319 lines
15 KiB
Plaintext
319 lines
15 KiB
Plaintext
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:
|
|
|
|
o A CPU looping in an RCU read-side critical section.
|
|
|
|
o A CPU looping with interrupts disabled.
|
|
|
|
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(). If the looping in the kernel is
|
|
really expected and desirable behavior, you might need to add
|
|
some calls to cond_resched().
|
|
|
|
o 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.
|
|
|
|
o 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.
|
|
|
|
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_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.
|
|
|
|
o 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.
|
|
|
|
o 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!
|
|
|
|
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_COMMON=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, RCU-bh, 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.
|
|
|
|
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 jiffies. 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 it 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=7073, c=7072, 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 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.
|
|
|
|
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 "fps=" 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 number of the last grace period
|
|
to start and to complete (7073 and 7072, respectively), and an estimate
|
|
of the total number of RCU callbacks queued across all CPUs (625 in
|
|
this case).
|
|
|
|
In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed
|
|
for each CPU:
|
|
|
|
0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
|
|
|
|
The "last_accelerate:" prints the low-order 16 bits (in hex) of the
|
|
jiffies counter when this CPU last invoked rcu_try_advance_all_cbs()
|
|
from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from
|
|
rcu_prepare_for_idle(). The "nonlazy_posted:" prints the number
|
|
of non-lazy callbacks posted since the last call to rcu_needs_cpu().
|
|
Finally, an "L" indicates that there are currently no non-lazy callbacks
|
|
("." is printed otherwise, as shown above) and "D" indicates that
|
|
dyntick-idle processing is enabled ("." is printed otherwise, for example,
|
|
if disabled via the "nohz=" kernel boot parameter).
|
|
|
|
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:
|
|
|
|
kthread starved for 23807 jiffies! g7073 c7072 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
|
|
|
|
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" and "c" numbers flag the
|
|
number of the last grace period started and completed, respectively,
|
|
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, and the "state" precedes value of the task_struct ->state field.
|
|
|
|
|
|
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.
|
|
|
|
|
|
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.
|