From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [RFC 0/12] introduce down_write_killable for rw_semaphore Date: Wed, 9 Mar 2016 14:17:10 +0100 Message-ID: <20160309131710.GB7978@gmail.com> References: <1454444369-2146-1-git-send-email-mhocko@kernel.org> <20160309121850.GA14915@gmail.com> <20160309125641.GH27018@dhcp22.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail-wm0-f65.google.com ([74.125.82.65]:34473 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932554AbcCINRQ (ORCPT ); Wed, 9 Mar 2016 08:17:16 -0500 Content-Disposition: inline In-Reply-To: <20160309125641.GH27018@dhcp22.suse.cz> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Michal Hocko Cc: LKML , Peter Zijlstra , Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , "David S. Miller" , Tony Luck , Andrew Morton , Chris Zankel , Max Filippov , x86@kernel.org, linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org, Linus Torvalds , Peter Zijlstra * Michal Hocko 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) 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! There's a (very) low number of system calls that are not interruptible, but the vast majority is. Thanks, Ingo