txt
===================================================================
--- /dev/null
+++ linux-2.6.20-rc4-mm1-bo/Documentation/hrtimers/hrtimers.txt
@@ -0,0 +1,178 @@
+
+hrtimers - subsystem for high-resolution kernel timers
+----------------------------------------------------
+
+This patch introduces a new subsystem for high-resolution kernel timers.
+
+One might ask the question: we already have a timer subsystem
+(kernel/timers.c), why do we need two timer subsystems? After a lot of
+back and forth trying to integrate high-resolution and high-precision
+features into the existing timer framework, and after testing various
+such high-resolution timer implementations in practice, we came to the
+conclusion that the timer wheel code is fundamentally not suitable for
+such an approach. We initially didn't believe this ('there must be a way
+to solve this'), and spent a considerable effort trying to integrate
+things into the timer wheel, but we failed. In hindsight, there are
+several reasons why such integration is hard/impossible:
+
+- the forced handling of low-resolution and high-resolution timers in
+  the same way leads to a lot of compromises, macro magic and #ifdef
+  mess. The timers.c code is very "tightly coded" around jiffies and
+  32-bitness assumptions, and has been honed and micro-optimized for a
+  relatively narrow use case (jiffies in a relatively narrow HZ range)
+  for many years - and thus even small extensions to it easily break
+  the wheel concept, leading to even worse compromises. The timer wheel
+  code is very good and tight code, there's zero problems with it in its
+  current usage - but it is simply not suitable to be extended for
+  high-res timers.
+
+- the unpredictable [O(N)] overhead of cascading leads to delays which
+  necessitate a more complex handling of high resolution timers, which
+  in turn decreases robustness. Such a design still led to rather large
+  timing inaccuracies. Cascading is a fundamental property of the timer
+  wheel concept, it cannot be 'designed out' without unevitably
+  degrading other portions of the timers.c code in an unacceptable way.
+
+- the implementation of the current posix-timer subsystem on top of
+  the timer wheel has already introduced a quite complex handling of
+  the required readjusting of absolute CLOCK_REALTIME timers at
+  settimeofday or NTP time - further underlying our experience by
+  example: that the timer wheel data structure is too rigid for high-res
+  timers.
+
+- the timer wheel code is most optimal for use cases which can be
+  identified as "timeouts". Such timeouts are usually set up to cover
+  error conditions in various I/O paths, such as networking and block
+  I/O. The vast majority of those timers never expire and are rarely
+  recascaded because the expected correct event arrives in time so they
+  can be removed from the timer wheel before any further processing of
+  them becomes necessary. Thus the users of these timeouts can accept
+  the granularity and precision tradeoffs of the timer wheel, and
+  largely expect the timer subsystem to have near-zero overhead.
+  Accurate timing for them is not a core purpose - in fact most of the
+  timeout values used are ad-hoc. For them it is at most a necessary
+  evil to guarantee the processing of actual timeout completions
+  (because most of the timeouts are deleted before completion), which
+  should thus be as cheap and unintrusive as possible.
+
+The primary users of precision timers are user-space applications that
+utilize nanosleep, posix-timers and itimer interfaces. Also, in-kernel
+users like drivers and subsystems which require precise timed events
+(e.g. multimedia) can benefit from the availability of a separate
+high-resolution timer subsystem as well.
+
+While this subsystem does not offer high-resolution clock sources just
+yet, the hrtimer subsystem can be easily extended with high-resolution
+clock capabilities, and patches for that exist and are maturing quickly.
+The increasing demand for realtime and multimedia applications along
+with other potential users for precise timers gives another reason to
+separate the "timeout" and "precise timer" subsystems.
+
+Another potential benefit is that such a separation allows even more
+special-purpose optimization of the existing timer wheel for the low
+resolution and low precision use cases - once the precision-sensitive
+APIs are separated from the timer wheel and are migrated over to
+hrtimers. E.g. we could decrease the frequency of the timeout subsystem
+from 250 Hz to 100 HZ (or even smaller).
+
+hrtimer subsystem implementation details
+----------------------------------------
+
+the basic design considerations were:
+
+- simplicity
+
+- data structure not bound to jiffies or any other granularity. All the
+  kernel logic works at 64-bit nanoseconds resolution - no compromises.
+
+- simplification of existing, timing related kernel code
+
+another basic requirement was the immediate enqueueing and ordering of
+timers at activation time. After looking at several possible solutions
+such as radix trees and hashes, we chose the red black tree as the basic
+data structure. Rbtrees are available as a library in the kernel and are
+used in various performance-critical areas of e.g. memory management and
+file systems. The rbtree is solely used for time sorted ordering, while
+a separate list is used to give the expiry code fast access to the
+queued timers, without having to walk the rbtree.
+
+(This separate list is also useful for later when we'll introduce
+high-resolution clocks, where we need separate pending and expired
+queues while keeping the time-order intact.)
+
+Time-ordered enqueueing is not purely for the purposes of
+high-resolution clocks though, it also simplifies the handling of
+absolute timers based on a low-resolution CLOCK_REALTIME. The existing
+implementation needed to keep an extra list of all armed absolute
+CLOCK_REALTIME timers along with complex locking. In case of
+settimeofday and NTP, all the timers (!) had to be dequeued, the
+time-changing code had to fix them up one by one, and all of them had to
+be enqueued again. The time-ordered enqueueing and the storage of the
+expiry time in absolute time units removes all this complex and poorly
+scaling code from the posix-timer implementation - the clock can simply
+be set without having to touch the rbtree. This also makes the handling
+of posix-timers simpler in general.
+
+The locking and per-CPU behavior of hrtimers was mostly taken from the
+existing timer wheel code, as it is mature and well suited. Sharing code
+was not really a win, due to the different data structures. Also, the
+hrtimer functions now have clearer behavior and clearer names - such as
+hrtimer_try_to_cancel() and hrtimer_cancel() [which are roughly
+equivalent to del_timer() and del_timer_sync()] - so there's no direct
+1:1 mapping between them on the algorithmical level, and thus no real
+potential for code sharing either.
+
+Basic data types: every time value, absolute or relative, is in a
+special nanosecond-resolution type: ktime_t. The kernel-internal
+representation of ktime_t values and operations is implemented via
+macros and inline functions, and can be switched between a "hybrid
+union" type and a plain "scalar" 64bit nanoseconds representation (at
+compile time). The hybrid union type optimizes time conversions on 32bit
+CPUs. This build-time-selectable ktime_t storage format was implemented
+to avoid the performance impact of 64-bit multiplications and divisions
+on 32bit CPUs. Such operations are frequently necessary to convert
+between the storage formats provided by kernel and userspace interfaces
+and the internal time format. (See include/linux/ktime.h for further
+details.)
+
+hrtimers - rounding of timer values
+-----------------------------------
+
+the hrtimer code will round timer events to lower-resolution clocks
+because it has to. Otherwise it will do no artificial rounding at all.
+
+one question is, what resolution value should be returned to the user by
+the clock_getres() interface. This will return whatever real resolution
+a given clock has - be it low-res, high-res, or artificially-low-res.
+
+hrtimers - testing and verification
+----------------------------------
+
+We used the high-resolution clock subsystem ontop of hrtimers to verify
+the hrtimer implementation details in praxis, and we also ran the posix
+timer tests in order to ensure specification compliance. We also ran
+tests on low-resolution clocks.
+
+The hrtimer patch converts the following kernel functionality to use
+hrtimers:
+
+ - nanosleep
+ - itimers
+ - posix-timers
+
+The conversion of nanosleep and posix-timers enabled the unification of
+nanosleep and clock_nanosleep.
+
+The code was successfully compiled for the following platforms:
+
+ i386, x86_64, ARM, PPC, PPC64, IA64
+
+The code was run-tested on the following platforms:
+
+ i386(UP/SMP), x86_64(UP/SMP), ARM, PPC
+
+hrtimers were also integrated into the -rt tree, along with a
+hrtimers-based high-resolution clock implementation, so the hrtimers
+code got a healthy amount of testing and use in practice.
+
+	Thomas Gleixner, Ingo Molnar

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