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Date:   Fri, 3 Apr 2020 11:16:55 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        RCU <rcu@vger.kernel.org>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: add emergency pool for headless case
Message-ID: <20200403181655.GR21484@bombadil.infradead.org>
References: <20200403173051.4081-1-urezki@gmail.com>
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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;
	}
}