From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Date: Wed, 09 Mar 2016 12:18:50 +0000 Subject: Re: [RFC 0/12] introduce down_write_killable for rw_semaphore Message-Id: <20160309121850.GA14915@gmail.com> List-Id: References: <1454444369-2146-1-git-send-email-mhocko@kernel.org> In-Reply-To: <1454444369-2146-1-git-send-email-mhocko@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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: > 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? 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. So there's no livelock scenario - it's "only" about latencies. 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'd love various processes to quit faster on Ctrl-C as well, not just on kill -9! This would also test the new code paths a lot better: kill -9 is a lot rarer than regular interruption. Thanks, Ingo From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932457AbcCIMTG (ORCPT ); Wed, 9 Mar 2016 07:19:06 -0500 Received: from mail-wm0-f66.google.com ([74.125.82.66]:33039 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932301AbcCIMS4 (ORCPT ); Wed, 9 Mar 2016 07:18:56 -0500 Date: Wed, 9 Mar 2016 13:18:50 +0100 From: Ingo Molnar 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 Subject: Re: [RFC 0/12] introduce down_write_killable for rw_semaphore Message-ID: <20160309121850.GA14915@gmail.com> References: <1454444369-2146-1-git-send-email-mhocko@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1454444369-2146-1-git-send-email-mhocko@kernel.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Michal Hocko 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? 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. So there's no livelock scenario - it's "only" about latencies. 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'd love various processes to quit faster on Ctrl-C as well, not just on kill -9! This would also test the new code paths a lot better: kill -9 is a lot rarer than regular interruption. Thanks, Ingo