Re: [PATCH 1/1] rcu/tree: add emergency pool for headless case

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Fri, Apr 03, 2020 at 07:30:51PM +0200, Uladzislau Rezki (Sony) wrote:
> @@ -2877,6 +2885,12 @@ struct kfree_rcu_cpu {
>  	bool initialized;
>  	// Number of objects for which GP not started
>  	int count;
> +
> +	/*
> +	 * Reserved emergency pool for headless variant.
> +	 */
> +	int nr_emergency;
> +	void **emergency;

This is a pretty expensive way to maintain an emergency pool.

Try something like this ...

struct emergency_pool_object {
	union {
		struct whatever foo;
		struct {
			int remaining;
			void *next;
		};
	};
};

struct kfree_rcu_cpu {
	...
	struct emergency_pool_object *epo;
};

struct whatever *get_emergency_object(struct kfree_rcu_cpu *krc)
{
	struct emergency_pool_object *epo = krc->epo;
	if (epo)
		krc->epo = epo->next;
	return &epo->foo;
}

void alloc_emergency_objects(struct kfree_rcu_cpu *krc, int n)
{
	int i = 0;

	if (krc->epo)
		i = krc->epo->remaining;

	while (++i < n) {
		struct emergency_pool_object *epo = kmalloc(sizeof(epo), GFP);
		epo->remaining = i;
		epo->next = krc->epo;
		krc->epo = epo;
	}
}