On Fri 07-09-18 11:36:55, Dmitry Vyukov wrote: > On Fri, Sep 7, 2018 at 10:27 AM, Michal Hocko <mhocko@xxxxxxxxxx> 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). > > > Should I add "vm.oom_dump_tasks = 0" to /etc/sysctl.conf on syzbot? > It looks like it will make things faster, not pollute console output, > prevent these stalls and that output does not seem to be too useful > for debugging. I think that oom_dump_tasks has only very limited usefulness for your testing. > But I am still concerned as to what has changed recently. Potentially > this happens only on linux-next, at least that's where I saw all > existing reports. > New tasks seem to be added to the tail of the tasks list, but this > part does not seem to be changed recently in linux-next.. Yes, that would be interesting to find out. -- Michal Hocko SUSE Labs
