On Thu 28-01-16 18:51:10, Johannes Weiner wrote: > On Thu, Jan 28, 2016 at 03:19:08PM -0800, David Rientjes wrote: > > On Thu, 28 Jan 2016, Johannes Weiner wrote: > > > > > The check has to happen while holding the OOM lock, otherwise we'll > > > end up killing much more than necessary when there are many racing > > > allocations. > > > > > > > Right, we need to try with ALLOC_WMARK_HIGH after oom_lock has been > > acquired. > > > > The situation is still somewhat fragile, however, but I think it's > > tangential to this patch series. If the ALLOC_WMARK_HIGH allocation fails > > because an oom victim hasn't freed its memory yet, and then the TIF_MEMDIE > > thread isn't visible during the oom killer's tasklist scan because it has > > exited, we still end up killing more than we should. The likelihood of > > this happening grows with the length of the tasklist. > > > > Perhaps we should try testing watermarks after a victim has been selected > > and immediately before killing? (Aside: we actually carry an internal > > patch to test mem_cgroup_margin() in the memcg oom path after selecting a > > victim because we have been hit with this before in the memcg path.) > > > > I would think that retrying with ALLOC_WMARK_HIGH would be enough memory > > to deem that we aren't going to immediately reenter an oom condition so > > the deferred killing is a waste of time. > > > > The downside is how sloppy this would be because it's blurring the line > > between oom killer and page allocator. We'd need the oom killer to return > > the selected victim to the page allocator, try the allocation, and then > > call oom_kill_process() if necessary. > > https://lkml.org/lkml/2015/3/25/40 > > We could have out_of_memory() wait until the number of outstanding OOM > victims drops to 0. Then __alloc_pages_may_oom() doesn't relinquish > the lock until its kill has been finalized: > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > index 914451a..4dc5b9d 100644 > --- a/mm/oom_kill.c > +++ b/mm/oom_kill.c > @@ -892,7 +892,9 @@ bool out_of_memory(struct oom_control *oc) > * Give the killed process a good chance to exit before trying > * to allocate memory again. > */ > - schedule_timeout_killable(1); > + if (!test_thread_flag(TIF_MEMDIE)) > + wait_event_timeout(oom_victims_wait, > + !atomic_read(&oom_victims), HZ); > } > return true; > }
Yes this makes sense to me -- Michal Hocko SUSE Labs
