On Wed 03-02-16 15:48:18, David Rientjes wrote: > On Wed, 3 Feb 2016, Michal Hocko wrote: > > > From: Michal Hocko <mhocko@xxxxxxxx> > > > > This is based on the idea from Mel Gorman discussed during LSFMM 2015 and > > independently brought up by Oleg Nesterov. > > > > The OOM killer currently allows to kill only a single task in a good > > hope that the task will terminate in a reasonable time and frees up its > > memory. Such a task (oom victim) will get an access to memory reserves > > via mark_oom_victim to allow a forward progress should there be a need > > for additional memory during exit path. > > > > It has been shown (e.g. by Tetsuo Handa) that it is not that hard to > > construct workloads which break the core assumption mentioned above and > > the OOM victim might take unbounded amount of time to exit because it > > might be blocked in the uninterruptible state waiting for an event > > (e.g. lock) which is blocked by another task looping in the page > > allocator. > > > > This patch reduces the probability of such a lockup by introducing a > > specialized kernel thread (oom_reaper) which tries to reclaim additional > > memory by preemptively reaping the anonymous or swapped out memory > > owned by the oom victim under an assumption that such a memory won't > > be needed when its owner is killed and kicked from the userspace anyway. > > There is one notable exception to this, though, if the OOM victim was > > in the process of coredumping the result would be incomplete. This is > > considered a reasonable constrain because the overall system health is > > more important than debugability of a particular application. > > > > A kernel thread has been chosen because we need a reliable way of > > invocation so workqueue context is not appropriate because all the > > workers might be busy (e.g. allocating memory). Kswapd which sounds > > like another good fit is not appropriate as well because it might get > > blocked on locks during reclaim as well. > > > > oom_reaper has to take mmap_sem on the target task for reading so the > > solution is not 100% because the semaphore might be held or blocked for > > write but the probability is reduced considerably wrt. basically any > > lock blocking forward progress as described above. In order to prevent > > from blocking on the lock without any forward progress we are using only > > a trylock and retry 10 times with a short sleep in between. > > Users of mmap_sem which need it for write should be carefully reviewed > > to use _killable waiting as much as possible and reduce allocations > > requests done with the lock held to absolute minimum to reduce the risk > > even further. > > > > The API between oom killer and oom reaper is quite trivial. wake_oom_reaper > > updates mm_to_reap with cmpxchg to guarantee only NULL->mm transition > > and oom_reaper clear this atomically once it is done with the work. This > > means that only a single mm_struct can be reaped at the time. As the > > operation is potentially disruptive we are trying to limit it to the > > ncessary minimum and the reaper blocks any updates while it operates on > > an mm. mm_struct is pinned by mm_count to allow parallel exit_mmap and a > > race is detected by atomic_inc_not_zero(mm_users). > > > > Chnages since v4 > > - drop MAX_RT_PRIO-1 as per David - memcg/cpuset/mempolicy OOM killing > > might interfere with the rest of the system > > Changes since v3 > > - many style/compile fixups by Andrew > > - unmap_mapping_range_tree needs full initialization of zap_details > > to prevent from missing unmaps and follow up BUG_ON during truncate > > resp. misaccounting - Kirill/Andrew > > - exclude mlocked pages because they need an explicit munlock by Kirill > > - use subsys_initcall instead of module_init - Paul Gortmaker > > - do not tear down mm if it is shared with the global init because this > > could lead to SEGV and panic - Tetsuo > > Changes since v2 > > - fix mm_count refernce leak reported by Tetsuo > > - make sure oom_reaper_th is NULL after kthread_run fails - Tetsuo > > - use wait_event_freezable rather than open coded wait loop - suggested > > by Tetsuo > > Changes since v1 > > - fix the screwed up detail->check_swap_entries - Johannes > > - do not use kthread_should_stop because that would need a cleanup > > and we do not have anybody to stop us - Tetsuo > > - move wake_oom_reaper to oom_kill_process because we have to wait > > for all tasks sharing the same mm to get killed - Tetsuo > > - do not reap mm structs which are shared with unkillable tasks - Tetsuo > > > > Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx> > > Suggested-by: Mel Gorman <mgorman@xxxxxxx> > > Acked-by: Mel Gorman <mgorman@xxxxxxx> > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > > Acked-by: David Rientjes <rientjes@xxxxxxxxxx> Thanks! > I think all the patches could really have been squashed together because > subsequent patches just overwrite already added code. The primary reason is a better bisectability and incremental nature of changes. > I was going to > suggest not doing atomic_inc(&mm->mm_count) in wake_oom_reaper() and > change oom_kill_process() to do I found it easier to follow the reference counting that way (pin the mm at the place when I hand over it to the async thread). > > if (can_oom_reap) > wake_oom_reaper(mm); > else > mmdrop(mm); > > but I see that we don't even touch mm->mm_count after the third patch. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>