From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755853Ab2IBCNV (ORCPT ); Sat, 1 Sep 2012 22:13:21 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:56376 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754662Ab2IBCNU (ORCPT ); Sat, 1 Sep 2012 22:13:20 -0400 X-Originating-IP: 217.70.178.148 X-Originating-IP: 50.43.46.74 Date: Sat, 1 Sep 2012 19:13:12 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org, "Paul E. McKenney" Subject: Re: [PATCH tip/core/rcu 07/23] rcu: Provide OOM handler to motivate lazy RCU callbacks Message-ID: <20120902021312.GG5713@leaf> References: <20120830181811.GA29154@linux.vnet.ibm.com> <1346350718-30937-1-git-send-email-paulmck@linux.vnet.ibm.com> <1346350718-30937-7-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1346350718-30937-7-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 30, 2012 at 11:18:22AM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > In kernels built with CONFIG_RCU_FAST_NO_HZ=y, CPUs can accumulate a > large number of lazy callbacks, which as the name implies will be slow > to be invoked. This can be a problem on small-memory systems, where the > default 6-second sleep for CPUs having only lazy RCU callbacks could well > be fatal. This commit therefore installs an OOM hander that ensures that > every CPU with non-lazy callbacks has at least one non-lazy callback, > in turn ensuring timely advancement for these callbacks. Did you mean "every CPU with lazy callbacks" here? > Signed-off-by: Paul E. McKenney > Signed-off-by: Paul E. McKenney > Tested-by: Sasha Levin Reviewed-by: Josh Triplett > kernel/rcutree.h | 5 ++- > kernel/rcutree_plugin.h | 80 +++++++++++++++++++++++++++++++++++++++++++++++ > 2 files changed, 84 insertions(+), 1 deletions(-) > > diff --git a/kernel/rcutree.h b/kernel/rcutree.h > index 117a150..effb273 100644 > --- a/kernel/rcutree.h > +++ b/kernel/rcutree.h > @@ -315,8 +315,11 @@ struct rcu_data { > unsigned long n_rp_need_fqs; > unsigned long n_rp_need_nothing; > > - /* 6) _rcu_barrier() callback. */ > + /* 6) _rcu_barrier() and OOM callbacks. */ > struct rcu_head barrier_head; > +#ifdef CONFIG_RCU_FAST_NO_HZ > + struct rcu_head oom_head; > +#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */ > > int cpu; > struct rcu_state *rsp; > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h > index 7f3244c..bac8cc1 100644 > --- a/kernel/rcutree_plugin.h > +++ b/kernel/rcutree_plugin.h > @@ -25,6 +25,7 @@ > */ > > #include > +#include > > #define RCU_KTHREAD_PRIO 1 > > @@ -2112,6 +2113,85 @@ 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 *flavor) > +{ > + struct rcu_state *rsp = flavor; > + struct rcu_data *rdp = __this_cpu_ptr(rsp->rda); > + > + if (rdp->qlen_lazy != 0) { > + 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. > + */ > +static int rcu_oom_notify(struct notifier_block *self, > + unsigned long notused, void *nfreed) > +{ > + int cpu; > + struct rcu_state *rsp; > + > + /* Wait for callbacks from earlier instance to complete. */ > + wait_event(oom_callback_wq, atomic_read(&oom_callback_count) == 0); > + > + /* > + * Prevent premature wakeup: ensure that all increments happen > + * before there is a chance of the counter reaching zero. > + */ > + atomic_set(&oom_callback_count, 1); > + > + get_online_cpus(); > + for_each_online_cpu(cpu) > + for_each_rcu_flavor(rsp) > + smp_call_function_single(cpu, rcu_oom_notify_cpu, > + rsp, 1); > + put_online_cpus(); > + > + /* Unconditionally decrement: no need to wake ourselves up. */ > + atomic_dec(&oom_callback_count); > + > + *(unsigned long *)nfreed = 1; > + 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_CPU_STALL_INFO > -- > 1.7.8 >