174 lines
8.4 KiB
ReStructuredText
174 lines
8.4 KiB
ReStructuredText
======================================================
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hrtimers - subsystem for high-resolution kernel timers
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======================================================
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This patch introduces a new subsystem for high-resolution kernel timers.
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One might ask the question: we already have a timer subsystem
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(kernel/timers.c), why do we need two timer subsystems? After a lot of
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back and forth trying to integrate high-resolution and high-precision
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features into the existing timer framework, and after testing various
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such high-resolution timer implementations in practice, we came to the
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conclusion that the timer wheel code is fundamentally not suitable for
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such an approach. We initially didn't believe this ('there must be a way
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to solve this'), and spent a considerable effort trying to integrate
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things into the timer wheel, but we failed. In hindsight, there are
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several reasons why such integration is hard/impossible:
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- the forced handling of low-resolution and high-resolution timers in
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the same way leads to a lot of compromises, macro magic and #ifdef
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mess. The timers.c code is very "tightly coded" around jiffies and
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32-bitness assumptions, and has been honed and micro-optimized for a
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relatively narrow use case (jiffies in a relatively narrow HZ range)
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for many years - and thus even small extensions to it easily break
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the wheel concept, leading to even worse compromises. The timer wheel
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code is very good and tight code, there's zero problems with it in its
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current usage - but it is simply not suitable to be extended for
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high-res timers.
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- the unpredictable [O(N)] overhead of cascading leads to delays which
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necessitate a more complex handling of high resolution timers, which
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in turn decreases robustness. Such a design still leads to rather large
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timing inaccuracies. Cascading is a fundamental property of the timer
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wheel concept, it cannot be 'designed out' without inevitably
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degrading other portions of the timers.c code in an unacceptable way.
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- the implementation of the current posix-timer subsystem on top of
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the timer wheel has already introduced a quite complex handling of
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the required readjusting of absolute CLOCK_REALTIME timers at
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settimeofday or NTP time - further underlying our experience by
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example: that the timer wheel data structure is too rigid for high-res
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timers.
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- the timer wheel code is most optimal for use cases which can be
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identified as "timeouts". Such timeouts are usually set up to cover
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error conditions in various I/O paths, such as networking and block
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I/O. The vast majority of those timers never expire and are rarely
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recascaded because the expected correct event arrives in time so they
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can be removed from the timer wheel before any further processing of
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them becomes necessary. Thus the users of these timeouts can accept
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the granularity and precision tradeoffs of the timer wheel, and
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largely expect the timer subsystem to have near-zero overhead.
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Accurate timing for them is not a core purpose - in fact most of the
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timeout values used are ad-hoc. For them it is at most a necessary
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evil to guarantee the processing of actual timeout completions
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(because most of the timeouts are deleted before completion), which
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should thus be as cheap and unintrusive as possible.
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The primary users of precision timers are user-space applications that
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utilize nanosleep, posix-timers and itimer interfaces. Also, in-kernel
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users like drivers and subsystems which require precise timed events
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(e.g. multimedia) can benefit from the availability of a separate
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high-resolution timer subsystem as well.
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While this subsystem does not offer high-resolution clock sources just
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yet, the hrtimer subsystem can be easily extended with high-resolution
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clock capabilities, and patches for that exist and are maturing quickly.
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The increasing demand for realtime and multimedia applications along
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with other potential users for precise timers gives another reason to
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separate the "timeout" and "precise timer" subsystems.
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Another potential benefit is that such a separation allows even more
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special-purpose optimization of the existing timer wheel for the low
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resolution and low precision use cases - once the precision-sensitive
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APIs are separated from the timer wheel and are migrated over to
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hrtimers. E.g. we could decrease the frequency of the timeout subsystem
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from 250 Hz to 100 HZ (or even smaller).
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hrtimer subsystem implementation details
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----------------------------------------
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the basic design considerations were:
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- simplicity
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- data structure not bound to jiffies or any other granularity. All the
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kernel logic works at 64-bit nanoseconds resolution - no compromises.
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- simplification of existing, timing related kernel code
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another basic requirement was the immediate enqueueing and ordering of
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timers at activation time. After looking at several possible solutions
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such as radix trees and hashes, we chose the red black tree as the basic
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data structure. Rbtrees are available as a library in the kernel and are
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used in various performance-critical areas of e.g. memory management and
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file systems. The rbtree is solely used for time sorted ordering, while
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a separate list is used to give the expiry code fast access to the
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queued timers, without having to walk the rbtree.
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(This separate list is also useful for later when we'll introduce
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high-resolution clocks, where we need separate pending and expired
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queues while keeping the time-order intact.)
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Time-ordered enqueueing is not purely for the purposes of
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high-resolution clocks though, it also simplifies the handling of
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absolute timers based on a low-resolution CLOCK_REALTIME. The existing
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implementation needed to keep an extra list of all armed absolute
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CLOCK_REALTIME timers along with complex locking. In case of
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settimeofday and NTP, all the timers (!) had to be dequeued, the
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time-changing code had to fix them up one by one, and all of them had to
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be enqueued again. The time-ordered enqueueing and the storage of the
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expiry time in absolute time units removes all this complex and poorly
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scaling code from the posix-timer implementation - the clock can simply
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be set without having to touch the rbtree. This also makes the handling
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of posix-timers simpler in general.
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The locking and per-CPU behavior of hrtimers was mostly taken from the
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existing timer wheel code, as it is mature and well suited. Sharing code
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was not really a win, due to the different data structures. Also, the
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hrtimer functions now have clearer behavior and clearer names - such as
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hrtimer_try_to_cancel() and hrtimer_cancel() [which are roughly
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equivalent to timer_delete() and timer_delete_sync()] - so there's no direct
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1:1 mapping between them on the algorithmic level, and thus no real
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potential for code sharing either.
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Basic data types: every time value, absolute or relative, is in a
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special nanosecond-resolution 64bit type: ktime_t.
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(Originally, the kernel-internal representation of ktime_t values and
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operations was implemented via macros and inline functions, and could be
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switched between a "hybrid union" type and a plain "scalar" 64bit
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nanoseconds representation (at compile time). This was abandoned in the
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context of the Y2038 work.)
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hrtimers - rounding of timer values
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-----------------------------------
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the hrtimer code will round timer events to lower-resolution clocks
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because it has to. Otherwise it will do no artificial rounding at all.
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one question is, what resolution value should be returned to the user by
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the clock_getres() interface. This will return whatever real resolution
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a given clock has - be it low-res, high-res, or artificially-low-res.
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hrtimers - testing and verification
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-----------------------------------
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We used the high-resolution clock subsystem on top of hrtimers to verify
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the hrtimer implementation details in praxis, and we also ran the posix
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timer tests in order to ensure specification compliance. We also ran
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tests on low-resolution clocks.
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The hrtimer patch converts the following kernel functionality to use
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hrtimers:
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- nanosleep
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- itimers
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- posix-timers
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The conversion of nanosleep and posix-timers enabled the unification of
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nanosleep and clock_nanosleep.
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The code was successfully compiled for the following platforms:
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i386, x86_64, ARM, PPC, PPC64, IA64
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The code was run-tested on the following platforms:
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i386(UP/SMP), x86_64(UP/SMP), ARM, PPC
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hrtimers were also integrated into the -rt tree, along with a
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hrtimers-based high-resolution clock implementation, so the hrtimers
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code got a healthy amount of testing and use in practice.
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Thomas Gleixner, Ingo Molnar
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