On Thu 09-07-20 15:31:23, Yafang Shao wrote: > On Thu, Jul 9, 2020 at 2:26 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > 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. > > No, I don't think anybody will use the reported 0 to get the > conclusion that it is a OOM_SCORE_ADJ_MIN task. > Because, > points = oom_badness(task, totalpages) * 1000 / totalpages; > so the points will always be 0 if the return value of > oom_badness(task, totalpages) is less than totalpages/1000. Correct, I've had the other implication in mind though. OOM_SCORE_ADJ_MIN -> score == 0 > If the user wants to know whether it is an OOM_SCORE_ADJ_MIN task, he > will always use /proc/[pid]/oom_score_adj to get it, that is more > reliable. > > > So you need to special case LONG_MIN at least. It would be also better > > to stick with [0, 2000] range. > > I don't know why it must stick with [0, 2000] range. Because this is a long term exported API and userspace might depend on it. -- Michal Hocko SUSE Labs
