Subject: Re: [RFC] net: release dst entry in dev_queue_xmit()

From: Jarek Poplawski <[email-masked]>
Date: Wed, 25 Mar 2009 20:18:50 +0100

On Wed, Mar 25, 2009 at 07:41:27PM +0100, Eric Dumazet wrote:
> Jarek Poplawski a écrit :
> > David Miller wrote, On 03/25/2009 08:17 AM:
> > 
> >> From: Eric Dumazet <[email-masked]>
> >> Date: Wed, 25 Mar 2009 08:13:30 +0100
> >>
> >>> If done in dev_hard_start_xmit(), skb could be requeued (because of
> >>> NETDEV_TX_BUSY).  Then if requeued, maybe at this time, dst being
> >>> NULL is not a problem ?
> >> Usually it should be OK because the packet schedulers have
> >> a sort-of one-behind state that allows them to reinsert
> >> the SKB into their queue datastructures without reclassifying.
> > 
> > 
> > Actually, since David has dumped requeuing there is no reinserting;
> > this last one "requeued" skb is buffered at the top in q->gso_skb
> > and waiting for better times.
> 
> Yes indeed, this is what I thought too, thanks Jarek.

Alas I'm a bit concerned with virtual devs, e.g. now I'm looking at
xmits in macvlan and pppoe. Maybe this patch should exclude them?

Jarek P.

> 
> I tested following patch today on my machine, but obviously could not try 
> all possible quirks :)
> 
> [PATCH] net: release dst entry in dev_hard_start_xmit()
> 
> One point of contention in high network loads is the dst_release() performed
> when a transmited skb is freed. This is because NIC tx completion calls skb free
> long after original call to dev_queue_xmit(skb).
> 
> CPU cache is cold and the atomic op in dst_release() stalls. On SMP, this is
> quite visible if one CPU is 100% handling softirqs for a network device,
> since dst_clone() is done by other cpus, involving cache line ping pongs.
> 
> I believe we can release dst in dev_hard_start_xmit(), while cpu cache is hot, since
> caller of dev_queue_xmit() had to hold a reference on dst right before. This reduce
> work to be done by softirq handler, and decrease cache misses.
> 
> I also believe only pktgen can call dev_queue_xmit() with skb which have
> a skb->users != 1. But pkthen skbs have a NULL dst entry.
> 
> I added a WARN_ON_ONCE() to catch other cases, and not release skb->dst
> if skb->users != 1
> 
> Signed-off-by: Eric Dumazet <[email-masked]>
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index e3fe5c7..a622db6 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -1664,6 +1664,26 @@ static int dev_gso_segment(struct sk_buff *skb)
>  	return 0;
>  }
>  
> +
> +/*
> + * Release dst while its refcnt is likely hot in CPU cache, instead
> + * of waiting NIC tx completion.
> + * We inline dst_release() code for performance reason
> + */
> +static void release_skb_dst(struct sk_buff *skb)
> +{
> +	if (likely(skb->dst)) {
> +		if (!WARN_ON_ONCE(atomic_read(&skb->users) != 1)) {
> +			int newrefcnt;
> +
> +			smp_mb__before_atomic_dec();
> +			newrefcnt = atomic_dec_return(&skb->dst->__refcnt);
> +			WARN_ON(newrefcnt < 0);
> +			skb->dst = NULL;
> +		}
> +	}
> +}
> +
>  int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
>  			struct netdev_queue *txq)
>  {
> @@ -1681,6 +1701,7 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
>  				goto gso;
>  		}
>  
> +		release_skb_dst(skb);
>  		return ops->ndo_start_xmit(skb, dev);
>  	}
>  
> @@ -1691,6 +1712,7 @@ gso:
>  
>  		skb->next = nskb->next;
>  		nskb->next = NULL;
> +		release_skb_dst(nskb);
>  		rc = ops->ndo_start_xmit(nskb, dev);
>  		if (unlikely(rc)) {
>  			nskb->next = skb->next;
>