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> I think all the patches could really have been squashed together because subsequent patches just overwrite already added code. I was going to suggest not doing atomic_inc(&mm->mm_count) in wake_oom_reaper() and change oom_kill_process() to do 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. -- 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>