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SubjectRe: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM killer.
On Mon 02-07-18 14:37:14, Paul E. McKenney wrote:
[...]
> commit d2b8d16b97ac2859919713b2d98b8a3ad22943a2
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Date: Mon Jul 2 14:30:37 2018 -0700
>
> rcu: Remove OOM code
>
> There is reason to believe that RCU's OOM code isn't really helping
> that much, given that the best it can hope to do is accelerate invoking
> callbacks by a few seconds, and even then only if some CPUs have no
> non-lazy callbacks, a condition that has been observed to be rare.
> This commit therefore removes RCU's OOM code. If this causes problems,
> it can easily be reinserted.
>
> Reported-by: Michal Hocko <mhocko@kernel.org>
> Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

I would also note that waiting in the notifier might be a problem on its
own because we are holding the oom_lock and the system cannot trigger
the OOM killer while we are holding it and waiting for oom_callback_wq
event. I am not familiar with the code to tell whether this can deadlock
but from a quick glance I _suspect_ that we might depend on __rcu_reclaim
and basically an arbitrary callback so no good.

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 3f3796b10c71..3d7ce73e7309 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1722,87 +1722,6 @@ 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 *unused)
> -{
> - struct rcu_state *rsp;
> - struct rcu_data *rdp;
> -
> - for_each_rcu_flavor(rsp) {
> - rdp = raw_cpu_ptr(rsp->rda);
> - if (rcu_segcblist_n_lazy_cbs(&rdp->cblist)) {
> - 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.
> - * Because an uncertain amount of memory will be freed in some uncertain
> - * timeframe, we do not claim to have freed anything.
> - */
> -static int rcu_oom_notify(struct notifier_block *self,
> - unsigned long notused, void *nfreed)
> -{
> - int cpu;
> -
> - /* Wait for callbacks from earlier instance to complete. */
> - wait_event(oom_callback_wq, atomic_read(&oom_callback_count) == 0);
> - smp_mb(); /* Ensure callback reuse happens after callback invocation. */
> -
> - /*
> - * Prevent premature wakeup: ensure that all increments happen
> - * before there is a chance of the counter reaching zero.
> - */
> - atomic_set(&oom_callback_count, 1);
> -
> - for_each_online_cpu(cpu) {
> - smp_call_function_single(cpu, rcu_oom_notify_cpu, NULL, 1);
> - cond_resched_tasks_rcu_qs();
> - }
> -
> - /* Unconditionally decrement: no need to wake ourselves up. */
> - atomic_dec(&oom_callback_count);
> -
> - 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_FAST_NO_HZ

--
Michal Hocko
SUSE Labs

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