On Fri 29-10-21 09:07:39, Suren Baghdasaryan wrote: > On Fri, Oct 29, 2021 at 6:03 AM Michal Hocko <mhocko@xxxxxxxx> wrote: [...] > > Well, I still do not see why that is a problem. This syscall is meant to > > release the address space not to do it fast. > > It's the same problem for a userspace memory reaper as for the > oom-reaper. The goal is to release the memory of the victim and to > quickly move on to the next one if needed. The purpose of the oom_reaper is to _guarantee_ a forward progress. It doesn't have to be quick or optimized for speed. [...] > > Btw. the above code will not really tell you much on a larger machine > > unless you manage to trigger mmap_sem contection. Otherwise you are > > measuring the mmap_sem writelock fast path and that should be really > > within a noise comparing to the whole address space destruction time. If > > that is not the case then we have a real problem with the locking... > > My understanding of that discussion is that the concern was that even > taking uncontended mmap_sem writelock would regress the exit path. > That was what I wanted to confirm. Am I misreading it? No, your reading match my recollection. I just think that code robustness in exchange of a rw semaphore write lock fast path is a reasonable price to pay even if that has some effect on micro benchmarks. -- Michal Hocko SUSE Labs
