Messages in this thread | | | Date | Mon, 03 Sep 2012 17:08:24 +0800 | From | Lai Jiangshan <> | Subject | Re: [PATCH tip/core/rcu 07/23] rcu: Provide OOM handler to motivate lazy RCU callbacks |
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On 08/31/2012 02:18 AM, Paul E. McKenney wrote: > From: "Paul E. McKenney" <paul.mckenney@linaro.org> > > 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. > > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > Tested-by: Sasha Levin <levinsasha928@gmail.com> > --- > 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 <linux/delay.h> > +#include <linux/oom.h> > > #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;
Hi, Paul
If you consider the above code has free some memory, you should use *(unsigned long *)nfreed = +1. ^^
And your code disable OOM actually, because it transfer *nfreed to NON-ZERO unconditionally.
I did not review the patch nor the whole series carefully.
And if it is possible, could you share the code with rcu_barrier()?
Thanks, Lai
> + 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
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