Subject: Re: [PATCH net-next v4 2/3] udp: implement memory accounting helpers

From: Paolo Abeni <[email-masked]>
Date: Thu, 20 Oct 2016 12:18:09 +0200

On Wed, 2016-10-19 at 21:43 -0700, Eric Dumazet wrote:
> On Wed, 2016-10-19 at 14:47 +0200, Paolo Abeni wrote:
> > Avoid using the generic helpers.
> > Adds a new spinlock_t to the udp_sock structure and use
> > it 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
> > 
> > The above needs custom memory reclaiming on shutdown, provided
> > by the udp_destruct_sock().
> > 
> > v3 -> v4:
> >   - reworked memory accunting, simplifying the schema
> >   - provide an helper for both memory scheduling and enqueuing
> > 
> > v1 -> v2:
> >   - use a udp specific destrctor to perform memory reclaiming
> >   - remove a couple of helpers, unneeded after the above cleanup
> >   - do not reclaim memory on dequeue if not under memory
> >     pressure
> >   - reworked the fwd accounting schema to avoid potential
> >     integer overflow
> > 
> > Acked-by: Hannes Frederic Sowa <[email-masked]>
> > Signed-off-by: Paolo Abeni <[email-masked]>
> > ---
> >  include/linux/udp.h |   1 +
> >  include/net/udp.h   |   4 ++
> >  net/ipv4/udp.c      | 111 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 116 insertions(+)
> > 
> > diff --git a/include/linux/udp.h b/include/linux/udp.h
> > index d1fd8cd..8ea7f8b 100644
> > --- a/include/linux/udp.h
> > +++ b/include/linux/udp.h
> > @@ -66,6 +66,7 @@ struct udp_sock {
> >  #define UDPLITE_RECV_CC  0x4		/* set via udplite setsocktopt        */
> >  	__u8		 pcflag;        /* marks socket as UDP-Lite if > 0    */
> >  	__u8		 unused[3];
> > +	spinlock_t	 mem_lock;	/* protects memory accounting         */
> >  	/*
> >  	 * For encapsulation sockets.
> >  	 */
> > diff --git a/include/net/udp.h b/include/net/udp.h
> > index ea53a87..18f1e6b 100644
> > --- a/include/net/udp.h
> > +++ b/include/net/udp.h
> > @@ -246,6 +246,9 @@ static inline __be16 udp_flow_src_port(struct net *net, struct sk_buff *skb,
> >  }
> >  
> >  /* net/ipv4/udp.c */
> > +void skb_consume_udp(struct sock *sk, struct sk_buff *skb, int len);
> > +int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb);
> > +
> >  void udp_v4_early_demux(struct sk_buff *skb);
> >  int udp_get_port(struct sock *sk, unsigned short snum,
> >  		 int (*saddr_cmp)(const struct sock *,
> > @@ -258,6 +261,7 @@ int udp_get_port(struct sock *sk, unsigned short snum,
> >  void udp4_hwcsum(struct sk_buff *skb, __be32 src, __be32 dst);
> >  int udp_rcv(struct sk_buff *skb);
> >  int udp_ioctl(struct sock *sk, int cmd, unsigned long arg);
> > +int udp_init_sock(struct sock *sk);
> >  int udp_disconnect(struct sock *sk, int flags);
> >  unsigned int udp_poll(struct file *file, struct socket *sock, poll_table *wait);
> >  struct sk_buff *skb_udp_tunnel_segment(struct sk_buff *skb,
> > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > index 7d96dc2..7047bb3 100644
> > --- a/net/ipv4/udp.c
> > +++ b/net/ipv4/udp.c
> > @@ -1172,6 +1172,117 @@ int udp_sendpage(struct sock *sk, struct page *page, int offset,
> >  	return ret;
> >  }
> >  
> > +static void udp_rmem_release(struct sock *sk, int size, int partial)
> > +{
> > +	struct udp_sock *up = udp_sk(sk);
> > +	int amt;
> > +
> > +	atomic_sub(size, &sk->sk_rmem_alloc);
> > +
> > +	spin_lock_bh(&up->mem_lock);
> > +	sk->sk_forward_alloc += size;
> > +	amt = (sk->sk_forward_alloc - partial) & ~(SK_MEM_QUANTUM - 1);
> > +	sk->sk_forward_alloc -= amt;
> > +	spin_unlock_bh(&up->mem_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);
> > +}
> > +
> > +int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
> > +{
> > +	struct sk_buff_head *list = &sk->sk_receive_queue;
> > +	int rmem, delta, amt, err = -ENOMEM;
> > +	struct udp_sock *up = udp_sk(sk);
> > +	int size = skb->truesize;
> > +
> > +	/* try to avoid the costly atomic add/sub pair when the receive
> > +	 * queue is full
> > +	 */
> > +	if (atomic_read(&sk->sk_rmem_alloc) + size > sk->sk_rcvbuf)
> > +		goto drop;
> > +
> > +	rmem = atomic_add_return(size, &sk->sk_rmem_alloc);
> > +	if (rmem > sk->sk_rcvbuf)
> > +		goto uncharge_drop;
> 
> This is dangerous : We used to accept one packet in receive queue, even
> if sk_rcvbuf is too small. (Check __sock_queue_rcv_skb())
> 
> With your code we might drop all packets even if receive queue is empty,
> this might add regressions for some apps.
> 
> > +
> > +	spin_lock(&up->mem_lock);
> > +	if (size < sk->sk_forward_alloc)
> > +		goto unlock;
> 
> Strange label name, since you do "sk->sk_forward_alloc -= size;"before
> unlock.
> 
> You could avoid the goto by :
> 
> 	if (size >= sk->sk_forward_alloc) {
>              amt = sk_mem_pages(size);
>              ...
>         }
>         sk->sk_forward_alloc += delta;k_mem_schedule(
> 
> > +
> > +	amt = sk_mem_pages(size);
> > +	delta = amt << SK_MEM_QUANTUM_SHIFT;
> > +	if (!__sk_mem_raise_allocated(sk, delta, amt, SK_MEM_RECV)) {
> > +		err = -ENOBUFS;
> > +		spin_unlock(&up->mem_lock);
> > +		goto uncharge_drop;
> > +	}
> > +
> > +	sk->sk_forward_alloc += delta;
> > +
> > +unlock:
> > +	sk->sk_forward_alloc -= size;
> > +	spin_unlock(&up->mem_lock);
> > +
> > +	/* the skb owner in now the udp socket */
> > +	skb_orphan(skb);
> > +	skb->sk = sk;
> > +	skb->destructor = udp_rmem_free;
> 
> > +
> > +	skb->dev = NULL;
> > +	sock_skb_set_dropcount(sk, skb);
> > +
> > +	spin_lock(&list->lock);
> 
> Since we acquire this lock anyway to store the packet,
> why not protecting sk_forward_alloc as well with this spinlock,
> and get rid of up->mem_lock.
> 
> Ideally a single spin_lock() should be enough.

I thought that would hit us back, since udp_recvmsg() needs to acquire
it twice and we are increasing the bh lock scope, but a quick test
showed otherwise!!!

> Then almost all other atomic ops could be transformed to non atomic ops.

I'm not sure which atomic operations you refer to ?!? AFAICS we still
need them to manipulate udp_memory_allocated, sk_rmem_alloc, and
sk_drops. Can you please hint which ones ?

Thank you,

Paolo

--
To unsubscribe from this list: send the line "unsubscribe linux-nfs" in
the body of a message to [email-masked]
More majordomo info at  http://vger.kernel.org/majordomo-info.html