Documentation/RCU/stallwarn.txt

   1 Using RCU's CPU Stall Detector
   2
   3 The rcu_cpu_stall_suppress module parameter enables RCU's CPU stall
   4 detector, which detects conditions that unduly delay RCU grace periods.
   5 This module parameter enables CPU stall detection by default, but
   6 may be overridden via boot-time parameter or at runtime via sysfs.
   7 The stall detector's idea of what constitutes "unduly delayed" is
   8 controlled by a set of kernel configuration variables and cpp macros:
   9
  10 CONFIG_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
  29 CONFIG_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
  35 RCU_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
  41 RCU_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
  48 RCU_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
  57 When a CPU detects that it is stalling, it will print a message similar
  58 to the following:
  59
  60 INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)
  61
  62 This message indicates that CPU 5 detected that it was causing a stall,
  63 and that the stall was affecting RCU-sched.  This message will normally be
  64 followed by a stack dump of the offending CPU.  On TREE_RCU kernel builds,
  65 RCU and RCU-sched are implemented by the same underlying mechanism,
  66 while on TREE_PREEMPT_RCU kernel builds, RCU is instead implemented
  67 by rcu_preempt_state.
  68
  69 On the other hand, if the offending CPU fails to print out a stall-warning
  70 message quickly enough, some other CPU will print a message similar to
  71 the following:
  72
  73 INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)
  74
  75 This message indicates that CPU 2 detected that CPUs 3 and 5 were both
  76 causing stalls, and that the stall was affecting RCU-bh.  This message
  77 will normally be followed by stack dumps for each CPU.  Please note that
  78 TREE_PREEMPT_RCU builds can be stalled by tasks as well as by CPUs,
  79 and that the tasks will be indicated by PID, for example, "P3421".
  80 It is even possible for a rcu_preempt_state stall to be caused by both
  81 CPUs -and- tasks, in which case the offending CPUs and tasks will all
  82 be called out in the list.
  83
  84 Finally, if the grace period ends just as the stall warning starts
  85 printing, there will be a spurious stall-warning message:
  86
  87 INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)
  88
  89 This is rare, but does happen from time to time in real life.
  90
  91 If the CONFIG_RCU_CPU_STALL_INFO kernel configuration parameter is set,
  92 more 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 softirq=82/543
  96            (t=65000 jiffies)
  97
  98 In kernels with CONFIG_RCU_FAST_NO_HZ, even more information is
  99 printed:
 100
 101         INFO: rcu_preempt detected stall on CPU
 102         0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
 103            (t=65000 jiffies)
 104
 105 The "(64628 ticks this GP)" indicates that this CPU has taken more
 106 than 64,000 scheduling-clock interrupts during the current stalled
 107 grace period.  If the CPU was not yet aware of the current grace
 108 period (for example, if it was offline), then this part of the message
 109 indicates how many grace periods behind the CPU is.
 110
 111 The "idle=" portion of the message prints the dyntick-idle state.
 112 The hex number before the first "/" is the low-order 12 bits of the
 113 dynticks counter, which will have an even-numbered value if the CPU is
 114 in dyntick-idle mode and an odd-numbered value otherwise.  The hex
 115 number between the two "/"s is the value of the nesting, which will
 116 be a small positive number if in the idle loop and a very large positive
 117 number (as shown above) otherwise.
 118
 119 The "softirq=" portion of the message tracks the number of RCU softirq
 120 handlers that the stalled CPU has executed.  The number before the "/"
 121 is the number that had executed since boot at the time that this CPU
 122 last noted the beginning of a grace period, which might be the current
 123 (stalled) grace period, or it might be some earlier grace period (for
 124 example, if the CPU might have been in dyntick-idle mode for an extended
 125 time period.  The number after the "/" is the number that have executed
 126 since boot until the current time.  If this latter number stays constant
 127 across repeated stall-warning messages, it is possible that RCU's softirq
 128 handlers are no longer able to execute on this CPU.  This can happen if
 129 the stalled CPU is spinning with interrupts are disabled, or, in -rt
 130 kernels, if a high-priority process is starving RCU's softirq handler.
 131
 132 For CONFIG_RCU_FAST_NO_HZ kernels, the "last_accelerate:" prints the
 133 low-order 16 bits (in hex) of the jiffies counter when this CPU last
 134 invoked rcu_try_advance_all_cbs() from rcu_needs_cpu() or last invoked
 135 rcu_accelerate_cbs() from rcu_prepare_for_idle().  The "nonlazy_posted:"
 136 prints the number of non-lazy callbacks posted since the last call to
 137 rcu_needs_cpu().  Finally, an "L" indicates that there are currently
 138 no non-lazy callbacks ("." is printed otherwise, as shown above) and
 139 "D" indicates that dyntick-idle processing is enabled ("." is printed
 140 otherwise, for example, if disabled via the "nohz=" kernel boot parameter).
 141
 142
 143 Multiple Warnings From One Stall
 144
 145 If a stall lasts long enough, multiple stall-warning messages will be
 146 printed for it.  The second and subsequent messages are printed at
 147 longer intervals, so that the time between (say) the first and second
 148 message will be about three times the interval between the beginning
 149 of the stall and the first message.
 150
 151
 152 What Causes RCU CPU Stall Warnings?
 153
 154 So your kernel printed an RCU CPU stall warning.  The next question is
 155 "What caused it?"  The following problems can result in RCU CPU stall
 156 warnings:
 157
 158 o       A CPU looping in an RCU read-side critical section.
 159
 160 o       A CPU looping with interrupts disabled.  This condition can
 161         result in RCU-sched and RCU-bh stalls.
 162
 163 o       A CPU looping with preemption disabled.  This condition can
 164         result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
 165         stalls.
 166
 167 o       A CPU looping with bottom halves disabled.  This condition can
 168         result in RCU-sched and RCU-bh stalls.
 169
 170 o       For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
 171         without invoking schedule().
 172
 173 o       A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
 174         happen to preempt a low-priority task in the middle of an RCU
 175         read-side critical section.   This is especially damaging if
 176         that low-priority task is not permitted to run on any other CPU,
 177         in which case the next RCU grace period can never complete, which
 178         will eventually cause the system to run out of memory and hang.
 179         While the system is in the process of running itself out of
 180         memory, you might see stall-warning messages.
 181
 182 o       A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
 183         is running at a higher priority than the RCU softirq threads.
 184         This will prevent RCU callbacks from ever being invoked,
 185         and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent
 186         RCU grace periods from ever completing.  Either way, the
 187         system will eventually run out of memory and hang.  In the
 188         CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning
 189         messages.
 190
 191 o       A hardware or software issue shuts off the scheduler-clock
 192         interrupt on a CPU that is not in dyntick-idle mode.  This
 193         problem really has happened, and seems to be most likely to
 194         result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
 195
 196 o       A bug in the RCU implementation.
 197
 198 o       A hardware failure.  This is quite unlikely, but has occurred
 199         at least once in real life.  A CPU failed in a running system,
 200         becoming unresponsive, but not causing an immediate crash.
 201         This resulted in a series of RCU CPU stall warnings, eventually
 202         leading the realization that the CPU had failed.
 203
 204 The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning.
 205 SRCU does not have its own CPU stall warnings, but its calls to
 206 synchronize_sched() will result in RCU-sched detecting RCU-sched-related
 207 CPU stalls.  Please note that RCU only detects CPU stalls when there is
 208 a grace period in progress.  No grace period, no CPU stall warnings.
 209
 210 To diagnose the cause of the stall, inspect the stack traces.
 211 The offending function will usually be near the top of the stack.
 212 If you have a series of stall warnings from a single extended stall,
 213 comparing the stack traces can often help determine where the stall
 214 is occurring, which will usually be in the function nearest the top of
 215 that portion of the stack which remains the same from trace to trace.
 216 If you can reliably trigger the stall, ftrace can be quite helpful.
 217
 218 RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
 219 and with RCU's event tracing.