On Tue 31-10-17 22:51:49, Tetsuo Handa wrote: > Michal Hocko wrote: > > On Tue 31-10-17 22:13:05, Tetsuo Handa wrote: > > > Michal Hocko wrote: > > > > On Tue 31-10-17 21:42:23, Tetsuo Handa wrote: > > > > > > While both have some merit, the first reason is mostly historical > > > > > > because we have the explicit locking now and it is really unlikely > > > > > > that > > > > > > the memory would be available right after we have given up trying. > > > > > > Last attempt allocation makes some sense of course but considering > > > > > > that > > > > > > the oom victim selection is quite an expensive operation which can > > > > > > take > > > > > > a considerable amount of time it makes much more sense to retry the > > > > > > allocation after the most expensive part rather than before. > > > > > > Therefore > > > > > > move the last attempt right before we are trying to kill an oom > > > > > > victim > > > > > > to rule potential races when somebody could have freed a lot of > > > > > > memory > > > > > > in the meantime. This will reduce the time window for potentially > > > > > > pre-mature OOM killing considerably. > > > > > > > > > > But this is about "doing last second allocation attempt after > > > > > selecting > > > > > an OOM victim". This is not about "allowing OOM victims to try > > > > > ALLOC_OOM > > > > > before selecting next OOM victim" which is the actual problem I'm > > > > > trying > > > > > to deal with. > > > > > > > > then split it into two. First make the general case and then add a more > > > > sophisticated on top. Dealing with multiple issues at once is what makes > > > > all those brain cells suffer. > > > > > > I'm failing to understand. I was dealing with single issue at once. > > > The single issue is "MMF_OOM_SKIP prematurely prevents OOM victims from > > > trying > > > ALLOC_OOM before selecting next OOM victims". Then, what are the general > > > case and > > > a more sophisticated? I wonder what other than "MMF_OOM_SKIP should allow > > > OOM > > > victims to try ALLOC_OOM for once before selecting next OOM victims" can > > > exist... > > > > Try to think little bit out of your very specific and borderline usecase > > and it will become obvious. ALLOC_OOM is a trivial update on top of > > moving get_page_from_freelist to oom_kill_process which is a more > > generic race window reducer. > > So, you meant "doing last second allocation attempt after selecting an OOM > victim" > as the general case and "using ALLOC_OOM at last second allocation attempt" > as a > more sophisticated. Then, you won't object conditionally switching > ALLOC_WMARK_HIGH > and ALLOC_OOM for last second allocation attempt, will you?
yes for oom_victims > But doing ALLOC_OOM for last second allocation attempt from out_of_memory() > involve > duplicating code (e.g. rebuilding zone list). Why would you do it? Do not blindly copy and paste code without a good reason. What kind of problem does this actually solve? > What is your preferred approach? > Duplicate relevant code? Use get_page_from_freelist() without rebuilding the > zone list? > Use __alloc_pages_nodemask() ? Just do what we do now with ALLOC_WMARK_HIGH and in a separate patch use ALLOC_OOM for oom victims. There shouldn't be any reasons to play additional tricks here. -- Michal Hocko SUSE Labs
