netdev - Re: 100% CPU load when generating traffic to destination network that nexthop is not reachable

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Date:   Tue, 15 Aug 2017 22:45:28 +0300 (EEST)
From:   Julian Anastasov <ja@....bg>
To:     Eric Dumazet <eric.dumazet@...il.com>
cc:     Paweł Staszewski <pstaszewski@...are.pl>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: 100% CPU load when generating traffic to destination network
 that nexthop is not reachable


	Hello,

On Tue, 15 Aug 2017, Eric Dumazet wrote:

> Please try this :
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index 16a1a4c4eb57fa1147f230916e2e62e18ef89562..95e0d7702029b583de8229e3c3eb923f6395b072 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -991,14 +991,18 @@ static void neigh_timer_handler(unsigned long arg)
>  
>  int __neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
>  {
> -	int rc;
>  	bool immediate_probe = false;
> +	int rc;
> +
> +	/* We _should_ test this under write_lock_bh(&neigh->lock),
> +	 * but this is too costly.
> +	 */
> +	if (READ_ONCE(neigh->nud_state) & (NUD_CONNECTED | NUD_DELAY | NUD_PROBE))
> +		return 0;

	The same fast check is already done in the only caller,
neigh_event_send. Now we risk to enter the
'if (!(neigh->nud_state & (NUD_STALE | NUD_INCOMPLETE))) {' block...

>  	write_lock_bh(&neigh->lock);
>  
>  	rc = 0;
> -	if (neigh->nud_state & (NUD_CONNECTED | NUD_DELAY | NUD_PROBE))
> -		goto out_unlock_bh;
>  	if (neigh->dead)
>  		goto out_dead;

Regards

--
Julian Anastasov <ja@....bg>