On Fri, Aug 25, 2017 at 10:14:03AM +0200, Michal Hocko wrote: > On Thu 24-08-17 15:58:01, Roman Gushchin wrote: > > On Thu, Aug 24, 2017 at 04:13:37PM +0200, Michal Hocko wrote: > > > On Thu 24-08-17 14:58:42, Roman Gushchin wrote: > [...] > > > > Both ways are not ideal, and sum of the processes is not ideal too. > > > > Especially, if you take oom_score_adj into account. Will you respect it? > > > > > > Yes, and I do not see any reason why we shouldn't. > > > > It makes things even more complicated. > > Right now task's oom_score can be in (~ -total_memory, ~ +2*total_memory) range, > > and it you're starting summing it, it can be multiplied by number of tasks... > > Weird. > > oom_score_adj is just a normalized bias so if tasks inside oom will use > it the whole memcg will get accumulated bias from all such tasks so it > is not completely off. I agree that the more tasks use the bias the more > biased the whole memcg will be. This might or might not be a problem. > As you are trying to reimplement the existing oom killer implementation > I do not think we cannot simply ignore API which people are used to. > > If this was a configurable oom policy then I could see how ignoring > oom_score_adj is acceptable because it would be an explicit opt-in. > > > It also will be different in case of system and memcg-wide OOM. > > Why, we do honor oom_score_adj for the memcg OOM now and in fact the > kernel memcg OOM killer shouldn't be very much different from the global > one except for the tasks scope. > > > > > I've started actually with such approach, but then found it weird. > > > > > > > > > Besides that you have > > > > > to check each task for over-killing anyway. So I do not see any > > > > > performance merits here. > > > > > > > > It's an implementation detail, and we can hopefully get rid of it at some point. > > > > > > Well, we might do some estimations and ignore oom scopes but I that > > > sounds really complicated and error prone. Unless we have anything like > > > that then I would start from tasks and build up the necessary to make a > > > decision at the higher level. > > > > Seriously speaking, do you have an example, when summing per-process > > oom_score will work better? > > The primary reason I am pushing for this is to have the common iterator > code path (which we have since Vladimir has unified memcg and global oom > paths) and only parametrize the value calculation and victim selection. > > > Especially, if we're talking about customizing oom_score calculation, > > it makes no sence to me. How you will sum process timestamps? > > Well, I meant you could sum oom_badness for your particular > implementation. If we need some other policy then this wouldn't work and > that's why I've said that I would like to preserve the current common > code and only parametrize value calculation and victim selection... I've spent some time to implement such a version. It really became shorter and more existing code were reused, howewer I've met a couple of serious issues: 1) Simple summing of per-task oom_score doesn't make sense. First, we calculate oom_score per-task, while should sum per-process values, or, better, per-mm struct. We can take only threa-group leader's score into account, but it's also not 100% accurate. And, again, we have a question what to do with per-task oom_score_adj, if we don't task the task's oom_score into account. Using memcg stats still looks to me as a more accurate and consistent way of estimating memcg memory footprint. 2) If we're treating tasks from not-kill-all cgroups as separate oom entities, and compare them with memcgs with kill-all flag, we definitely need per-task oom_priority to provide a clear way to compare entities. Otherwise we need per-memcg size-based oom_score_adj, which is not the best idea, as we agreed earlier. Thanks! Roman -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
