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SubjectRe: [PATCH] rcu: Release per-cpu krcp page cache when CPU going offline
On Thu, Jan 21, 2021 at 02:49:49PM +0800, qiang.zhang@windriver.com wrote:
> From: Zqiang <qiang.zhang@windriver.com>
>
> If CPUs go offline, the corresponding krcp's page cache can
> not be use util the CPU come back online, or maybe the CPU
> will never go online again, this commit therefore free krcp's
> page cache when CPUs go offline.
>
> Signed-off-by: Zqiang <qiang.zhang@windriver.com>

Adding Uladzislau on CC for his thoughts.

Thanx, Paul

> ---
> kernel/rcu/tree.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 46 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index e04e336bee42..2eaf6f287483 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -158,6 +158,9 @@ static void sync_sched_exp_online_cleanup(int cpu);
> static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp);
> static bool rcu_rdp_is_offloaded(struct rcu_data *rdp);
>
> +static void krc_offline(unsigned int cpu, bool off);
> +static void free_krc_page_cache(int cpu);
> +
> /* rcuc/rcub kthread realtime priority */
> static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
> module_param(kthread_prio, int, 0444);
> @@ -2457,6 +2460,9 @@ int rcutree_dead_cpu(unsigned int cpu)
>
> // Stop-machine done, so allow nohz_full to disable tick.
> tick_dep_clear(TICK_DEP_BIT_RCU);
> +
> + krc_offline(cpu, true);
> + free_krc_page_cache(cpu);
> return 0;
> }
>
> @@ -3169,6 +3175,7 @@ struct kfree_rcu_cpu {
>
> struct llist_head bkvcache;
> int nr_bkv_objs;
> + bool offline;
> };
>
> static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc) = {
> @@ -3220,6 +3227,8 @@ static inline bool
> put_cached_bnode(struct kfree_rcu_cpu *krcp,
> struct kvfree_rcu_bulk_data *bnode)
> {
> + if (krcp->offline)
> + return false;
> // Check the limit.
> if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> return false;
> @@ -3230,6 +3239,39 @@ put_cached_bnode(struct kfree_rcu_cpu *krcp,
>
> }
>
> +static void krc_offline(unsigned int cpu, bool off)
> +{
> + unsigned long flags;
> + struct kfree_rcu_cpu *krcp;
> +
> + krcp = per_cpu_ptr(&krc, cpu);
> + raw_spin_lock_irqsave(&krcp->lock, flags);
> + if (off)
> + krcp->offline = true;
> + else
> + krcp->offline = false;
> + raw_spin_unlock_irqrestore(&krcp->lock, flags);
> +}
> +
> +static void free_krc_page_cache(int cpu)
> +{
> + unsigned long flags;
> + struct kfree_rcu_cpu *krcp;
> + int i;
> + struct kvfree_rcu_bulk_data *bnode;
> +
> + krcp = per_cpu_ptr(&krc, cpu);
> +
> + for (i = 0; i < rcu_min_cached_objs; i++) {
> + raw_spin_lock_irqsave(&krcp->lock, flags);
> + bnode = get_cached_bnode(krcp);
> + raw_spin_unlock_irqrestore(&krcp->lock, flags);
> + if (!bnode)
> + break;
> + free_page((unsigned long)bnode);
> + }
> +}
> +
> /*
> * This function is invoked in workqueue context after a grace period.
> * It frees all the objects queued on ->bhead_free or ->head_free.
> @@ -3549,7 +3591,8 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> kasan_record_aux_stack(ptr);
> success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr);
> if (!success) {
> - run_page_cache_worker(krcp);
> + if (!krcp->offline)
> + run_page_cache_worker(krcp);
>
> if (head == NULL)
> // Inline if kvfree_rcu(one_arg) call.
> @@ -4086,6 +4129,7 @@ int rcutree_prepare_cpu(unsigned int cpu)
> rcu_spawn_cpu_nocb_kthread(cpu);
> WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus + 1);
>
> + krc_offline(cpu, false);
> return 0;
> }
>
> @@ -4591,6 +4635,7 @@ static void __init kfree_rcu_batch_init(void)
> INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
> INIT_WORK(&krcp->page_cache_work, fill_page_cache_func);
> krcp->initialized = true;
> + krcp->offline = true;
> }
> if (register_shrinker(&kfree_rcu_shrinker))
> pr_err("Failed to register kfree_rcu() shrinker!\n");
> --
> 2.29.2
>

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