Re: rcu_preempt detected stalls

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Aug 31, 2021 at 05:21:44PM +0200, Jorge Ramirez-Ortiz, Foundries 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.

The message above really does mean what it says:  If your workload
prevents RCU's grace-period kthread ("rcu_preempt" in this case) from
running, you just bought yourself an OOM.

> [   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?).

RCU priority boosting sets the rcu_preempt kthread's scheduling priority
to SCHED_FIFO priority level 1 instead of the normal SCHED_OTHER.
Therefore, if you build with CONFIG_RCU_BOOST=n, but manually set the
priority of rcu_preempt to SCHED_FIFO priority level 1, you might also
see this RCU CPU stall warning go away.

> 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?

I have not looked at stress-ng, but it is possible to configure your
system so that rcu_preempt gets little or no CPU time, for example,
by placing it into a CPU-poor cgroup on the one hand or by disabling
throttling and running a heavy real-time workload on the other.
Is stress-ng doing something like this?

There could of course also be an arm64 problem that affect scheduling,
but I suggest looking closely at what stress-ng is doing first.

Please let me know how it goes!

							Thanx, Paul

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