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1 High resolution timers and dynamic ticks design notes 2 ----------------------------------------------------- 3 4 Further information can be found in the paper of the OLS 2006 talk "hrtimers 5 and beyond". The paper is part of the OLS 2006 Proceedings Volume 1, which can 6 be found on the OLS website: 7 http://www.linuxsymposium.org/2006/linuxsymposium_procv1.pdf 8 9 The slides to this talk are available from: 10 http://tglx.de/projects/hrtimers/ols2006-hrtimers.pdf 11 12 The slides contain five figures (pages 2, 15, 18, 20, 22), which illustrate the 13 changes in the time(r) related Linux subsystems. Figure #1 (p. 2) shows the 14 design of the Linux time(r) system before hrtimers and other building blocks 15 got merged into mainline. 16 17 Note: the paper and the slides are talking about "clock event source", while we 18 switched to the name "clock event devices" in meantime. 19 20 The design contains the following basic building blocks: 21 22 - hrtimer base infrastructure 23 - timeofday and clock source management 24 - clock event management 25 - high resolution timer functionality 26 - dynamic ticks 27 28 29 hrtimer base infrastructure 30 --------------------------- 31 32 The hrtimer base infrastructure was merged into the 2.6.16 kernel. Details of 33 the base implementation are covered in Documentation/timers/hrtimers.txt. See 34 also figure #2 (OLS slides p. 15) 35 36 The main differences to the timer wheel, which holds the armed timer_list type 37 timers are: 38 - time ordered enqueueing into a rb-tree 39 - independent of ticks (the processing is based on nanoseconds) 40 41 42 timeofday and clock source management 43 ------------------------------------- 44 45 John Stultz's Generic Time Of Day (GTOD) framework moves a large portion of 46 code out of the architecture-specific areas into a generic management 47 framework, as illustrated in figure #3 (OLS slides p. 18). The architecture 48 specific portion is reduced to the low level hardware details of the clock 49 sources, which are registered in the framework and selected on a quality based 50 decision. The low level code provides hardware setup and readout routines and 51 initializes data structures, which are used by the generic time keeping code to 52 convert the clock ticks to nanosecond based time values. All other time keeping 53 related functionality is moved into the generic code. The GTOD base patch got 54 merged into the 2.6.18 kernel. 55 56 Further information about the Generic Time Of Day framework is available in the 57 OLS 2005 Proceedings Volume 1: 58 http://www.linuxsymposium.org/2005/linuxsymposium_procv1.pdf 59 60 The paper "We Are Not Getting Any Younger: A New Approach to Time and 61 Timers" was written by J. Stultz, D.V. Hart, & N. Aravamudan. 62 63 Figure #3 (OLS slides p.18) illustrates the transformation. 64 65 66 clock event management 67 ---------------------- 68 69 While clock sources provide read access to the monotonically increasing time 70 value, clock event devices are used to schedule the next event 71 interrupt(s). The next event is currently defined to be periodic, with its 72 period defined at compile time. The setup and selection of the event device 73 for various event driven functionalities is hardwired into the architecture 74 dependent code. This results in duplicated code across all architectures and 75 makes it extremely difficult to change the configuration of the system to use 76 event interrupt devices other than those already built into the 77 architecture. Another implication of the current design is that it is necessary 78 to touch all the architecture-specific implementations in order to provide new 79 functionality like high resolution timers or dynamic ticks. 80 81 The clock events subsystem tries to address this problem by providing a generic 82 solution to manage clock event devices and their usage for the various clock 83 event driven kernel functionalities. The goal of the clock event subsystem is 84 to minimize the clock event related architecture dependent code to the pure 85 hardware related handling and to allow easy addition and utilization of new 86 clock event devices. It also minimizes the duplicated code across the 87 architectures as it provides generic functionality down to the interrupt 88 service handler, which is almost inherently hardware dependent. 89 90 Clock event devices are registered either by the architecture dependent boot 91 code or at module insertion time. Each clock event device fills a data 92 structure with clock-specific property parameters and callback functions. The 93 clock event management decides, by using the specified property parameters, the 94 set of system functions a clock event device will be used to support. This 95 includes the distinction of per-CPU and per-system global event devices. 96 97 System-level global event devices are used for the Linux periodic tick. Per-CPU 98 event devices are used to provide local CPU functionality such as process 99 accounting, profiling, and high resolution timers. 100 101 The management layer assigns one or more of the following functions to a clock 102 event device: 103 - system global periodic tick (jiffies update) 104 - cpu local update_process_times 105 - cpu local profiling 106 - cpu local next event interrupt (non periodic mode) 107 108 The clock event device delegates the selection of those timer interrupt related 109 functions completely to the management layer. The clock management layer stores 110 a function pointer in the device description structure, which has to be called 111 from the hardware level handler. This removes a lot of duplicated code from the 112 architecture specific timer interrupt handlers and hands the control over the 113 clock event devices and the assignment of timer interrupt related functionality 114 to the core code. 115 116 The clock event layer API is rather small. Aside from the clock event device 117 registration interface it provides functions to schedule the next event 118 interrupt, clock event device notification service and support for suspend and 119 resume. 120 121 The framework adds about 700 lines of code which results in a 2KB increase of 122 the kernel binary size. The conversion of i386 removes about 100 lines of 123 code. The binary size decrease is in the range of 400 byte. We believe that the 124 increase of flexibility and the avoidance of duplicated code across 125 architectures justifies the slight increase of the binary size. 126 127 The conversion of an architecture has no functional impact, but allows to 128 utilize the high resolution and dynamic tick functionalities without any change 129 to the clock event device and timer interrupt code. After the conversion the 130 enabling of high resolution timers and dynamic ticks is simply provided by 131 adding the kernel/time/Kconfig file to the architecture specific Kconfig and 132 adding the dynamic tick specific calls to the idle routine (a total of 3 lines 133 added to the idle function and the Kconfig file) 134 135 Figure #4 (OLS slides p.20) illustrates the transformation. 136 137 138 high resolution timer functionality 139 ----------------------------------- 140 141 During system boot it is not possible to use the high resolution timer 142 functionality, while making it possible would be difficult and would serve no 143 useful function. The initialization of the clock event device framework, the 144 clock source framework (GTOD) and hrtimers itself has to be done and 145 appropriate clock sources and clock event devices have to be registered before 146 the high resolution functionality can work. Up to the point where hrtimers are 147 initialized, the system works in the usual low resolution periodic mode. The 148 clock source and the clock event device layers provide notification functions 149 which inform hrtimers about availability of new hardware. hrtimers validates 150 the usability of the registered clock sources and clock event devices before 151 switching to high resolution mode. This ensures also that a kernel which is 152 configured for high resolution timers can run on a system which lacks the 153 necessary hardware support. 154 155 The high resolution timer code does not support SMP machines which have only 156 global clock event devices. The support of such hardware would involve IPI 157 calls when an interrupt happens. The overhead would be much larger than the 158 benefit. This is the reason why we currently disable high resolution and 159 dynamic ticks on i386 SMP systems which stop the local APIC in C3 power 160 state. A workaround is available as an idea, but the problem has not been 161 tackled yet. 162 163 The time ordered insertion of timers provides all the infrastructure to decide 164 whether the event device has to be reprogrammed when a timer is added. The 165 decision is made per timer base and synchronized across per-cpu timer bases in 166 a support function. The design allows the system to utilize separate per-CPU 167 clock event devices for the per-CPU timer bases, but currently only one 168 reprogrammable clock event device per-CPU is utilized. 169 170 When the timer interrupt happens, the next event interrupt handler is called 171 from the clock event distribution code and moves expired timers from the 172 red-black tree to a separate double linked list and invokes the softirq 173 handler. An additional mode field in the hrtimer structure allows the system to 174 execute callback functions directly from the next event interrupt handler. This 175 is restricted to code which can safely be executed in the hard interrupt 176 context. This applies, for example, to the common case of a wakeup function as 177 used by nanosleep. The advantage of executing the handler in the interrupt 178 context is the avoidance of up to two context switches - from the interrupted 179 context to the softirq and to the task which is woken up by the expired 180 timer. 181 182 Once a system has switched to high resolution mode, the periodic tick is 183 switched off. This disables the per system global periodic clock event device - 184 e.g. the PIT on i386 SMP systems. 185 186 The periodic tick functionality is provided by an per-cpu hrtimer. The callback 187 function is executed in the next event interrupt context and updates jiffies 188 and calls update_process_times and profiling. The implementation of the hrtimer 189 based periodic tick is designed to be extended with dynamic tick functionality. 190 This allows to use a single clock event device to schedule high resolution 191 timer and periodic events (jiffies tick, profiling, process accounting) on UP 192 systems. This has been proved to work with the PIT on i386 and the Incrementer 193 on PPC. 194 195 The softirq for running the hrtimer queues and executing the callbacks has been 196 separated from the tick bound timer softirq to allow accurate delivery of high 197 resolution timer signals which are used by itimer and POSIX interval 198 timers. The execution of this softirq can still be delayed by other softirqs, 199 but the overall latencies have been significantly improved by this separation. 200 201 Figure #5 (OLS slides p.22) illustrates the transformation. 202 203 204 dynamic ticks 205 ------------- 206 207 Dynamic ticks are the logical consequence of the hrtimer based periodic tick 208 replacement (sched_tick). The functionality of the sched_tick hrtimer is 209 extended by three functions: 210 211 - hrtimer_stop_sched_tick 212 - hrtimer_restart_sched_tick 213 - hrtimer_update_jiffies 214 215 hrtimer_stop_sched_tick() is called when a CPU goes into idle state. The code 216 evaluates the next scheduled timer event (from both hrtimers and the timer 217 wheel) and in case that the next event is further away than the next tick it 218 reprograms the sched_tick to this future event, to allow longer idle sleeps 219 without worthless interruption by the periodic tick. The function is also 220 called when an interrupt happens during the idle period, which does not cause a 221 reschedule. The call is necessary as the interrupt handler might have armed a 222 new timer whose expiry time is before the time which was identified as the 223 nearest event in the previous call to hrtimer_stop_sched_tick. 224 225 hrtimer_restart_sched_tick() is called when the CPU leaves the idle state before 226 it calls schedule(). hrtimer_restart_sched_tick() resumes the periodic tick, 227 which is kept active until the next call to hrtimer_stop_sched_tick(). 228 229 hrtimer_update_jiffies() is called from irq_enter() when an interrupt happens 230 in the idle period to make sure that jiffies are up to date and the interrupt 231 handler has not to deal with an eventually stale jiffy value. 232 233 The dynamic tick feature provides statistical values which are exported to 234 userspace via /proc/stats and can be made available for enhanced power 235 management control. 236 237 The implementation leaves room for further development like full tickless 238 systems, where the time slice is controlled by the scheduler, variable 239 frequency profiling, and a complete removal of jiffies in the future. 240 241 242 Aside the current initial submission of i386 support, the patchset has been 243 extended to x86_64 and ARM already. Initial (work in progress) support is also 244 available for MIPS and PowerPC. 245 246 Thomas, Ingo 247 248