Using RCU's CPU Stall Detector The rcu_cpu_stall_suppress module parameter enables RCU's CPU stall detector, which detects conditions that unduly delay RCU grace periods. This module parameter enables CPU stall detection by default, but may be overridden via boot-time parameter or at runtime via sysfs. The stall detector's idea of what constitutes "unduly delayed" is controlled by a set of kernel configuration variables and cpp macros: CONFIG_RCU_CPU_STALL_TIMEOUT This kernel configuration parameter defines the period of time that RCU will wait from the beginning of a grace period until it issues an RCU CPU stall warning. This time period is normally sixty seconds. This configuration parameter may be changed at runtime via the /sys/module/rcutree/parameters/rcu_cpu_stall_timeout, however this parameter is checked only at the beginning of a cycle. So if you are 30 seconds into a 70-second stall, setting this sysfs parameter to (say) five will shorten the timeout for the -next- stall, or the following warning for the current stall (assuming the stall lasts long enough). It will not affect the timing of the next warning for the current stall. Stall-warning messages may be enabled and disabled completely via /sys/module/rcutree/parameters/rcu_cpu_stall_suppress. CONFIG_RCU_CPU_STALL_VERBOSE This kernel configuration parameter causes the stall warning to also dump the stacks of any tasks that are blocking the current RCU-preempt grace period. RCU_CPU_STALL_INFO This kernel configuration parameter causes the stall warning to print out additional per-CPU diagnostic information, including information on scheduling-clock ticks and RCU's idle-CPU tracking. RCU_STALL_DELAY_DELTA Although the lockdep facility is extremely useful, it does add some overhead. Therefore, under CONFIG_PROVE_RCU, the RCU_STALL_DELAY_DELTA macro allows five extra seconds before giving an RCU CPU stall warning message. RCU_STALL_RAT_DELAY The CPU stall detector tries to make the offending CPU print its own warnings, as this often gives better-quality stack traces. However, if the offending CPU does not detect its own stall in the number of jiffies specified by RCU_STALL_RAT_DELAY, then some other CPU will complain. This delay is normally set to two jiffies. When a CPU detects that it is stalling, it will print a message similar to the following: INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies) This message indicates that CPU 5 detected that it was causing a stall, and that the stall was affecting RCU-sched. This message will normally be followed by a stack dump of the offending CPU. On TREE_RCU kernel builds, RCU and RCU-sched are implemented by the same underlying mechanism, while on TREE_PREEMPT_RCU kernel builds, RCU is instead implemented by rcu_preempt_state. On the other hand, if the offending CPU fails to print out a stall-warning message quickly enough, some other CPU will print a message similar to the following: INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies) This message indicates that CPU 2 detected that CPUs 3 and 5 were both causing stalls, and that the stall was affecting RCU-bh. This message will normally be followed by stack dumps for each CPU. Please note that TREE_PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that the tasks will be indicated by PID, for example, "P3421". It is even possible for a rcu_preempt_state stall to be caused by both CPUs -and- tasks, in which case the offending CPUs and tasks will all be called out in the list. Finally, if the grace period ends just as the stall warning starts printing, there will be a spurious stall-warning message: INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies) This is rare, but does happen from time to time in real life. If the CONFIG_RCU_CPU_STALL_INFO kernel configuration parameter is set, more information is printed with the stall-warning message, for example: INFO: rcu_preempt detected stall on CPU 0: (63959 ticks this GP) idle=241/3fffffffffffffff/0 (t=65000 jiffies) In kernels with CONFIG_RCU_FAST_NO_HZ, even more information is printed: INFO: rcu_preempt detected stall on CPU 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 drain=0 . timer not pending (t=65000 jiffies) The "(64628 ticks this GP)" indicates that this CPU has taken more than 64,000 scheduling-clock interrupts during the current stalled grace period. If the CPU was not yet aware of the current grace period (for example, if it was offline), then this part of the message indicates how many grace periods behind the CPU is. The "idle=" portion of the message prints the dyntick-idle state. The hex number before the first "/" is the low-order 12 bits of the dynticks counter, which will have an even-numbered value if the CPU is in dyntick-idle mode and an odd-numbered value otherwise. The hex number between the two "/"s is the value of the nesting, which will be a small positive number if in the idle loop and a very large positive number (as shown above) otherwise. For CONFIG_RCU_FAST_NO_HZ kernels, the "drain=0" indicates that the CPU is not in the process of trying to force itself into dyntick-idle state, the "." indicates that the CPU has not given up forcing RCU into dyntick-idle mode (it would be "H" otherwise), and the "timer not pending" indicates that the CPU has not recently forced RCU into dyntick-idle mode (it would otherwise indicate the number of microseconds remaining in this forced state). Multiple Warnings From One Stall If a stall lasts long enough, multiple stall-warning messages will be printed for it. The second and subsequent messages are printed at longer intervals, so that the time between (say) the first and second message will be about three times the interval between the beginning of the stall and the first message. What Causes RCU CPU Stall Warnings? So your kernel printed an RCU CPU stall warning. The next question is "What caused it?" The following problems can result in RCU CPU stall warnings: o A CPU looping in an RCU read-side critical section. o A CPU looping with interrupts disabled. This condition can result in RCU-sched and RCU-bh stalls. o A CPU looping with preemption disabled. This condition can result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh stalls. o A CPU looping with bottom halves disabled. This condition can result in RCU-sched and RCU-bh stalls. o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel without invoking schedule(). o A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might happen to preempt a low-priority task in the middle of an RCU read-side critical section. This is especially damaging if that low-priority task is not permitted to run on any other CPU, in which case the next RCU grace period can never complete, which will eventually cause the system to run out of memory and hang. While the system is in the process of running itself out of memory, you might see stall-warning messages. o A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that is running at a higher priority than the RCU softirq threads. This will prevent RCU callbacks from ever being invoked, and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent RCU grace periods from ever completing. Either way, the system will eventually run out of memory and hang. In the CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning messages. o A hardware or software issue shuts off the scheduler-clock interrupt on a CPU that is not in dyntick-idle mode. This problem really has happened, and seems to be most likely to result in RCU CPU stall warnings for CONFIG_NO_HZ=n kernels. o A bug in the RCU implementation. o A hardware failure. This is quite unlikely, but has occurred at least once in real life. A CPU failed in a running system, becoming unresponsive, but not causing an immediate crash. This resulted in a series of RCU CPU stall warnings, eventually leading the realization that the CPU had failed. The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning. SRCU does not have its own CPU stall warnings, but its calls to synchronize_sched() will result in RCU-sched detecting RCU-sched-related CPU stalls. Please note that RCU only detects CPU stalls when there is a grace period in progress. No grace period, no CPU stall warnings. To diagnose the cause of the stall, inspect the stack traces. The offending function will usually be near the top of the stack. If you have a series of stall warnings from a single extended stall, comparing the stack traces can often help determine where the stall is occurring, which will usually be in the function nearest the top of that portion of the stack which remains the same from trace to trace. If you can reliably trigger the stall, ftrace can be quite helpful. RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE and with RCU's event tracing.