rcu_preempt detected stalls

Tue Aug 31 10:11:59 PDT 2021

followings are some of my kernel config options:
CONFIG_PREEMPT=y
CONFIG_PREEMPT_COUNT=y
CONFIG_PREEMPTION=y
CONFIG_PREEMPT_DYNAMIC=y
CONFIG_PREEMPT_RCU=y
CONFIG_HAVE_PREEMPT_DYNAMIC=y
CONFIG_TREE_RCU=y
CONFIG_PREEMPT_RCU=y
CONFIG_RCU_EXPERT=y
CONFIG_SRCU=y
CONFIG_TREE_SRCU=y
CONFIG_TASKS_RCU_GENERIC=y
CONFIG_TASKS_RCU=y
CONFIG_TASKS_RUDE_RCU=y
CONFIG_TASKS_TRACE_RCU=y
CONFIG_RCU_STALL_COMMON=y
CONFIG_RCU_NEED_SEGCBLIST=y
CONFIG_RCU_FANOUT=64
CONFIG_RCU_FANOUT_LEAF=16
# CONFIG_RCU_FAST_NO_HZ is not set
# CONFIG_RCU_BOOST is not set
CONFIG_RCU_NOCB_CPU=y
# CONFIG_TASKS_TRACE_RCU_READ_MB is not set
# end of RCU Subsystem
CONFIG_MMU_GATHER_RCU_TABLE_FREE=y
CONFIG_LOCKDEP_CIRCULAR_QUEUE_BITS=12
# RCU Debugging
CONFIG_PROVE_RCU=y
# CONFIG_PROVE_RCU_LIST is not set
# CONFIG_RCU_SCALE_TEST is not set
CONFIG_RCU_TORTURE_TEST=m
# CONFIG_RCU_REF_SCALE_TEST is not set
CONFIG_RCU_CPU_STALL_TIMEOUT=21
# CONFIG_RCU_TRACE is not set
# CONFIG_RCU_EQS_DEBUG is not set

Thanks
Zhouyi

On Wed, Sep 1, 2021 at 1:01 AM Zhouyi Zhou <zhouzhouyi at gmail.com> wrote:
>
> I did an experiment just now on x86_64 virtual machines, rcu did not
> complain after 10 minutes's test, I hope my effort can provide some
> clue.
>
> 1. I clone the fresh new linux kernel (git clone
> https://kernel.source.codeaurora.cn/pub/scm/linux/kernel/git/torvalds/linux.git)
> 2. compile the kernel without CONFIG_RCU_BOOST (: # CONFIG_RCU_BOOST is not set)
> 3. boot the kernel on a x86_64 VM (kvm -cpu host -smp 16  -hda
> ./debian10.qcow2 -m 4096 -net
> user,hostfwd=tcp::5556-:22,hostfwd=tcp::5555-:19 -net nic,model=e1000
> -vnc :30)
> 4. run the test (stress-ng --sequential 16  --class scheduler -t 5m --times)
> 5. monitor the system by constantly typing top and dmesg
> 6. after 10 minutes, nothing else happens except that the dmesg report
> following two messages
> [  672.528192] sched: DL replenish lagged too much
> [  751.127790] hrtimer: interrupt took 12143 ns
>
> So, I guess CONFIG_RCU_BOOST is not necessary for x86_64 virtual machines
>
> Zhouyi
>
> On Tue, Aug 31, 2021 at 11:24 PM Jorge Ramirez-Ortiz, Foundries
> <jorge at foundries.io> wrote:
> >
> > Hi
> >
> > When enabling CONFIG_PREEMPT and running the stress-ng scheduler class
> > tests on arm64 (xilinx zynqmp and imx imx8mm SoCs) we are observing the following.
> >
> > [   62.578917] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> > [   62.585015]  (detected by 0, t=5253 jiffies, g=3017, q=2972)
> > [   62.590663] rcu: All QSes seen, last rcu_preempt kthread activity 5254 (4294907943-4294902689), jiffies_till_next_fqs=1, root
> > +->qsmask 0x0
> > [   62.603086] rcu: rcu_preempt kthread starved for 5258 jiffies! g3017 f0x2 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
> > [   62.613246] rcu:     Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> > [   62.622359] rcu: RCU grace-period kthread stack dump:
> > [   62.627395] task:rcu_preempt     state:R  running task     stack:    0 pid:   14 ppid:     2 flags:0x00000028
> > [   62.637308] Call trace:
> > [   62.639748]  __switch_to+0x11c/0x190
> > [   62.643319]  __schedule+0x3b8/0x8d8
> > [   62.646796]  schedule+0x4c/0x108
> > [   62.650018]  schedule_timeout+0x1ac/0x358
> > [   62.654021]  rcu_gp_kthread+0x6a8/0x12b8
> > [   62.657933]  kthread+0x14c/0x158
> > [   62.661153]  ret_from_fork+0x10/0x18
> > [   62.682919] BUG: scheduling while atomic: stress-ng-hrtim/831/0x00000002
> > [   62.689604] Preemption disabled at:
> > [   62.689614] [<ffffffc010059418>] irq_enter_rcu+0x30/0x58
> > [   62.698393] CPU: 0 PID: 831 Comm: stress-ng-hrtim Not tainted 5.10.42+ #5
> > [   62.706296] Hardware name: Zynqmp new (DT)
> > [   62.710115] Call trace:
> > [   62.712548]  dump_backtrace+0x0/0x240
> > [   62.716202]  show_stack+0x2c/0x38
> > [   62.719510]  dump_stack+0xcc/0x104
> > [   62.722904]  __schedule_bug+0x78/0xc8
> > [   62.726556]  __schedule+0x70c/0x8d8
> > [   62.730037]  schedule+0x4c/0x108
> > [   62.733259]  do_notify_resume+0x224/0x5d8
> > [   62.737259]  work_pending+0xc/0x2a4
> >
> > The error results in OOM eventually.
> >
> > RCU priority boosting does work around this issue but it seems to me
> > a workaround more than a fix (otherwise boosting would be enabled
> > by CONFIG_PREEMPT for arm64 I guess?).
> >
> > The question is: is this an arm64 bug that should be investigated? or
> > is this some known corner case of running stress-ng that is already
> > understood?
> >
> > thanks
> > Jorge
> >
> >
> >