On Thu 09-07-20 10:14:14, Yafang Shao wrote: > On Thu, Jul 9, 2020 at 3:02 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > On Wed 08-07-20 10:57:27, David Rientjes wrote: > > > On Wed, 8 Jul 2020, Michal Hocko wrote: > > > > > > > I have only now realized that David is not on Cc. Add him here. The > > > > patch is http://lkml.kernel.org/r/1594214649-9837-1-git-send-email-laoar.shao@xxxxxxxxx. > > > > > > > > I believe the main problem is that we are normalizing to oom_score_adj > > > > units rather than usage/total. I have a very vague recollection this has > > > > been done in the past but I didn't get to dig into details yet. > > > > > > > > > > The memcg max is 4194304 pages, and an oom_score_adj of -998 would yield a > > > page adjustment of: > > > > > > adj = -998 * 4194304 / 1000 = −4185915 pages > > > > > > The largest pid 58406 (data_sim) has rss 3967322 pages, > > > pgtables 37101568 / 4096 = 9058 pages, and swapents 0. So it's unadjusted > > > badness is > > > > > > 3967322 + 9058 pages = 3976380 pages > > > > > > Factoring in oom_score_adj, all of these processes will have a badness of > > > 1 because oom_badness() doesn't underflow, which I think is the point of > > > Yafang's proposal. > > > > > > I think the patch can work but, as you mention, also needs an update to > > > proc_oom_score(). proc_oom_score() is using the global amount of memory > > > so Yafang is likely not seeing it go negative for that reason but it could > > > happen. > > > > Yes, memcg just makes it more obvious but the same might happen for the > > global case. I am not sure how we can both alow underflow and present > > the value that would fit the existing model. The exported value should > > really reflect what the oom killer is using for the calculation or we > > are going to see discrepancies between the real oom decision and > > presented values. So I believe we really have to change the calculation > > rather than just make it tolerant to underflows. > > > > Hi Michal, > > - Before my patch, > The result of oom_badness() is [1, 2 * totalpages), > and the result of proc_oom_score() is [0, 2000). > > While the badness score in the Documentation/filesystems/proc.rst is: [0, 1000] > "The badness heuristic assigns a value to each candidate task ranging from 0 > (never kill) to 1000 (always kill) to determine which process is targeted" > > That means, we need to update the documentation anyway unless my > calculation is wrong. No, your calculation is correct. The documentation is correct albeit slightly misleading. The net score calculation is indeed in range of [0, 1000]. It is the oom_score_adj added on top which skews it. This is documented as "The value of /proc/<pid>/oom_score_adj is added to the badness score before it is used to determine which task to kill." This is the exported value but paragraph "3.2 /proc/<pid>/oom_score" only says "This file can be used to check the current score used by the oom-killer is for any given <pid>." which is not really explicit about the exported range. Maybe clarifying that would be helpful. I will post a patch. There are few other things to sync up with the current state. > So the point will be how to change it ? > > - After my patch > oom_badness(): (-totalpages, 2 * totalpages) > proc_oom_score(): (-1000, 2000) > > If we allow underflow, we can change the documentation as "from -1000 > (never kill) to 2000(always kill)". > While if we don't allow underflow, we can make bellow simple change, > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index 774784587..0da8efa41 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -528,7 +528,7 @@ static int proc_oom_score(struct seq_file *m, > struct pid_namespace *ns, > unsigned long totalpages = totalram_pages + total_swap_pages; > unsigned long points = 0; > > - points = oom_badness(task, NULL, NULL, totalpages) * > + points = 1000 + oom_badness(task, NULL, NULL, totalpages) * > 1000 / totalpages; > seq_printf(m, "%lu\n", points); > > And then update the documentation as "from 0 (never kill) to 3000 > (always kill)" This is still not quite there yet, I am afraid. OOM_SCORE_ADJ_MIN tasks have always reported 0 and I can imagine somebody might depend on this fact. So you need to special case LONG_MIN at least. It would be also better to stick with [0, 2000] range. -- Michal Hocko SUSE Labs
