On Wed 09-03-16 13:18:50, Ingo Molnar wrote: > > * Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > Hi, > > > > the following patchset implements a killable variant of write lock for > > rw_semaphore. My usecase is to turn as many mmap_sem write users to use a > > killable variant which will be helpful for the oom_reaper [1] to asynchronously > > tear down the oom victim address space which requires mmap_sem for read. This > > will reduce a likelihood of OOM livelocks caused by oom victim being stuck on a > > lock or other resource which prevents it to reach its exit path and release the > > memory. [...] > > So I'm a tiny bit concerned about this arguments. > > AFAICS killability here just makes existing system calls more interruptible - > right? see below > In that sense that's not really a livelock scenario: it just takes shorter > time for resources to be released. > > If a livelock is possible (where resources are never released) then I'd like to > see a specific example of such a livelock. > > You have the other patch-set: > > [PATCH 0/18] change mmap_sem taken for write killable > > that makes use of down_write_killable(), and there you argue: > > [...] this is a follow up work for oom_reaper [1]. As the async OOM killing > depends on oom_sem for read we would really appreciate if a holder for write > stood in the way. This patchset is changing many of down_write calls to be > killable to help those cases when the writer is blocked and waiting for readers > to release the lock and so help __oom_reap_task to process the oom victim. > > there seems to be a misunderstanding: if a writer is blocked waiting for readers > then no new readers are allowed - the writer will get its turn the moment all > existing readers drop the lock. Readers might be blocked e.g. on the memory allocation which cannot proceed due to OOM. Such a reader might be operating on a remote mm. > So there's no livelock scenario - it's "only" about latencies. Latency is certainly one aspect of it as well because the sooner the mmap_sem gets released for other readers to sooner the oom_reaper can tear down the victims address space and release the memory and free up some memory so that we do not have to wait for the victim to exit. > And once we realize that it's about latencies (assuming I'm right!), not about > correctness per se, I'm wondering whether it would be a good idea to introduce > down_write_interruptible(), instead of down_write_killable(). I am not against interruptible variant as well but I suspect that some paths are not expected to return EINTR. I haven't checked them for this but killable is sufficient for the problem I am trying to solve. That problem is real while latencies do not seem to be that eminent. down_write_interruptible will be trivial to do on top. -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
