Re: [PATCH 4/3] mm, oom: drop the last allocation attempt before out_of_memory
From: Michal Hocko
Date: Fri Jan 29 2016 - 10:32:57 EST
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.
> 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