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 ==============================
 Using RCU's CPU Stall Detector
 ==============================
 
 This document first discusses what sorts of issues RCU's CPU stall
 detector can locate, and then discusses kernel parameters and Kconfig
 options that can be used to fine-tune the detector's operation.  Finally,
 this document explains the stall detector's "splat" format.
 
 
 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:
 
 -       A CPU looping in an RCU read-side critical section.
 
 -       A CPU looping with interrupts disabled.
 
 -       A CPU looping with preemption disabled.
 
 -       A CPU looping with bottom halves disabled.
 
 -       For !CONFIG_PREEMPTION kernels, a CPU looping anywhere in the kernel
         without invoking schedule().  If the looping in the kernel is
         really expected and desirable behavior, you might need to add
         some calls to cond_resched().
 
 -       Booting Linux using a console connection that is too slow to
         keep up with the boot-time console-message rate.  For example,
         a 115Kbaud serial console can be *way* too slow to keep up
         with boot-time message rates, and will frequently result in
         RCU CPU stall warning messages.  Especially if you have added
         debug printk()s.
 
 -       Anything that prevents RCU's grace-period kthreads from running.
         This can result in the "All QSes seen" console-log message.
         This message will include information on when the kthread last
         ran and how often it should be expected to run.  It can also
         result in the ``rcu_.*kthread starved for`` console-log message,
         which will include additional debugging information.
 
 -       A CPU-bound real-time task in a CONFIG_PREEMPTION 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.
 
 -       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_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_PREEMPT_RCU case, you might see stall-warning
         messages.
 
         You can use the rcutree.kthread_prio kernel boot parameter to
         increase the scheduling priority of RCU's kthreads, which can
         help avoid this problem.  However, please note that doing this
         can increase your system's context-switch rate and thus degrade
         performance.
 
 -       A periodic interrupt whose handler takes longer than the time
         interval between successive pairs of interrupts.  This can
         prevent RCU's kthreads and softirq handlers from running.
         Note that certain high-overhead debugging options, for example
         the function_graph tracer, can result in interrupt handler taking
         considerably longer than normal, which can in turn result in
         RCU CPU stall warnings.
 
 -       Testing a workload on a fast system, tuning the stall-warning
         timeout down to just barely avoid RCU CPU stall warnings, and then
         running the same workload with the same stall-warning timeout on a
         slow system.  Note that thermal throttling and on-demand governors
         can cause a single system to be sometimes fast and sometimes slow!
 
 -       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_COMMON=n kernels.
 
 -       A hardware or software issue that prevents time-based wakeups
         from occurring.  These issues can range from misconfigured or
         buggy timer hardware through bugs in the interrupt or exception
         path (whether hardware, firmware, or software) through bugs
         in Linux's timer subsystem through bugs in the scheduler, and,
         yes, even including bugs in RCU itself.  It can also result in
         the ``rcu_.*timer wakeup didn't happen for`` console-log message,
         which will include additional debugging information.
 
 -       A low-level kernel issue that either fails to invoke one of the
         variants of rcu_eqs_enter(true), rcu_eqs_exit(true), ct_idle_enter(),
         ct_idle_exit(), ct_irq_enter(), or ct_irq_exit() on the one
         hand, or that invokes one of them too many times on the other.
         Historically, the most frequent issue has been an omission
         of either irq_enter() or irq_exit(), which in turn invoke
         ct_irq_enter() or ct_irq_exit(), respectively.  Building your
         kernel with CONFIG_RCU_EQS_DEBUG=y can help track down these types
         of issues, which sometimes arise in architecture-specific code.
 
 -       A bug in the RCU implementation.
 
 -       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-tasks implementations have CPU stall warning.
 Note that SRCU does *not* have CPU stall warnings.  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.  For information on RCU's event tracing,
 see include/trace/events/rcu.h.
 
 
 Fine-Tuning the RCU CPU Stall Detector
 ======================================
 
 The rcuupdate.rcu_cpu_stall_suppress module parameter disables 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
         21 seconds.
 
         This configuration parameter may be changed at runtime via the
         /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however
         this parameter is checked only at the beginning of a cycle.
         So if you are 10 seconds into a 40-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/rcupdate/parameters/rcu_cpu_stall_suppress.
 
 CONFIG_RCU_EXP_CPU_STALL_TIMEOUT
 --------------------------------
 
         Same as the CONFIG_RCU_CPU_STALL_TIMEOUT parameter but only for
         the expedited grace period. This parameter defines the period
         of time that RCU will wait from the beginning of an expedited
         grace period until it issues an RCU CPU stall warning. This time
         period is normally 20 milliseconds on Android devices.  A zero
         value causes the CONFIG_RCU_CPU_STALL_TIMEOUT value to be used,
         after conversion to milliseconds.
 
         This configuration parameter may be changed at runtime via the
         /sys/module/rcupdate/parameters/rcu_exp_cpu_stall_timeout, however
         this parameter is checked only at the beginning of a cycle. If you
         are in a current stall cycle, setting it to a new value will change
         the timeout for the -next- stall.
 
         Stall-warning messages may be enabled and disabled completely via
         /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
 
 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.  (This is a cpp
         macro, not a kernel configuration parameter.)
 
 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.  (This is a cpp macro, not a kernel configuration
         parameter.)
 
 rcupdate.rcu_task_stall_timeout
 -------------------------------
 
         This boot/sysfs parameter controls the RCU-tasks stall warning
         interval.  A value of zero or less suppresses RCU-tasks stall
         warnings.  A positive value sets the stall-warning interval
         in seconds.  An RCU-tasks stall warning starts with the line:
 
                 INFO: rcu_tasks detected stalls on tasks:
 
         And continues with the output of sched_show_task() for each
         task stalling the current RCU-tasks grace period.
 
 
 Interpreting RCU's CPU Stall-Detector "Splats"
 ==============================================
 
 For non-RCU-tasks flavors of RCU, when a CPU detects that some other
 CPU is stalling, it will print a message similar to the following::
 
         INFO: rcu_sched detected stalls on CPUs/tasks:
         2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0
         16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0
         (detected by 32, t=2603 jiffies, g=7075, q=625)
 
 This message indicates that CPU 32 detected that CPUs 2 and 16 were both
 causing stalls, and that the stall was affecting RCU-sched.  This message
 will normally be followed by stack dumps for each CPU.  Please note that
 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 an rcu_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.
 In some cases, CPUs will detect themselves stalling, which will result
 in a self-detected stall.
 
 CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with
 the RCU core for the past three grace periods.  In contrast, CPU 16's "(0
 ticks this GP)" indicates that this CPU has not taken any scheduling-clock
 interrupts during the current stalled grace period.
 
 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 non-negative number if in the idle loop (as shown above) and a
 very large positive number otherwise.
 
 The "softirq=" portion of the message tracks the number of RCU softirq
 handlers that the stalled CPU has executed.  The number before the "/"
 is the number that had executed since boot at the time that this CPU
 last noted the beginning of a grace period, which might be the current
 (stalled) grace period, or it might be some earlier grace period (for
 example, if the CPU might have been in dyntick-idle mode for an extended
 time period).  The number after the "/" is the number that have executed
 since boot until the current time.  If this latter number stays constant
 across repeated stall-warning messages, it is possible that RCU's softirq
 handlers are no longer able to execute on this CPU.  This can happen if
 the stalled CPU is spinning with interrupts are disabled, or, in -rt
 kernels, if a high-priority process is starving RCU's softirq handler.
 
 The "fqs=" shows the number of force-quiescent-state idle/offline
 detection passes that the grace-period kthread has made across this
 CPU since the last time that this CPU noted the beginning of a grace
 period.
 
 The "detected by" line indicates which CPU detected the stall (in this
 case, CPU 32), how many jiffies have elapsed since the start of the grace
 period (in this case 2603), the grace-period sequence number (7075), and
 an estimate of the total number of RCU callbacks queued across all CPUs
 (625 in this case).
 
 If the grace period ends just as the stall warning starts printing,
 there will be a spurious stall-warning message, which will include
 the following::
 
         INFO: Stall ended before state dump start
 
 This is rare, but does happen from time to time in real life.  It is also
 possible for a zero-jiffy stall to be flagged in this case, depending
 on how the stall warning and the grace-period initialization happen to
 interact.  Please note that it is not possible to entirely eliminate this
 sort of false positive without resorting to things like stop_machine(),
 which is overkill for this sort of problem.
 
 If all CPUs and tasks have passed through quiescent states, but the
 grace period has nevertheless failed to end, the stall-warning splat
 will include something like the following::
 
         All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0
 
 The "23807" indicates that it has been more than 23 thousand jiffies
 since the grace-period kthread ran.  The "jiffies_till_next_fqs"
 indicates how frequently that kthread should run, giving the number
 of jiffies between force-quiescent-state scans, in this case three,
 which is way less than 23807.  Finally, the root rcu_node structure's
 ->qsmask field is printed, which will normally be zero.
 
 If the relevant grace-period kthread has been unable to run prior to
 the stall warning, as was the case in the "All QSes seen" line above,
 the following additional line is printed::
 
         rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5
         Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior.
 
 Starving the grace-period kthreads of CPU time can of course result
 in RCU CPU stall warnings even when all CPUs and tasks have passed
 through the required quiescent states.  The "g" number shows the current
 grace-period sequence number, the "f" precedes the ->gp_flags command
 to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the
 kthread is waiting for a short timeout, the "state" precedes value of the
 task_struct ->state field, and the "cpu" indicates that the grace-period
 kthread last ran on CPU 5.
 
 If the relevant grace-period kthread does not wake from FQS wait in a
 reasonable time, then the following additional line is printed::
 
         kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402
 
 The "23804" indicates that kthread's timer expired more than 23 thousand
 jiffies ago.  The rest of the line has meaning similar to the kthread
 starvation case.
 
 Additionally, the following line is printed::
 
         Possible timer handling issue on cpu=4 timer-softirq=11142
 
 Here "cpu" indicates that the grace-period kthread last ran on CPU 4,
 where it queued the fqs timer.  The number following the "timer-softirq"
 is the current ``TIMER_SOFTIRQ`` count on cpu 4.  If this value does not
 change on successive RCU CPU stall warnings, there is further reason to
 suspect a timer problem.
 
 These messages are usually followed by stack dumps of the CPUs and tasks
 involved in the stall.  These stack traces can help you locate the cause
 of the stall, keeping in mind that the CPU detecting the stall will have
 an interrupt frame that is mainly devoted to detecting the stall.
 
 
 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.  It can be helpful to compare the
 stack dumps for the different messages for the same stalled grace period.
 
 
 Stall Warnings for Expedited Grace Periods
 ==========================================
 
 If an expedited grace period detects a stall, it will place a message
 like the following in dmesg::
 
         INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/.
 
 This indicates that CPU 7 has failed to respond to a reschedule IPI.
 The three periods (".") following the CPU number indicate that the CPU
 is online (otherwise the first period would instead have been "O"),
 that the CPU was online at the beginning of the expedited grace period
 (otherwise the second period would have instead been "o"), and that
 the CPU has been online at least once since boot (otherwise, the third
 period would instead have been "N").  The number before the "jiffies"
 indicates that the expedited grace period has been going on for 21,119
 jiffies.  The number following the "s:" indicates that the expedited
 grace-period sequence counter is 73.  The fact that this last value is
 odd indicates that an expedited grace period is in flight.  The number
 following "root:" is a bitmask that indicates which children of the root
 rcu_node structure correspond to CPUs and/or tasks that are blocking the
 current expedited grace period.  If the tree had more than one level,
 additional hex numbers would be printed for the states of the other
 rcu_node structures in the tree.
 
 As with normal grace periods, 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 entirely possible to see stall warnings from normal and from
 expedited grace periods at about the same time during the same run.

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