On Wed, Apr 21, 2021 at 12:16 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > [...] > > To decide when to kill, the oom-killer has to read a lot of metrics. > > It has to open a lot of files to read them and there will definitely > > be new allocations involved in those operations. For example reading > > memory.stat does a page size allocation. Similarly, to perform action > > the oom-killer may have to read cgroup.procs file which again has > > allocation inside it. > > True but many of those can be avoided by opening the file early. At > least seq_file based ones will not allocate later if the output size > doesn't increase. Which should be the case for many. I think it is a > general improvement to push those who allocate during read to an open > time allocation. > I agree that this would be a general improvement but it is not always possible (see below). > > Regarding sophisticated oom policy, I can give one example of our > > cluster level policy. For robustness, many user facing jobs run a lot > > of instances in a cluster to handle failures. Such jobs are tolerant > > to some amount of failures but they still have requirements to not let > > the number of running instances below some threshold. Normally killing > > such jobs is fine but we do want to make sure that we do not violate > > their cluster level agreement. So, the userspace oom-killer may > > dynamically need to confirm if such a job can be killed. > > What kind of data do you need to examine to make those decisions? > Most of the time the cluster level scheduler pushes the information to the node controller which transfers that information to the oom-killer. However based on the freshness of the information the oom-killer might request to pull the latest information (IPC and RPC). [...] > > > > I was thinking of simply prctl(SET_MEMPOOL, bytes) to assign mempool > > to a thread (not shared between threads) and prctl(RESET_MEMPOOL) to > > free the mempool. > > I am not a great fan of prctl. It has become a dumping ground for all > mix of unrelated functionality. But let's say this is a minor detail at > this stage. I agree this does not have to be prctl(). > So you are proposing to have a per mm mem pool that would be I was thinking of per-task_struct instead of per-mm_struct just for simplicity. > used as a fallback for an allocation which cannot make a forward > progress, right? Correct > Would that pool be preallocated and sitting idle? Correct > What kind of allocations would be allowed to use the pool? I was thinking of any type of allocation from the oom-killer (or specific threads). Please note that the mempool is the backup and only used in the slowpath. > What if the pool is depleted? This would mean that either the estimate of mempool size is bad or oom-killer is buggy and leaking memory. I am open to any design directions for mempool or some other way where we can provide a notion of memory guarantee to oom-killer. thanks, Shakeel
