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. 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
