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    Subject[PATCH tip/core/rcu 07/23] rcu: Provide OOM handler to motivate lazy RCU callbacks
    Date
    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;
    + 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


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