linux-kernel - Re: RCU stall

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Date:	Tue, 22 Mar 2016 16:53:26 -0700
From:	Bart Van Assche <bart.vanassche@...disk.com>
To:	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RCU stall

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.

Thanks,

Bart.