Re: [PATCH] rcu: Fix soft lockup for rcu_nocb_kthread

[Paul E. McKenney]

On Fri, Nov 18, 2016 at 08:37:28PM +0800, Ding Tianhong wrote:
> 
> 
> On 2016/8/10 9:59, Paul E. McKenney wrote:
> > On Wed, Aug 10, 2016 at 09:13:14AM +0800, Ding Tianhong wrote:
> >> On 2016/6/16 22:19, Paul E. McKenney wrote:
> >>> On Thu, Jun 16, 2016 at 02:09:47PM +0800, Ding Tianhong wrote:
> >>>> On 2016/6/15 23:49, Paul E. McKenney wrote:
> >>>>> On Wed, Jun 15, 2016 at 03:27:36PM +0800, Ding Tianhong wrote:
> >>>>>> I met this problem when using the Testgine to send package to ixgbevf nic
> >>>>>> by this steps:
> >>>>>> 1. Connect to ixgbevf, and set the speed to 10Gb/s, it could work fine.
> >>>>>> 2. Then use ifconfig to down the nic and up again, loop for several times.
> >>>>>> 3. The system panic by soft lockup.
> >>>>>
> >>>>> Good catch, queued for review and testing.  But what .config was your
> >>>>> kernel built with?
> >>>>>
> >>>>
> >>>> I use the redhat7.1 defconfig to build my kernel, and the RCU config is this:
> >>>>  120 #
> >>>>  121 # RCU Subsystem
> >>>>  122 #
> >>>>  123 CONFIG_TREE_RCU=y
> >>>>  124 # CONFIG_PREEMPT_RCU is not set
> >>>>  125 CONFIG_RCU_STALL_COMMON=y
> >>>>  126 CONFIG_CONTEXT_TRACKING=y
> >>>>  127 CONFIG_RCU_USER_QS=y
> >>>>  128 # CONFIG_CONTEXT_TRACKING_FORCE is not set
> >>>>  129 CONFIG_RCU_FANOUT=64
> >>>>  130 CONFIG_RCU_FANOUT_LEAF=16
> >>>>  131 # CONFIG_RCU_FANOUT_EXACT is not set
> >>>>  132 # CONFIG_RCU_FAST_NO_HZ is not set
> >>>>  133 # CONFIG_TREE_RCU_TRACE is not set
> >>>>  134 CONFIG_RCU_NOCB_CPU=y
> >>>>  135 CONFIG_RCU_NOCB_CPU_ALL=y
> >>>>  136 CONFIG_BUILD_BIN2C=y
> >>>
> >>> Thank you!  You were running with preemption disabled, so your system
> >>> would indeed be very susceptible to this problem.
> >>>
> >>>>> Also, I did tweak both the commit log and the patch.  Your cond_resched()
> >>>>> would prevent soft lockups, but not RCU stalls, so I substituted
> >>>>> cond_resched_rcu_qs().  Please let me know if either of those changes
> >>>>> causes problems at your end.
> >>>>
> >>>> Looks fine to me, I will apply this to my branch and test it, thanks.
> >>>
> >>> Please let me know how it goes!
> >>>
> >>> 							Thanx, Paul
> >>>
> >>
> >> Hi Paul:
> >>
> >> It has been a long time after applying this patch, and didn't found any problem, I believe this patch is fine, thanks.
> > 
> > Very good!  I will push this one upstream during the next merge window.
> > 
> > 							Thanx, Paul
> > 
> 
> Hi Paul:
> 
> Sorry to say that I have found this patch will introduce an OOM problem, it will be triggered by huge IP abnormal packet
> arrived, it looks that avoid process any pending softirqs in the rcuos kthread is the best way to fix this problem, I will
> send a new patch to revert this and fix the problem.

Interesting...

Could you please let me know exactly how the added cond_resched_rcu_qs()
leads to an OOM?  Is it that the softirqs prevent the grace-period kthread
from making progress?

							Thanx, Paul