On Mon, Jan 23, 2023 at 1:59 AM 'Michal Hocko' via kernel-team <kernel-team@xxxxxxxxxxx> wrote: > > On Fri 20-01-23 08:20:43, Suren Baghdasaryan wrote: > > 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. > > I am still not sure we are on the same page here. No, vmas shouldn't lay > around un ntil the process exit. I am really suggesting queuing only for > remove_vma paths. You can have a different rcu callback than the one > used for trivial single vma removal paths. Oh, I also missed the remove_mt() part and thought you want to drain the list only in exit_mmap(). I think that's a good option! > > -- > Michal Hocko > SUSE Labs > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. >
