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 14:06:25 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Julian Anastasov <ja@....bg>
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

On Tue, 2017-08-15 at 22:45 +0300, Julian Anastasov wrote:
> 	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...


Right you are.

It must be possible to add a fast path without locks.

(say if jiffies has not changed before last state change)