1Using RCU's CPU Stall Detector 2 3The rcu_cpu_stall_suppress module parameter enables RCU's CPU stall 4detector, which detects conditions that unduly delay RCU grace periods. 5This module parameter enables CPU stall detection by default, but 6may be overridden via boot-time parameter or at runtime via sysfs. 7The stall detector's idea of what constitutes "unduly delayed" is 8controlled by a set of kernel configuration variables and cpp macros: 9 10CONFIG_RCU_CPU_STALL_TIMEOUT 11 12 This kernel configuration parameter defines the period of time 13 that RCU will wait from the beginning of a grace period until it 14 issues an RCU CPU stall warning. This time period is normally 15 sixty seconds. 16 17 This configuration parameter may be changed at runtime via the 18 /sys/module/rcutree/parameters/rcu_cpu_stall_timeout, however 19 this parameter is checked only at the beginning of a cycle. 20 So if you are 30 seconds into a 70-second stall, setting this 21 sysfs parameter to (say) five will shorten the timeout for the 22 -next- stall, or the following warning for the current stall 23 (assuming the stall lasts long enough). It will not affect the 24 timing of the next warning for the current stall. 25 26 Stall-warning messages may be enabled and disabled completely via 27 /sys/module/rcutree/parameters/rcu_cpu_stall_suppress. 28 29CONFIG_RCU_CPU_STALL_VERBOSE 30 31 This kernel configuration parameter causes the stall warning to 32 also dump the stacks of any tasks that are blocking the current 33 RCU-preempt grace period. 34 35RCU_CPU_STALL_INFO 36 37 This kernel configuration parameter causes the stall warning to 38 print out additional per-CPU diagnostic information, including 39 information on scheduling-clock ticks and RCU's idle-CPU tracking. 40 41RCU_STALL_DELAY_DELTA 42 43 Although the lockdep facility is extremely useful, it does add 44 some overhead. Therefore, under CONFIG_PROVE_RCU, the 45 RCU_STALL_DELAY_DELTA macro allows five extra seconds before 46 giving an RCU CPU stall warning message. 47 48RCU_STALL_RAT_DELAY 49 50 The CPU stall detector tries to make the offending CPU print its 51 own warnings, as this often gives better-quality stack traces. 52 However, if the offending CPU does not detect its own stall in 53 the number of jiffies specified by RCU_STALL_RAT_DELAY, then 54 some other CPU will complain. This delay is normally set to 55 two jiffies. 56 57When a CPU detects that it is stalling, it will print a message similar 58to the following: 59 60INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies) 61 62This message indicates that CPU 5 detected that it was causing a stall, 63and that the stall was affecting RCU-sched. This message will normally be 64followed by a stack dump of the offending CPU. On TREE_RCU kernel builds, 65RCU and RCU-sched are implemented by the same underlying mechanism, 66while on TREE_PREEMPT_RCU kernel builds, RCU is instead implemented 67by rcu_preempt_state. 68 69On the other hand, if the offending CPU fails to print out a stall-warning 70message quickly enough, some other CPU will print a message similar to 71the following: 72 73INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies) 74 75This message indicates that CPU 2 detected that CPUs 3 and 5 were both 76causing stalls, and that the stall was affecting RCU-bh. This message 77will normally be followed by stack dumps for each CPU. Please note that 78TREE_PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, 79and that the tasks will be indicated by PID, for example, "P3421". 80It is even possible for a rcu_preempt_state stall to be caused by both 81CPUs -and- tasks, in which case the offending CPUs and tasks will all 82be called out in the list. 83 84Finally, if the grace period ends just as the stall warning starts 85printing, there will be a spurious stall-warning message: 86 87INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies) 88 89This is rare, but does happen from time to time in real life. 90 91If the CONFIG_RCU_CPU_STALL_INFO kernel configuration parameter is set, 92more information is printed with the stall-warning message, for example: 93 94 INFO: rcu_preempt detected stall on CPU 95 0: (63959 ticks this GP) idle=241/3fffffffffffffff/0 96 (t=65000 jiffies) 97 98In kernels with CONFIG_RCU_FAST_NO_HZ, even more information is 99printed: 100 101 INFO: rcu_preempt detected stall on CPU 102 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 drain=0 . timer=-1 103 (t=65000 jiffies) 104 105The "(64628 ticks this GP)" indicates that this CPU has taken more 106than 64,000 scheduling-clock interrupts during the current stalled 107grace period. If the CPU was not yet aware of the current grace 108period (for example, if it was offline), then this part of the message 109indicates how many grace periods behind the CPU is. 110 111The "idle=" portion of the message prints the dyntick-idle state. 112The hex number before the first "/" is the low-order 12 bits of the 113dynticks counter, which will have an even-numbered value if the CPU is 114in dyntick-idle mode and an odd-numbered value otherwise. The hex 115number between the two "/"s is the value of the nesting, which will 116be a small positive number if in the idle loop and a very large positive 117number (as shown above) otherwise. 118 119For CONFIG_RCU_FAST_NO_HZ kernels, the "drain=0" indicates that the 120CPU is not in the process of trying to force itself into dyntick-idle 121state, the "." indicates that the CPU has not given up forcing RCU 122into dyntick-idle mode (it would be "H" otherwise), and the "timer=-1" 123indicates that the CPU has not recented forced RCU into dyntick-idle 124mode (it would otherwise indicate the number of microseconds remaining 125in this forced state). 126 127 128Multiple Warnings From One Stall 129 130If a stall lasts long enough, multiple stall-warning messages will be 131printed for it. The second and subsequent messages are printed at 132longer intervals, so that the time between (say) the first and second 133message will be about three times the interval between the beginning 134of the stall and the first message. 135 136 137What Causes RCU CPU Stall Warnings? 138 139So your kernel printed an RCU CPU stall warning. The next question is 140"What caused it?" The following problems can result in RCU CPU stall 141warnings: 142 143o A CPU looping in an RCU read-side critical section. 144 145o A CPU looping with interrupts disabled. This condition can 146 result in RCU-sched and RCU-bh stalls. 147 148o A CPU looping with preemption disabled. This condition can 149 result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh 150 stalls. 151 152o A CPU looping with bottom halves disabled. This condition can 153 result in RCU-sched and RCU-bh stalls. 154 155o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel 156 without invoking schedule(). 157 158o A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might 159 happen to preempt a low-priority task in the middle of an RCU 160 read-side critical section. This is especially damaging if 161 that low-priority task is not permitted to run on any other CPU, 162 in which case the next RCU grace period can never complete, which 163 will eventually cause the system to run out of memory and hang. 164 While the system is in the process of running itself out of 165 memory, you might see stall-warning messages. 166 167o A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that 168 is running at a higher priority than the RCU softirq threads. 169 This will prevent RCU callbacks from ever being invoked, 170 and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent 171 RCU grace periods from ever completing. Either way, the 172 system will eventually run out of memory and hang. In the 173 CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning 174 messages. 175 176o A hardware or software issue shuts off the scheduler-clock 177 interrupt on a CPU that is not in dyntick-idle mode. This 178 problem really has happened, and seems to be most likely to 179 result in RCU CPU stall warnings for CONFIG_NO_HZ=n kernels. 180 181o A bug in the RCU implementation. 182 183o A hardware failure. This is quite unlikely, but has occurred 184 at least once in real life. A CPU failed in a running system, 185 becoming unresponsive, but not causing an immediate crash. 186 This resulted in a series of RCU CPU stall warnings, eventually 187 leading the realization that the CPU had failed. 188 189The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning. 190SRCU does not have its own CPU stall warnings, but its calls to 191synchronize_sched() will result in RCU-sched detecting RCU-sched-related 192CPU stalls. Please note that RCU only detects CPU stalls when there is 193a grace period in progress. No grace period, no CPU stall warnings. 194 195To diagnose the cause of the stall, inspect the stack traces. 196The offending function will usually be near the top of the stack. 197If you have a series of stall warnings from a single extended stall, 198comparing the stack traces can often help determine where the stall 199is occurring, which will usually be in the function nearest the top of 200that portion of the stack which remains the same from trace to trace. 201If you can reliably trigger the stall, ftrace can be quite helpful. 202 203RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE 204and with RCU's event tracing. 205