From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f71.google.com (mail-ed1-f71.google.com [209.85.208.71]) by kanga.kvack.org (Postfix) with ESMTP id 0426F6B0005 for ; Fri, 29 Jun 2018 09:26:43 -0400 (EDT) Received: by mail-ed1-f71.google.com with SMTP id c8-v6so2986643edr.16 for ; Fri, 29 Jun 2018 06:26:42 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id l56-v6si2033136edd.239.2018.06.29.06.26.41 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 29 Jun 2018 06:26:41 -0700 (PDT) Date: Fri, 29 Jun 2018 15:26:38 +0200 From: Michal Hocko Subject: Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM killer. Message-ID: <20180629132638.GD5963@dhcp22.suse.cz> References: <20180621073142.GA10465@dhcp22.suse.cz> <2d8c3056-1bc2-9a32-d745-ab328fd587a1@i-love.sakura.ne.jp> <20180626170345.GA3593@linux.vnet.ibm.com> <20180627072207.GB32348@dhcp22.suse.cz> <20180627143125.GW3593@linux.vnet.ibm.com> <20180628113942.GD32348@dhcp22.suse.cz> <20180628213105.GP3593@linux.vnet.ibm.com> <20180629090419.GD13860@dhcp22.suse.cz> <20180629125218.GX3593@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180629125218.GX3593@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: "Paul E. McKenney" Cc: Tetsuo Handa , David Rientjes , linux-mm@kvack.org, Andrew Morton , linux-kernel@vger.kernel.org On Fri 29-06-18 05:52:18, Paul E. McKenney wrote: > On Fri, Jun 29, 2018 at 11:04:19AM +0200, Michal Hocko wrote: > > On Thu 28-06-18 14:31:05, Paul E. McKenney wrote: > > > On Thu, Jun 28, 2018 at 01:39:42PM +0200, Michal Hocko wrote: [...] > > > > Well, I am not really sure what is the objective of the oom notifier to > > > > point you to the right direction. IIUC you just want to kick callbacks > > > > to be handled sooner under a heavy memory pressure, right? How is that > > > > achieved? Kick a worker? > > > > > > That is achieved by enqueuing a non-lazy callback on each CPU's callback > > > list, but only for those CPUs having non-empty lists. This causes > > > CPUs with lists containing only lazy callbacks to be more aggressive, > > > in particular, it prevents such CPUs from hanging out idle for seconds > > > at a time while they have callbacks on their lists. > > > > > > The enqueuing happens via an IPI to the CPU in question. > > > > I am afraid this is too low level for my to understand what is going on > > here. What are lazy callbacks and why do they need any specific action > > when we are getting close to OOM? I mean, I do understand that we might > > have many callers of call_rcu and free memory lazily. But there is quite > > a long way before we start the reclaim until we reach the OOM killer path. > > So why don't those callbacks get called during that time period? How are > > their triggered when we are not hitting the OOM path? They surely cannot > > sit there for ever, right? Can we trigger them sooner? Maybe the > > shrinker is not the best fit but we have a retry feedback loop in the page > > allocator, maybe we can kick this processing from there. > > The effect of RCU's current OOM code is to speed up callback invocation > by at most a few seconds (assuming no stalled CPUs, in which case > it is not possible to speed up callback invocation). > > Given that, I should just remove RCU's OOM code entirely? Yeah, it seems so. I do not see how this would really help much. If we really need some way to kick callbacks then we should do so much earlier in the reclaim process - e.g. when we start struggling to reclaim any memory. I am curious. Has the notifier been motivated by a real world use case or it was "nice thing to do"? -- Michal Hocko SUSE Labs