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-
-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