netdev - Re: [PATCH v2 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu() similar to kfree

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Date: Sat, 24 Jun 2023 15:53:38 +0800
From: Hou Tao <houtao@...weicloud.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>, daniel@...earbox.net,
 andrii@...nel.org, void@...ifault.com, paulmck@...nel.org
Cc: tj@...nel.org, rcu@...r.kernel.org, netdev@...r.kernel.org,
 bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v2 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu()
 similar to kfree_rcu().

Hi,

On 6/24/2023 11:13 AM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
>
> Introduce bpf_mem_[cache_]free_rcu() similar to kfree_rcu().
> Unlike bpf_mem_[cache_]free() that links objects for immediate reuse into
> per-cpu free list the _rcu() flavor waits for RCU grace period and then moves
> objects into free_by_rcu_ttrace list where they are waiting for RCU
> task trace grace period to be freed into slab.
>
SNIP
> +static void check_free_by_rcu(struct bpf_mem_cache *c)
> +{
> +	struct llist_node *llnode, *t;
> +
> +	if (llist_empty(&c->free_by_rcu) && llist_empty(&c->free_llist_extra_rcu))
> +		return;
> +
> +	/* drain free_llist_extra_rcu */
> +	llist_for_each_safe(llnode, t, llist_del_all(&c->free_llist_extra_rcu))
> +		if (__llist_add(llnode, &c->free_by_rcu))
> +			c->free_by_rcu_tail = llnode;

Just like add_obj_to_free_list(),  we should do conditional
local_irq_save(flags) and local_inc_return(&c->active) as well for
free_by_rcu, otherwise free_by_rcu may be corrupted by bpf program
running in a NMI context.
> +
> +	if (atomic_xchg(&c->call_rcu_in_progress, 1)) {
> +		/*
> +		 * Instead of kmalloc-ing new rcu_head and triggering 10k
> +		 * call_rcu() to hit rcutree.qhimark and force RCU to notice
> +		 * the overload just ask RCU to hurry up. There could be many
> +		 * objects in free_by_rcu list.
> +		 * This hint reduces memory consumption for an artifical
> +		 * benchmark from 2 Gbyte to 150 Mbyte.
> +		 */
> +		rcu_request_urgent_qs_task(current);
> +		return;
> +	}
> +
> +	WARN_ON_ONCE(!llist_empty(&c->waiting_for_gp));
> +
> +	WRITE_ONCE(c->waiting_for_gp.first, __llist_del_all(&c->free_by_rcu));
> +	c->waiting_for_gp_tail = c->free_by_rcu_tail;
> +	call_rcu_hurry(&c->rcu, __free_by_rcu);

The same protection is needed for c->free_by_rcu_tail as well.