1.. SPDX-License-Identifier: GPL-2.0 2 3============================== 4Using RCU's CPU Stall Detector 5============================== 6 7This document first discusses what sorts of issues RCU's CPU stall 8detector can locate, and then discusses kernel parameters and Kconfig 9options that can be used to fine-tune the detector's operation. Finally, 10this document explains the stall detector's "splat" format. 11 12 13What Causes RCU CPU Stall Warnings? 14=================================== 15 16So your kernel printed an RCU CPU stall warning. The next question is 17"What caused it?" The following problems can result in RCU CPU stall 18warnings: 19 20- A CPU looping in an RCU read-side critical section. 21 22- A CPU looping with interrupts disabled. 23 24- A CPU looping with preemption disabled. 25 26- A CPU looping with bottom halves disabled. 27 28- For !CONFIG_PREEMPTION kernels, a CPU looping anywhere in the kernel 29 without invoking schedule(). If the looping in the kernel is 30 really expected and desirable behavior, you might need to add 31 some calls to cond_resched(). 32 33- Booting Linux using a console connection that is too slow to 34 keep up with the boot-time console-message rate. For example, 35 a 115Kbaud serial console can be *way* too slow to keep up 36 with boot-time message rates, and will frequently result in 37 RCU CPU stall warning messages. Especially if you have added 38 debug printk()s. 39 40- Anything that prevents RCU's grace-period kthreads from running. 41 This can result in the "All QSes seen" console-log message. 42 This message will include information on when the kthread last 43 ran and how often it should be expected to run. It can also 44 result in the ``rcu_.*kthread starved for`` console-log message, 45 which will include additional debugging information. 46 47- A CPU-bound real-time task in a CONFIG_PREEMPTION kernel, which might 48 happen to preempt a low-priority task in the middle of an RCU 49 read-side critical section. This is especially damaging if 50 that low-priority task is not permitted to run on any other CPU, 51 in which case the next RCU grace period can never complete, which 52 will eventually cause the system to run out of memory and hang. 53 While the system is in the process of running itself out of 54 memory, you might see stall-warning messages. 55 56- A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that 57 is running at a higher priority than the RCU softirq threads. 58 This will prevent RCU callbacks from ever being invoked, 59 and in a CONFIG_PREEMPT_RCU kernel will further prevent 60 RCU grace periods from ever completing. Either way, the 61 system will eventually run out of memory and hang. In the 62 CONFIG_PREEMPT_RCU case, you might see stall-warning 63 messages. 64 65 You can use the rcutree.kthread_prio kernel boot parameter to 66 increase the scheduling priority of RCU's kthreads, which can 67 help avoid this problem. However, please note that doing this 68 can increase your system's context-switch rate and thus degrade 69 performance. 70 71- A periodic interrupt whose handler takes longer than the time 72 interval between successive pairs of interrupts. This can 73 prevent RCU's kthreads and softirq handlers from running. 74 Note that certain high-overhead debugging options, for example 75 the function_graph tracer, can result in interrupt handler taking 76 considerably longer than normal, which can in turn result in 77 RCU CPU stall warnings. 78 79- Testing a workload on a fast system, tuning the stall-warning 80 timeout down to just barely avoid RCU CPU stall warnings, and then 81 running the same workload with the same stall-warning timeout on a 82 slow system. Note that thermal throttling and on-demand governors 83 can cause a single system to be sometimes fast and sometimes slow! 84 85- A hardware or software issue shuts off the scheduler-clock 86 interrupt on a CPU that is not in dyntick-idle mode. This 87 problem really has happened, and seems to be most likely to 88 result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels. 89 90- A hardware or software issue that prevents time-based wakeups 91 from occurring. These issues can range from misconfigured or 92 buggy timer hardware through bugs in the interrupt or exception 93 path (whether hardware, firmware, or software) through bugs 94 in Linux's timer subsystem through bugs in the scheduler, and, 95 yes, even including bugs in RCU itself. It can also result in 96 the ``rcu_.*timer wakeup didn't happen for`` console-log message, 97 which will include additional debugging information. 98 99- A low-level kernel issue that either fails to invoke one of the 100 variants of rcu_user_enter(), rcu_user_exit(), rcu_idle_enter(), 101 rcu_idle_exit(), rcu_irq_enter(), or rcu_irq_exit() on the one 102 hand, or that invokes one of them too many times on the other. 103 Historically, the most frequent issue has been an omission 104 of either irq_enter() or irq_exit(), which in turn invoke 105 rcu_irq_enter() or rcu_irq_exit(), respectively. Building your 106 kernel with CONFIG_RCU_EQS_DEBUG=y can help track down these types 107 of issues, which sometimes arise in architecture-specific code. 108 109- A bug in the RCU implementation. 110 111- A hardware failure. This is quite unlikely, but has occurred 112 at least once in real life. A CPU failed in a running system, 113 becoming unresponsive, but not causing an immediate crash. 114 This resulted in a series of RCU CPU stall warnings, eventually 115 leading the realization that the CPU had failed. 116 117The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning. 118Note that SRCU does *not* have CPU stall warnings. Please note that 119RCU only detects CPU stalls when there is a grace period in progress. 120No grace period, no CPU stall warnings. 121 122To diagnose the cause of the stall, inspect the stack traces. 123The offending function will usually be near the top of the stack. 124If you have a series of stall warnings from a single extended stall, 125comparing the stack traces can often help determine where the stall 126is occurring, which will usually be in the function nearest the top of 127that portion of the stack which remains the same from trace to trace. 128If you can reliably trigger the stall, ftrace can be quite helpful. 129 130RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE 131and with RCU's event tracing. For information on RCU's event tracing, 132see include/trace/events/rcu.h. 133 134 135Fine-Tuning the RCU CPU Stall Detector 136====================================== 137 138The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's 139CPU stall detector, which detects conditions that unduly delay RCU grace 140periods. This module parameter enables CPU stall detection by default, 141but may be overridden via boot-time parameter or at runtime via sysfs. 142The stall detector's idea of what constitutes "unduly delayed" is 143controlled by a set of kernel configuration variables and cpp macros: 144 145CONFIG_RCU_CPU_STALL_TIMEOUT 146---------------------------- 147 148 This kernel configuration parameter defines the period of time 149 that RCU will wait from the beginning of a grace period until it 150 issues an RCU CPU stall warning. This time period is normally 151 21 seconds. 152 153 This configuration parameter may be changed at runtime via the 154 /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however 155 this parameter is checked only at the beginning of a cycle. 156 So if you are 10 seconds into a 40-second stall, setting this 157 sysfs parameter to (say) five will shorten the timeout for the 158 *next* stall, or the following warning for the current stall 159 (assuming the stall lasts long enough). It will not affect the 160 timing of the next warning for the current stall. 161 162 Stall-warning messages may be enabled and disabled completely via 163 /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress. 164 165RCU_STALL_DELAY_DELTA 166--------------------- 167 168 Although the lockdep facility is extremely useful, it does add 169 some overhead. Therefore, under CONFIG_PROVE_RCU, the 170 RCU_STALL_DELAY_DELTA macro allows five extra seconds before 171 giving an RCU CPU stall warning message. (This is a cpp 172 macro, not a kernel configuration parameter.) 173 174RCU_STALL_RAT_DELAY 175------------------- 176 177 The CPU stall detector tries to make the offending CPU print its 178 own warnings, as this often gives better-quality stack traces. 179 However, if the offending CPU does not detect its own stall in 180 the number of jiffies specified by RCU_STALL_RAT_DELAY, then 181 some other CPU will complain. This delay is normally set to 182 two jiffies. (This is a cpp macro, not a kernel configuration 183 parameter.) 184 185rcupdate.rcu_task_stall_timeout 186------------------------------- 187 188 This boot/sysfs parameter controls the RCU-tasks stall warning 189 interval. A value of zero or less suppresses RCU-tasks stall 190 warnings. A positive value sets the stall-warning interval 191 in seconds. An RCU-tasks stall warning starts with the line: 192 193 INFO: rcu_tasks detected stalls on tasks: 194 195 And continues with the output of sched_show_task() for each 196 task stalling the current RCU-tasks grace period. 197 198 199Interpreting RCU's CPU Stall-Detector "Splats" 200============================================== 201 202For non-RCU-tasks flavors of RCU, when a CPU detects that some other 203CPU is stalling, it will print a message similar to the following:: 204 205 INFO: rcu_sched detected stalls on CPUs/tasks: 206 2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0 207 16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0 208 (detected by 32, t=2603 jiffies, g=7075, q=625) 209 210This message indicates that CPU 32 detected that CPUs 2 and 16 were both 211causing stalls, and that the stall was affecting RCU-sched. This message 212will normally be followed by stack dumps for each CPU. Please note that 213PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that 214the tasks will be indicated by PID, for example, "P3421". It is even 215possible for an rcu_state stall to be caused by both CPUs *and* tasks, 216in which case the offending CPUs and tasks will all be called out in the list. 217In some cases, CPUs will detect themselves stalling, which will result 218in a self-detected stall. 219 220CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with 221the RCU core for the past three grace periods. In contrast, CPU 16's "(0 222ticks this GP)" indicates that this CPU has not taken any scheduling-clock 223interrupts during the current stalled grace period. 224 225The "idle=" portion of the message prints the dyntick-idle state. 226The hex number before the first "/" is the low-order 12 bits of the 227dynticks counter, which will have an even-numbered value if the CPU 228is in dyntick-idle mode and an odd-numbered value otherwise. The hex 229number between the two "/"s is the value of the nesting, which will be 230a small non-negative number if in the idle loop (as shown above) and a 231very large positive number otherwise. 232 233The "softirq=" portion of the message tracks the number of RCU softirq 234handlers that the stalled CPU has executed. The number before the "/" 235is the number that had executed since boot at the time that this CPU 236last noted the beginning of a grace period, which might be the current 237(stalled) grace period, or it might be some earlier grace period (for 238example, if the CPU might have been in dyntick-idle mode for an extended 239time period). The number after the "/" is the number that have executed 240since boot until the current time. If this latter number stays constant 241across repeated stall-warning messages, it is possible that RCU's softirq 242handlers are no longer able to execute on this CPU. This can happen if 243the stalled CPU is spinning with interrupts are disabled, or, in -rt 244kernels, if a high-priority process is starving RCU's softirq handler. 245 246The "fqs=" shows the number of force-quiescent-state idle/offline 247detection passes that the grace-period kthread has made across this 248CPU since the last time that this CPU noted the beginning of a grace 249period. 250 251The "detected by" line indicates which CPU detected the stall (in this 252case, CPU 32), how many jiffies have elapsed since the start of the grace 253period (in this case 2603), the grace-period sequence number (7075), and 254an estimate of the total number of RCU callbacks queued across all CPUs 255(625 in this case). 256 257If the grace period ends just as the stall warning starts printing, 258there will be a spurious stall-warning message, which will include 259the following:: 260 261 INFO: Stall ended before state dump start 262 263This is rare, but does happen from time to time in real life. It is also 264possible for a zero-jiffy stall to be flagged in this case, depending 265on how the stall warning and the grace-period initialization happen to 266interact. Please note that it is not possible to entirely eliminate this 267sort of false positive without resorting to things like stop_machine(), 268which is overkill for this sort of problem. 269 270If all CPUs and tasks have passed through quiescent states, but the 271grace period has nevertheless failed to end, the stall-warning splat 272will include something like the following:: 273 274 All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0 275 276The "23807" indicates that it has been more than 23 thousand jiffies 277since the grace-period kthread ran. The "jiffies_till_next_fqs" 278indicates how frequently that kthread should run, giving the number 279of jiffies between force-quiescent-state scans, in this case three, 280which is way less than 23807. Finally, the root rcu_node structure's 281->qsmask field is printed, which will normally be zero. 282 283If the relevant grace-period kthread has been unable to run prior to 284the stall warning, as was the case in the "All QSes seen" line above, 285the following additional line is printed:: 286 287 rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5 288 Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior. 289 290Starving the grace-period kthreads of CPU time can of course result 291in RCU CPU stall warnings even when all CPUs and tasks have passed 292through the required quiescent states. The "g" number shows the current 293grace-period sequence number, the "f" precedes the ->gp_flags command 294to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the 295kthread is waiting for a short timeout, the "state" precedes value of the 296task_struct ->state field, and the "cpu" indicates that the grace-period 297kthread last ran on CPU 5. 298 299If the relevant grace-period kthread does not wake from FQS wait in a 300reasonable time, then the following additional line is printed:: 301 302 kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 303 304The "23804" indicates that kthread's timer expired more than 23 thousand 305jiffies ago. The rest of the line has meaning similar to the kthread 306starvation case. 307 308Additionally, the following line is printed:: 309 310 Possible timer handling issue on cpu=4 timer-softirq=11142 311 312Here "cpu" indicates that the grace-period kthread last ran on CPU 4, 313where it queued the fqs timer. The number following the "timer-softirq" 314is the current ``TIMER_SOFTIRQ`` count on cpu 4. If this value does not 315change on successive RCU CPU stall warnings, there is further reason to 316suspect a timer problem. 317 318These messages are usually followed by stack dumps of the CPUs and tasks 319involved in the stall. These stack traces can help you locate the cause 320of the stall, keeping in mind that the CPU detecting the stall will have 321an interrupt frame that is mainly devoted to detecting the stall. 322 323 324Multiple Warnings From One Stall 325================================ 326 327If a stall lasts long enough, multiple stall-warning messages will 328be printed for it. The second and subsequent messages are printed at 329longer intervals, so that the time between (say) the first and second 330message will be about three times the interval between the beginning 331of the stall and the first message. It can be helpful to compare the 332stack dumps for the different messages for the same stalled grace period. 333 334 335Stall Warnings for Expedited Grace Periods 336========================================== 337 338If an expedited grace period detects a stall, it will place a message 339like the following in dmesg:: 340 341 INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/. 342 343This indicates that CPU 7 has failed to respond to a reschedule IPI. 344The three periods (".") following the CPU number indicate that the CPU 345is online (otherwise the first period would instead have been "O"), 346that the CPU was online at the beginning of the expedited grace period 347(otherwise the second period would have instead been "o"), and that 348the CPU has been online at least once since boot (otherwise, the third 349period would instead have been "N"). The number before the "jiffies" 350indicates that the expedited grace period has been going on for 21,119 351jiffies. The number following the "s:" indicates that the expedited 352grace-period sequence counter is 73. The fact that this last value is 353odd indicates that an expedited grace period is in flight. The number 354following "root:" is a bitmask that indicates which children of the root 355rcu_node structure correspond to CPUs and/or tasks that are blocking the 356current expedited grace period. If the tree had more than one level, 357additional hex numbers would be printed for the states of the other 358rcu_node structures in the tree. 359 360As with normal grace periods, PREEMPT_RCU builds can be stalled by 361tasks as well as by CPUs, and that the tasks will be indicated by PID, 362for example, "P3421". 363 364It is entirely possible to see stall warnings from normal and from 365expedited grace periods at about the same time during the same run. 366