Re: [PATCH v6 0/2] rcu: Add RCU stall diagnosis information

Leizhen (ThunderTown)
2022-11-15T09:12:22Z

On 2022/11/13 2:59, Paul E. McKenney wrote:
> On Thu, Nov 10, 2022 at 10:27:44AM +0800, Leizhen (ThunderTown) wrote:
>> On 2022/11/10 1:22, Paul E. McKenney wrote:
>>> On Wed, Nov 09, 2022 at 06:03:17PM +0100, Frederic Weisbecker wrote:
>>>> On Wed, Nov 09, 2022 at 07:59:01AM -0800, Paul E. McKenney wrote:
>>>>> On Wed, Nov 09, 2022 at 04:26:21PM +0100, Frederic Weisbecker wrote:
>>>>>> Hi Zhen Lei,
>>>>>>
>>>>>> On Wed, Nov 09, 2022 at 05:37:36PM +0800, Zhen Lei wrote:
>>>>>>> v5 --> v6:
>>>>>>> 1. When there are more than two continuous RCU stallings, correctly handle the
>>>>>>> value of the second and subsequent sampling periods. Update comments and
>>>>>>> document.
>>>>>>> Thanks to Elliott, Robert for the test.
>>>>>>> 2. Change "rcu stall" to "RCU stall".
>>>>>>>
>>>>>>> v4 --> v5:
>>>>>>> 1. Resolve a git am conflict. No code change.
>>>>>>>
>>>>>>> v3 --> v4:
>>>>>>> 1. Rename rcu_cpu_stall_deep_debug to rcu_cpu_stall_cputime.
>>>>>>>
>>>>>>> v2 --> v3:
>>>>>>> 1. Fix the return type of kstat_cpu_irqs_sum()
>>>>>>> 2. Add Kconfig option CONFIG_RCU_CPU_STALL_DEEP_DEBUG and boot parameter
>>>>>>> rcupdate.rcu_cpu_stall_deep_debug.
>>>>>>> 3. Add comments and normalize local variable name
>>>>>>>
>>>>>>>
>>>>>>> v1 --> v2:
>>>>>>> 1. Fixed a bug in the code. If the rcu stall is detected by another CPU,
>>>>>>> kcpustat_this_cpu cannot be used.
>>>>>>> @@ -451,7 +451,7 @@ static void print_cpu_stat_info(int cpu)
>>>>>>> if (r->gp_seq != rdp->gp_seq)
>>>>>>> return;
>>>>>>>
>>>>>>> - cpustat = kcpustat_this_cpu->cpustat;
>>>>>>> + cpustat = kcpustat_cpu(cpu).cpustat;
>>>>>>> 2. Move the start point of statistics from rcu_stall_kick_kthreads() to
>>>>>>> rcu_implicit_dynticks_qs(), removing the dependency on irq_work.
>>>>>>>
>>>>>>> v1:
>>>>>>> In some extreme cases, such as the I/O pressure test, the CPU usage may
>>>>>>> be 100%, causing RCU stall. In this case, the printed information about
>>>>>>> current is not useful. Displays the number and usage of hard interrupts,
>>>>>>> soft interrupts, and context switches that are generated within half of
>>>>>>> the CPU stall timeout, can help us make a general judgment. In other
>>>>>>> cases, we can preliminarily determine whether an infinite loop occurs
>>>>>>> when local_irq, local_bh or preempt is disabled.
>>>>>>
>>>>>> That looks useful but I have to ask: what does it bring that the softlockup
>>>>>> and hardlockup watchdog can not already solve?
>>>>>
>>>>> This is a good point. One possible benefit is putting the needed information
>>>>> in one spot, for example, in cases where the soft/hard lockup timeouts are
>>>>> significantly different than the RCU CPU stall warning timeout.
>>>>
>>>> Arguably, the hardlockup/softlockup detectors usually trigger after RCU stall,
>>>> unless all CPUs are caught into a hardlockup, in which case only the hardlockup
>>>> detector has a chance.
>>>>
>>>> Anyway I would say that in this case just lower the delay for the lockup
>>>> detectors to consider the situation is a lockup?
>>>
>>> Try it both ways and see how it works? The rcutorture module parameters
>>> stall_cpu and stall_cpu_irqsoff are easy ways to generate these sorts
>>> of scenarios.
>>>
>>> Actually, that does remind me of something. Back when I was chasing
>>> that interrupt storm, would this patch have helped me? In that case, the
>>
>> Yes, this patch series originally addressed an RCU stall issue caused by an
>> interruption storm. The serial port driver written by another project team
>> failed to write the register in a specific condition. As a result, interrupts
>> were repeatedly reported.
>
> Very good!
>
>>> half-way point would have been reached while all online CPUs were spinning
>>> with interrupts disabled and the incoming CPU was getting hammered with
>>> continual scheduling-clock interrupts. So I suspect that the answer is
>>> "no" because the incoming CPU was not blocking the grace period.
>>>
>>> Instead of being snapshot halfway to the RCU CPU stall warning, should
>>> the values be snapshot when the CPU notices the beginning or end of an
>>> RCU grace period and when a CPU goes offline?
>>
>> This won't work. Those normal counts that occurred before the failure
>> have an impact on our analysis. For example, some software interrupts
>> may have been generated before local_bh_disable() is called.
>
> Fair enough, and thank you for considering this option. But please be
> prepared to adjust (somehow or another) as needed to accommodate other
> failure scenarios as they arise.

Except the document has an warning of "make htmldocs". I can't think of
anything to improve on the v7 at the moment.

Change the type of softirqs[NR_SOFTIRQS] from "unsigned int" to
"unsigned long", I will post a separate patch in future. Because
more people may join the discussion.

How about I post v8 tomorrow?

>
> Thanx, Paul
>
>>> But that would not suffice, because detailed information would not have
>>> been dumped for the incoming CPU.
>>>
>>> However, the lack of context switches and interrupts on the rest of the
>>> CPUs would likely have been a big cluebat, so there is that. It might
>>> be better to rework the warning at the beginning of rcu_sched_clock_irq()
>>> to complain if more than (say) 10 scheduling-clock interrupts occur on
>>> a given CPU during a single jiffy.
>>>
>>> Independent of Zhen Lei patch.
>>>
>>> Thoughts?
>>>
>>> Thanx, Paul
>>>
>>>> Thanks.
>>>>
>>>>
>>>>>
>>>>> Thoughts?
>>>>>
>>>>> Thanx, Paul
>>>>>
>>>>>> Thanks.
>>>>>>
>>>>>>>
>>>>>>> Zhen Lei (2):
>>>>>>> rcu: Add RCU stall diagnosis information
>>>>>>> doc: Document CONFIG_RCU_CPU_STALL_CPUTIME=y stall information
>>>>>>>
>>>>>>> Documentation/RCU/stallwarn.rst | 88 +++++++++++++++++++
>>>>>>> .../admin-guide/kernel-parameters.txt | 6 ++
>>>>>>> kernel/rcu/Kconfig.debug | 11 +++
>>>>>>> kernel/rcu/rcu.h | 1 +
>>>>>>> kernel/rcu/tree.c | 17 ++++
>>>>>>> kernel/rcu/tree.h | 19 ++++
>>>>>>> kernel/rcu/tree_stall.h | 29 ++++++
>>>>>>> kernel/rcu/update.c | 2 +
>>>>>>> 8 files changed, 173 insertions(+)
>>>>>>>
>>>>>>> --
>>>>>>> 2.25.1
>>>>>>>
>>> .
>>>
>>
>> --
>> Regards,
>> Zhen Lei
> .
>

--
Regards,
Zhen Lei