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