On Wed 09-03-16 14:17:10, Ingo Molnar wrote: > > * Michal Hocko <[email protected]> wrote: > > > > [...] 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. > > Doing complex allocations with the mm locked looks fragile no matter what: we > should add debugging code that warns if allocations are done with a remote mm > locked. (it should be trivial)
No matter how fragile is that it is not something non-existent. Just have a look at use_mm for example. We definitely do not want to warn about those, right? > In fact people were thining about turning the mm semaphore into a rwlock - > with > that no blocking call should be possible with the lock held. > > So I maintain: > > > > So there's no livelock scenario - it's "only" about latencies. > > With a qualification: s/only/mostly ;-) > > > 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. > > If they don't expect EINTR then they sure don't expect SIGKILL either! Why? Each syscall already is killable as the task might be killed by the OOM killer. > There's a (very) low number of system calls that are not interruptible, but > the > vast majority is. That might be true. I just fail to see how this is related to the particular problem I am trying to solve. As I've said those callsites which cause problems with latencies can be later converted to interruptible waiting trivially. -- Michal Hocko SUSE Labs
