On Fri, Jan 20, 2023 at 12:52 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Thu 19-01-23 10:52:03, Suren Baghdasaryan wrote: > > On Thu, Jan 19, 2023 at 4:59 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Mon 09-01-23 12:53:34, Suren Baghdasaryan wrote: > > > > call_rcu() can take a long time when callback offloading is enabled. > > > > Its use in the vm_area_free can cause regressions in the exit path when > > > > multiple VMAs are being freed. To minimize that impact, place VMAs into > > > > a list and free them in groups using one call_rcu() call per group. > > > > > > After some more clarification I can understand how call_rcu might not be > > > super happy about thousands of callbacks to be invoked and I do agree > > > that this is not really optimal. > > > > > > On the other hand I do not like this solution much either. > > > VM_AREA_FREE_LIST_MAX is arbitrary and it won't really help all that > > > much with processes with a huge number of vmas either. It would still be > > > in housands of callbacks to be scheduled without a good reason. > > > > > > Instead, are there any other cases than remove_vma that need this > > > batching? We could easily just link all the vmas into linked list and > > > use a single call_rcu instead, no? This would both simplify the > > > implementation, remove the scaling issue as well and we do not have to > > > argue whether VM_AREA_FREE_LIST_MAX should be epsilon or epsilon + 1. > > > > Yes, I agree the solution is not stellar. I wanted something simple > > but this is probably too simple. OTOH keeping all dead vm_area_structs > > on the list without hooking up a shrinker (additional complexity) does > > not sound too appealing either. > > I suspect you have missed my idea. I do not really want to keep the list > around or any shrinker. It is dead simple. Collect all vmas in > remove_vma and then call_rcu the whole list at once after the whole list > (be it from exit_mmap or remove_mt). See? Yes, I understood your idea but keeping dead objects until the process exits even when the system is low on memory (no shrinkers attached) seems too wasteful. If we do this I would advocate for attaching a shrinker. > > -- > Michal Hocko > SUSE Labs
