On 2018/09/07 17:27, Michal Hocko wrote: > On Fri 07-09-18 05:58:06, Tetsuo Handa wrote: >> On 2018/09/06 23:39, Michal Hocko wrote: >>>>>> I know /proc/sys/vm/oom_dump_tasks . Showing some entries while not always >>>>>> printing all entries might be helpful. >>>>> >>>>> Not really. It could be more confusing than helpful. The main purpose of >>>>> the listing is to double check the list to understand the oom victim >>>>> selection. If you have a partial list you simply cannot do that. >>>> >>>> It serves as a safeguard for avoiding RCU stall warnings. >>>> >>>>> >>>>> If the iteration takes too long and I can imagine it does with zillions >>>>> of tasks then the proper way around it is either release the lock >>>>> periodically after N tasks is processed or outright skip the whole thing >>>>> if there are too many tasks. The first option is obviously tricky to >>>>> prevent from duplicate entries or other artifacts. >>>>> >>>> >>>> Can we add rcu_lock_break() like check_hung_uninterruptible_tasks() does? >>> >>> This would be a better variant of your timeout based approach. But it >>> can still produce an incomplete task list so it still consumes a lot of >>> resources to print a long list of tasks potentially while that list is not >>> useful for any evaluation. Maybe that is good enough. I don't know. I >>> would generally recommend to disable the whole thing with workloads with >>> many tasks though. >>> >> >> The "safeguard" is useful when there are _unexpectedly_ many tasks (like >> syzbot in this case). Why not to allow those who want to avoid lockup to >> avoid lockup rather than forcing them to disable the whole thing? > > So you get an rcu lockup splat and what? Unless you have panic_on_rcu_stall > then this should be recoverable thing (assuming we cannot really > livelock as described by Dmitry). > syzbot is getting hung task panic (140 seconds) because one dump_tasks() from out_of_memory() consumes 52 seconds on a 2 CPU machine because we have only cond_resched() which can yield CPU resource to tasks which need CPU resource. This is similar to a bug shown below. [upstream] INFO: task hung in fsnotify_mark_destroy_workfn https://syzkaller.appspot.com/bug?id=0e75779a6f0faac461510c6330514e8f0e893038 [upstream] INFO: task hung in fsnotify_connector_destroy_workfn https://syzkaller.appspot.com/bug?id=aa11d2d767f3750ef9a40d156a149e9cfa735b73 Continuing printk() until khungtaskd fires is a stupid behavior.
