On Wed, 6 May 2020 17:42:40 -0700 "Paul E. McKenney" <paul...@kernel.org> wrote:
> This commit adds a shrinker so as to inform RCU when memory is scarce.
> RCU responds by shifting into the same fast and inefficient mode that is
> used in the presence of excessive numbers of RCU callbacks. RCU remains
> in this state for one-tenth of a second, though this time window can be
> extended by another call to the shrinker.
>
> If it proves feasible, a later commit might add a function call directly
> indicating the end of the period of scarce memory.
(Cc David Chinner, who often has opinions on shrinkers ;))
It's a bit abusive of the intent of the slab shrinkers, but I don't
immediately see a problem with it. Always returning 0 from
->scan_objects might cause a problem in some situations(?).
Perhaps we should have a formal "system getting low on memory, please
do something" notification API.
How significant is this? How much memory can RCU consume?
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2368,8 +2368,15 @@ static void force_qs_rnp(int (*f)(struct rcu_data
> *rdp))
> struct rcu_data *rdp;
> struct rcu_node *rnp;
>
> - rcu_state.cbovld = rcu_state.cbovldnext;
> + // Load .oomovld before .oomovldend, pairing with .oomovld set.
> + rcu_state.cbovld = smp_load_acquire(&rcu_state.oomovld) || // ^^^
> + rcu_state.cbovldnext;
> rcu_state.cbovldnext = false;
> + if (READ_ONCE(rcu_state.oomovld) &&
> + time_after(jiffies, READ_ONCE(rcu_state.oomovldend))) {
> + WRITE_ONCE(rcu_state.oomovld, false);
> + pr_info("%s: Ending OOM-mode grace periods.\n", __func__);
> + }
> rcu_for_each_leaf_node(rnp) {
> cond_resched_tasks_rcu_qs();
> mask = 0;
> @@ -2697,6 +2704,35 @@ static void check_cb_ovld(struct rcu_data *rdp)
> raw_spin_unlock_rcu_node(rnp);
> }
>
> +/* Return a rough count of the RCU callbacks outstanding. */
> +static unsigned long rcu_oom_count(struct shrinker *unused1,
> + struct shrink_control *unused2)
> +{
> + int cpu;
> + unsigned long ncbs = 0;
> +
> + for_each_possible_cpu(cpu)
> + ncbs += rcu_get_n_cbs_cpu(cpu);
> + return ncbs;
> +}
> +
> +/* Start up an interval of fast high-overhead grace periods. */
> +static unsigned long rcu_oom_scan(struct shrinker *unused1,
> + struct shrink_control *unused2)
> +{
> + pr_info("%s: Starting OOM-mode grace periods.\n", __func__);
> + WRITE_ONCE(rcu_state.oomovldend, jiffies + HZ / 10);
> + smp_store_release(&rcu_state.oomovld, true); // After .oomovldend
> + rcu_force_quiescent_state(); // Kick grace period
> + return 0; // We haven't actually reclaimed anything yet.
> +}
> +
> +static struct shrinker rcu_shrinker = {
> + .count_objects = rcu_oom_count,
> + .scan_objects = rcu_oom_scan,
> + .seeks = DEFAULT_SEEKS,
> +};
> +
> /* Helper function for call_rcu() and friends. */
> static void
> __call_rcu(struct rcu_head *head, rcu_callback_t func)
> @@ -4146,6 +4182,7 @@ void __init rcu_init(void)
> qovld_calc = DEFAULT_RCU_QOVLD_MULT * qhimark;
> else
> qovld_calc = qovld;
> + WARN_ON(register_shrinker(&rcu_shrinker));
> }
>
> #include "tree_stall.h"
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 2d7fcb9..c4d8e96 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -326,6 +326,8 @@ struct rcu_state {
> int ncpus_snap; /* # CPUs seen last time. */
> u8 cbovld; /* Callback overload now? */
> u8 cbovldnext; /* ^ ^ next time? */
> + u8 oomovld; /* OOM overload? */
> + unsigned long oomovldend; /* OOM ovld end, jiffies. */
>
> unsigned long jiffies_force_qs; /* Time at which to invoke */
> /* force_quiescent_state(). */
