Re: [PATCH tip/core/rcu 11/15] rcu: Avoid spurious RCU CPU stallwarnings

[Steven Rostedt]

On Thu, 2012-09-06 at 14:03 -0700, Paul E. McKenney wrote:

> Here are a few other ways that stalls can happen:
> 
> o	A CPU looping in an RCU read-side critical section.

For a minute? That's a bug.

> 	
> o	A CPU looping with interrupts disabled.  This condition can
> 	result in RCU-sched and RCU-bh stalls.

Also a bug.

> 
> o	A CPU looping with preemption disabled.  This condition can
> 	result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
> 	stalls.

Bug as well.

> 
> o	A CPU looping with bottom halves disabled.  This condition can
> 	result in RCU-sched and RCU-bh stalls.

Bug too.

> 
> o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
> 	without invoking schedule().

Another bug.

> 
> 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.

Buggy system.

> 
> 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.

Not really a bug, but the developers need a spanking.

> 
> 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=n kernels.

Driving the bug.

> 
> o	A bug in the RCU implementation.

Bug in the name.

> 
> 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.

Hardware bug.

So, where's the "spurious RCU CPU stall warnings"?

All these cases deserve a warning.

-- Steve


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