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Date:   Fri, 1 May 2020 14:27:49 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 09/24] rcu/tree: cache specified number of objects

On Tue, Apr 28, 2020 at 10:58:48PM +0200, Uladzislau Rezki (Sony) wrote:
> Cache some extra objects per-CPU. During reclaim process
> some pages are cached instead of releasing by linking them
> into the list. Such approach provides O(1) access time to
> the cache.
> 
> That reduces number of requests to the page allocator, also
> that makes it more helpful if a low memory condition occurs.
> 
> A parameter reflecting the minimum allowed pages to be
> cached per one CPU is propagated via sysfs, it is read
> only, the name is "rcu_min_cached_objs".
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  kernel/rcu/tree.c | 64 ++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 60 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 89e9ca3f4e3e..d8975819b1c9 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -178,6 +178,14 @@ module_param(gp_init_delay, int, 0444);
>  static int gp_cleanup_delay;
>  module_param(gp_cleanup_delay, int, 0444);
>  
> +/*
> + * This rcu parameter is read-only, but can be write also.

You mean that although the parameter is read-only, you see no reason
why it could not be converted to writeable?

If it was writeable, and a given CPU had the maximum numbr of cached
objects, the rcu_min_cached_objs value was decreased, but that CPU never
saw another kfree_rcu(), would the number of cached objects change?

(Just curious, not asking for a change in functionality.)

> + * It reflects the minimum allowed number of objects which
> + * can be cached per-CPU. Object size is equal to one page.
> + */
> +int rcu_min_cached_objs = 2;
> +module_param(rcu_min_cached_objs, int, 0444);
> +
>  /* Retrieve RCU kthreads priority for rcutorture */
>  int rcu_get_gp_kthreads_prio(void)
>  {
> @@ -2887,7 +2895,6 @@ struct kfree_rcu_cpu_work {
>   * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period
>   * @head: List of kfree_rcu() objects not yet waiting for a grace period
>   * @bhead: Bulk-List of kfree_rcu() objects not yet waiting for a grace period
> - * @bcached: Keeps at most one object for later reuse when build chain blocks
>   * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
>   * @lock: Synchronize access to this structure
>   * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
> @@ -2902,7 +2909,6 @@ struct kfree_rcu_cpu_work {
>  struct kfree_rcu_cpu {
>  	struct rcu_head *head;
>  	struct kfree_rcu_bulk_data *bhead;
> -	struct kfree_rcu_bulk_data *bcached;
>  	struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
>  	raw_spinlock_t lock;
>  	struct delayed_work monitor_work;
> @@ -2910,6 +2916,15 @@ struct kfree_rcu_cpu {
>  	bool initialized;
>  	// Number of objects for which GP not started
>  	int count;
> +
> +	/*
> +	 * Number of cached objects which are queued into
> +	 * the lock-less list. This cache is used by the
> +	 * kvfree_call_rcu() function and as of now its
> +	 * size is static.
> +	 */
> +	struct llist_head bkvcache;
> +	int nr_bkv_objs;
>  };
>  
>  static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc) = {
> @@ -2946,6 +2961,31 @@ krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
>  	local_irq_restore(flags);
>  }
>  
> +static inline struct kfree_rcu_bulk_data *
> +get_cached_bnode(struct kfree_rcu_cpu *krcp)
> +{
> +	if (!krcp->nr_bkv_objs)
> +		return NULL;
> +
> +	krcp->nr_bkv_objs--;
> +	return (struct kfree_rcu_bulk_data *)
> +		llist_del_first(&krcp->bkvcache);
> +}
> +
> +static inline bool
> +put_cached_bnode(struct kfree_rcu_cpu *krcp,
> +	struct kfree_rcu_bulk_data *bnode)
> +{
> +	/* Check the limit. */
> +	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> +		return false;
> +
> +	llist_add((struct llist_node *) bnode, &krcp->bkvcache);
> +	krcp->nr_bkv_objs++;
> +	return true;
> +
> +}
> +
>  /*
>   * This function is invoked in workqueue context after a grace period.
>   * It frees all the objects queued on ->bhead_free or ->head_free.
> @@ -2981,7 +3021,12 @@ static void kfree_rcu_work(struct work_struct *work)
>  		kfree_bulk(bhead->nr_records, bhead->records);
>  		rcu_lock_release(&rcu_callback_map);
>  
> -		if (cmpxchg(&krcp->bcached, NULL, bhead))
> +		krcp = krc_this_cpu_lock(&flags);

Presumably the list can also be accessed without holding this lock,
because otherwise we shouldn't need llist...

							Thanx, Paul

> +		if (put_cached_bnode(krcp, bhead))
> +			bhead = NULL;
> +		krc_this_cpu_unlock(krcp, flags);
> +
> +		if (bhead)
>  			free_page((unsigned long) bhead);
>  
>  		cond_resched_tasks_rcu_qs();
> @@ -3114,7 +3159,7 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
>  	/* Check if a new block is required. */
>  	if (!krcp->bhead ||
>  			krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) {
> -		bnode = xchg(&krcp->bcached, NULL);
> +		bnode = get_cached_bnode(krcp);
>  		if (!bnode) {
>  			WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
>  
> @@ -4167,12 +4212,23 @@ static void __init kfree_rcu_batch_init(void)
>  
>  	for_each_possible_cpu(cpu) {
>  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> +		struct kfree_rcu_bulk_data *bnode;
>  
>  		for (i = 0; i < KFREE_N_BATCHES; i++) {
>  			INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
>  			krcp->krw_arr[i].krcp = krcp;
>  		}
>  
> +		for (i = 0; i < rcu_min_cached_objs; i++) {
> +			bnode = (struct kfree_rcu_bulk_data *)
> +				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> +
> +			if (bnode)
> +				put_cached_bnode(krcp, bnode);
> +			else
> +				pr_err("Failed to preallocate for %d CPU!\n", cpu);
> +		}
> +
>  		INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
>  		krcp->initialized = true;
>  	}
> -- 
> 2.20.1
> 

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