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[148.163.156.1]) by mx.google.com with ESMTPS id b11-v6si1107202edf.140.2018.06.29.05.50.17 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 29 Jun 2018 05:50:18 -0700 (PDT) Received: from pps.filterd (m0098394.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w5TCeAJS011700 for ; Fri, 29 Jun 2018 08:50:16 -0400 Received: from e17.ny.us.ibm.com (e17.ny.us.ibm.com [129.33.205.207]) by mx0a-001b2d01.pphosted.com with ESMTP id 2jwm41arpy-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Fri, 29 Jun 2018 08:50:16 -0400 Received: from localhost by e17.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 29 Jun 2018 08:50:14 -0400 Date: Fri, 29 Jun 2018 05:52:18 -0700 From: "Paul E. McKenney" Subject: Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM killer. Reply-To: paulmck@linux.vnet.ibm.com References: <1529493638-6389-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180629090419.GD13860@dhcp22.suse.cz> Message-Id: <20180629125218.GX3593@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Tetsuo Handa , David Rientjes , linux-mm@kvack.org, Andrew Morton , linux-kernel@vger.kernel.org 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: > > > On Wed 27-06-18 07:31:25, Paul E. McKenney wrote: > > > > On Wed, Jun 27, 2018 at 09:22:07AM +0200, Michal Hocko wrote: > > > > > On Tue 26-06-18 10:03:45, Paul E. McKenney wrote: > > > > > [...] > > > > > > 3. Something else? > > > > > > > > > > How hard it would be to use a different API than oom notifiers? E.g. a > > > > > shrinker which just kicks all the pending callbacks if the reclaim > > > > > priority reaches low values (e.g. 0)? > > > > > > > > Beats me. What is a shrinker? ;-) > > > > > > This is a generich mechanism to reclaim memory that is not on standard > > > LRU lists. Lwn.net surely has some nice coverage (e.g. > > > https://lwn.net/Articles/548092/). > > > > "In addition, there is little agreement over what a call to a shrinker > > really means or how the called subsystem should respond." ;-) > > > > Is this set up using register_shrinker() in mm/vmscan.c? I am guessing > > Yes, exactly. You are supposed to implement the two methods in struct > shrink_control > > > that the many mentions of shrinker in DRM are irrelevant. > > > > If my guess is correct, the API seems a poor fit for RCU. I can > > produce an approximate number of RCU callbacks for ->count_objects(), > > but a given callback might free a lot of memory or none at all. Plus, > > to actually have ->scan_objects() free them before returning, I would > > need to use something like rcu_barrier(), which might involve longer > > delays than desired.` > > Well, I am not yet sure how good fit this is because I still do not > understand the underlying problem your notifier is trying to solve. So I > will get back to this once that is settled. > > > > Or am I missing something here? > > > > > > More seriously, could you please point me at an exemplary shrinker > > > > use case so I can see what is involved? > > > > > > 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? Thanx, Paul