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diff --git a/Documentation/timers/hrtimers.rst b/Documentation/timers/hrtimers.rst new file mode 100644 index 000000000000..c1c20a693e8f --- /dev/null +++ b/Documentation/timers/hrtimers.rst @@ -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 leads to rather large + timing inaccuracies. Cascading is a fundamental property of the timer + wheel concept, it cannot be 'designed out' without inevitably + 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 algorithmic 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 |