128 lines
5.6 KiB
Plaintext
128 lines
5.6 KiB
Plaintext
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
|
|
ten seconds.
|
|
|
|
RCU_SECONDS_TILL_STALL_RECHECK
|
|
|
|
This macro defines the period of time that RCU will wait after
|
|
issuing a stall warning until it issues another stall warning
|
|
for the same stall. This time period is normally set to three
|
|
times the check interval plus thirty seconds.
|
|
|
|
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.
|
|
|
|
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 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.
|