Re: RCU stall

From: Paul E. McKenney
Date: Tue Mar 22 2016 - 21:59:40 EST

On Tue, Mar 22, 2016 at 04:53:26PM -0700, Bart Van Assche wrote:
> On 03/22/2016 01:45 PM, Paul E. McKenney wrote:
> >You are getting a soft lockup as well as an RCU CPU stall warning, so
> >it looks like something is taking a very long time in blk_done_softirq().
> >
> >You have multiple occurrences at different times, so it looks to be
> >a long time as opposed to an infinite time. Are you perhaps doing
> >something that would make a huge amount of work for blk_done_softirq()?
> >
> >See Documentation/RCU/stallwarn.txt in the kernel source tree for more
> >info on how to debug this sort of thing.
> Hello Paul,
> None of the drivers involved in the test I ran contain RCU code that
> has been changed recently. The block and SCSI subsystems processes
> I/O completions in softirq context but until last week I hadn't seen
> any RCU lockup complaints when I ran an SRP test against a kernel
> with lockdep and several other kernel debugging options enabled.
> This is why I sent an e-mail to you. I have read
> Documentation/RCU/stallwarn.txt after I received your reply but this
> didn't provide me any clue about where to look for the root cause.
> Any further help would be appreciated.

My suggestion would be to check the block/SCSI softirq handler for
event traces. If there are some, enable them and see what the loop
is doing. Documentation/trace/ftrace.txt describes how to enable
existing event tracing.

If there is no event tracing, consider adding some in your local
view. Failing that, there is always printk(). ;-)

Or perhaps you have some sort of debug setup.

Either way, the next step is to work out why that CPU is spending
so much time in that loop.

Thanx, Paul