Re: [PATCH tip/core/rcu 07/23] rcu: Provide OOM handler to motivate lazy RCU callbacks

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Sep 03, 2012 at 05:08:24PM +0800, Lai Jiangshan wrote:
> 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.

Hmmm...  That does indeed cause out_of_memory() to unconditionally
return, doesn't it?

So I should really just leave *nfreed alone, since I cannot be sure
whether or not anything will actually get freed.  I -could- count
callbacks, but they might well be allocated as fast as they are freed.

Good catch!!!

> I did not review the patch nor the whole series carefully.
> 
> And if it is possible, could you share the code with rcu_barrier()?

At the moment, it adds more code than it saves.

							Thanx, Paul

> 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
>