netdev - Re: [PATCH net-next v6 2/3] udp: implement memory accounting helpers

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Date:   Fri, 21 Oct 2016 05:24:02 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        James Morris <jmorris@...ei.org>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        Alexander Duyck <aduyck@...antis.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Tom Herbert <tom@...bertland.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Edward Cree <ecree@...arflare.com>, linux-nfs@...r.kernel.org
Subject: Re: [PATCH net-next v6 2/3] udp: implement memory accounting helpers

On Fri, 2016-10-21 at 13:55 +0200, Paolo Abeni wrote:
> Avoid using the generic helpers.
> Use the receive queue spin lock to protect the memory
> accounting operation, both on enqueue and on dequeue.
> 
> On dequeue perform partial memory reclaiming, trying to
> leave a quantum of forward allocated memory.
> 
> On enqueue use a custom helper, to allow some optimizations:
> - use a plain spin_lock() variant instead of the slightly
>   costly spin_lock_irqsave(),
> - avoid dst_force check, since the calling code has already
>   dropped the skb dst
> - avoid orphaning the skb, since skb_steal_sock() already did
>   the work for us

>  
> +static void udp_rmem_release(struct sock *sk, int size, int partial)
> +{
> +	int amt;
> +
> +	atomic_sub(size, &sk->sk_rmem_alloc);
> +
> +	spin_lock_bh(&sk->sk_receive_queue.lock);
> +	sk->sk_forward_alloc += size;
> +	amt = (sk->sk_forward_alloc - partial) & ~(SK_MEM_QUANTUM - 1);
> +	sk->sk_forward_alloc -= amt;
> +	spin_unlock_bh(&sk->sk_receive_queue.lock);
> +
> +	if (amt)
> +		__sk_mem_reduce_allocated(sk, amt >> SK_MEM_QUANTUM_SHIFT);
> +}
> +
> +static void udp_rmem_free(struct sk_buff *skb)
> +{
> +	udp_rmem_release(skb->sk, skb->truesize, 1);
> +}
> +


It looks like you are acquiring/releasing sk_receive_queue.lock twice
per packet in recvmsg() (the second time in the destructor above)

We could do slightly better if :

We do not set skb->destructor at all, and manage
sk_rmem_alloc/sk_forward_alloc changes at the time we dequeue skb
 (if !MSG_PEEK), before copy to user space.