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
