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). -- Michal Hocko SUSE Labs
