On Mon, Jul 24, 2017 at 04:15:26PM +0200, Michal Hocko wrote: > On Mon 24-07-17 17:00:08, Kirill A. Shutemov wrote: > > On Mon, Jul 24, 2017 at 09:23:32AM +0200, Michal Hocko wrote: > > > From: Michal Hocko <mhocko@xxxxxxxx> > > > > > > David has noticed that the oom killer might kill additional tasks while > > > the exiting oom victim hasn't terminated yet because the oom_reaper marks > > > the curent victim MMF_OOM_SKIP too early when mm->mm_users dropped down > > > to 0. The race is as follows > > > > > > oom_reap_task do_exit > > > exit_mm > > > __oom_reap_task_mm > > > mmput > > > __mmput > > > mmget_not_zero # fails > > > exit_mmap # frees memory > > > set_bit(MMF_OOM_SKIP) > > > > > > The victim is still visible to the OOM killer until it is unhashed. > > > > > > Currently we try to reduce a risk of this race by taking oom_lock > > > and wait for out_of_memory sleep while holding the lock to give the > > > victim some time to exit. This is quite suboptimal approach because > > > there is no guarantee the victim (especially a large one) will manage > > > to unmap its address space and free enough memory to the particular oom > > > domain which needs a memory (e.g. a specific NUMA node). > > > > > > Fix this problem by allowing __oom_reap_task_mm and __mmput path to > > > race. __oom_reap_task_mm is basically MADV_DONTNEED and that is allowed > > > to run in parallel with other unmappers (hence the mmap_sem for read). > > > > > > The only tricky part is to exclude page tables tear down and all > > > operations which modify the address space in the __mmput path. exit_mmap > > > doesn't expect any other users so it doesn't use any locking. Nothing > > > really forbids us to use mmap_sem for write, though. In fact we are > > > already relying on this lock earlier in the __mmput path to synchronize > > > with ksm and khugepaged. > > > > That's true, but we take mmap_sem there for small portion of cases. > > > > It's quite different from taking the lock unconditionally. I'm worry about > > scalability implication of such move. On bigger machines it can be big > > hit. > > What kind of scalability implication you have in mind? There is > basically a zero contention on the mmap_sem that late in the exit path > so this should be pretty much a fast path of the down_write. I agree it > is not 0 cost but the cost of the address space freeing should basically > make it a noise. Even in fast path case, it adds two atomic operation per-process. If the cache line is not exclusive to the core by the time of exit(2) it can be noticible. ... but I guess it's not very hot scenario. I guess I'm just too cautious here. :) > > Should we do performance/scalability evaluation of the patch before > > getting it applied? > > What kind of test(s) would you be interested in? Can we at lest check that number of /bin/true we can spawn per second wouldn't be harmed by the patch? ;) -- Kirill A. Shutemov -- 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>
