From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id 21DF86B0008 for ; Tue, 3 Jul 2018 03:24:19 -0400 (EDT) Received: by mail-pf0-f198.google.com with SMTP id j8-v6so660868pfn.6 for ; Tue, 03 Jul 2018 00:24:19 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id f34-v6si517876ple.52.2018.07.03.00.24.16 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 03 Jul 2018 00:24:16 -0700 (PDT) Date: Tue, 3 Jul 2018 09:24:13 +0200 From: Michal Hocko Subject: Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM killer. Message-ID: <20180703072413.GD16767@dhcp22.suse.cz> References: <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> <20180629132638.GD5963@dhcp22.suse.cz> <20180630170522.GZ3593@linux.vnet.ibm.com> <20180702213714.GA7604@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180702213714.GA7604@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 Mon 02-07-18 14:37:14, Paul E. McKenney wrote: [...] > commit d2b8d16b97ac2859919713b2d98b8a3ad22943a2 > Author: Paul E. McKenney > Date: Mon Jul 2 14:30:37 2018 -0700 > > rcu: Remove OOM code > > There is reason to believe that RCU's OOM code isn't really helping > that much, given that the best it can hope to do is accelerate invoking > callbacks by a few seconds, and even then only if some CPUs have no > non-lazy callbacks, a condition that has been observed to be rare. > This commit therefore removes RCU's OOM code. If this causes problems, > it can easily be reinserted. > > Reported-by: Michal Hocko > Reported-by: Tetsuo Handa > Signed-off-by: Paul E. McKenney I would also note that waiting in the notifier might be a problem on its own because we are holding the oom_lock and the system cannot trigger the OOM killer while we are holding it and waiting for oom_callback_wq event. I am not familiar with the code to tell whether this can deadlock but from a quick glance I _suspect_ that we might depend on __rcu_reclaim and basically an arbitrary callback so no good. Acked-by: Michal Hocko Thanks! > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index 3f3796b10c71..3d7ce73e7309 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -1722,87 +1722,6 @@ static void rcu_idle_count_callbacks_posted(void) > __this_cpu_add(rcu_dynticks.nonlazy_posted, 1); > } > > -/* > - * Data for flushing lazy RCU callbacks at OOM time. > - */ > -static atomic_t oom_callback_count; > -static DECLARE_WAIT_QUEUE_HEAD(oom_callback_wq); > - > -/* > - * RCU OOM callback -- decrement the outstanding count and deliver the > - * wake-up if we are the last one. > - */ > -static void rcu_oom_callback(struct rcu_head *rhp) > -{ > - if (atomic_dec_and_test(&oom_callback_count)) > - wake_up(&oom_callback_wq); > -} > - > -/* > - * Post an rcu_oom_notify callback on the current CPU if it has at > - * least one lazy callback. This will unnecessarily post callbacks > - * to CPUs that already have a non-lazy callback at the end of their > - * callback list, but this is an infrequent operation, so accept some > - * extra overhead to keep things simple. > - */ > -static void rcu_oom_notify_cpu(void *unused) > -{ > - struct rcu_state *rsp; > - struct rcu_data *rdp; > - > - for_each_rcu_flavor(rsp) { > - rdp = raw_cpu_ptr(rsp->rda); > - if (rcu_segcblist_n_lazy_cbs(&rdp->cblist)) { > - atomic_inc(&oom_callback_count); > - rsp->call(&rdp->oom_head, rcu_oom_callback); > - } > - } > -} > - > -/* > - * If low on memory, ensure that each CPU has a non-lazy callback. > - * This will wake up CPUs that have only lazy callbacks, in turn > - * ensuring that they free up the corresponding memory in a timely manner. > - * Because an uncertain amount of memory will be freed in some uncertain > - * timeframe, we do not claim to have freed anything. > - */ > -static int rcu_oom_notify(struct notifier_block *self, > - unsigned long notused, void *nfreed) > -{ > - int cpu; > - > - /* Wait for callbacks from earlier instance to complete. */ > - wait_event(oom_callback_wq, atomic_read(&oom_callback_count) == 0); > - smp_mb(); /* Ensure callback reuse happens after callback invocation. */ > - > - /* > - * Prevent premature wakeup: ensure that all increments happen > - * before there is a chance of the counter reaching zero. > - */ > - atomic_set(&oom_callback_count, 1); > - > - for_each_online_cpu(cpu) { > - smp_call_function_single(cpu, rcu_oom_notify_cpu, NULL, 1); > - cond_resched_tasks_rcu_qs(); > - } > - > - /* Unconditionally decrement: no need to wake ourselves up. */ > - atomic_dec(&oom_callback_count); > - > - return NOTIFY_OK; > -} > - > -static struct notifier_block rcu_oom_nb = { > - .notifier_call = rcu_oom_notify > -}; > - > -static int __init rcu_register_oom_notifier(void) > -{ > - register_oom_notifier(&rcu_oom_nb); > - return 0; > -} > -early_initcall(rcu_register_oom_notifier); > - > #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */ > > #ifdef CONFIG_RCU_FAST_NO_HZ -- Michal Hocko SUSE Labs