two jiffies. (This is a cpp macro, not a kernel configuration
parameter.)
-When a CPU detects that it is stalling, it will print a message similar
-to the following:
+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 wtih 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.
+
+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_state detected stall on CPU 5 (t=2500 jiffies)
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
+while on 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
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,
+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
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 For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the
+ kernel without invoking schedule(). Note that cond_resched()
+ does not necessarily prevent RCU CPU stall warnings. Therefore,
+ if the looping in the kernel is really expected and desirable
+ behavior, you might need to replace some of the cond_resched()
+ calls with calls to cond_resched_rcu_qs().
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
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
+ 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_TREE_PREEMPT_RCU case, you might see stall-warning
+ CONFIG_PREEMPT_RCU case, you might see stall-warning
messages.
o A hardware or software issue shuts off the scheduler-clock
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.
+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.
projects / mirror_ubuntu-zesty-kernel.git / blobdiff