netdev - Re: [PATCH] IPv6: Fix CPU contention on FIB6 GC

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Date:   Mon, 22 Jun 2020 15:55:21 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Oliver Herms <oliver.peter.herms@...il.com>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        kuba@...nel.org
Subject: Re: [PATCH] IPv6: Fix CPU contention on FIB6 GC



On 6/22/20 1:53 PM, Oliver Herms wrote:
> When fib6_run_gc is called with parameter force=true the spinlock in
> /net/ipv6/ip6_fib.c:2310 can lock all CPUs in softirq when
> net.ipv6.route.max_size is exceeded (seen this multiple times).
> One sotirq/CPU get's the lock. All others spin to get it. It takes
> substantial time until all are done. Effectively it's a DOS vector.
> 
> As the splinlock is only enforcing that there is at most one GC running
> at a time, it should IMHO be safe to use force=false here resulting
> in spin_trylock_bh instead of spin_lock_bh, thus avoiding the lock
> contention.
> 
> Finding a locked spinlock means some GC is going on already so it is
> save to just skip another execution of the GC.
> 
> Signed-off-by: Oliver Herms <oliver.peter.herms@...il.com>
> ---
>  net/ipv6/route.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index 82cbb46a2a4f..7e6fbaf43549 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -3205,7 +3205,7 @@ static int ip6_dst_gc(struct dst_ops *ops)
>  		goto out;
>  
>  	net->ipv6.ip6_rt_gc_expire++;
> -	fib6_run_gc(net->ipv6.ip6_rt_gc_expire, net, true);
> +	fib6_run_gc(net->ipv6.ip6_rt_gc_expire, net, false);
>  	entries = dst_entries_get_slow(ops);
>  	if (entries < ops->gc_thresh)
>  		net->ipv6.ip6_rt_gc_expire = rt_gc_timeout>>1;


On which kernel have you seen a contention ?

I am asking this because I recently pushed a patch that basically should have
been enough to take care of the problem.

commit d8882935fcae28bceb5f6f56f09cded8d36d85e6
Author: Eric Dumazet <edumazet@...gle.com>
Date:   Fri May 8 07:34:14 2020 -0700

    ipv6: use DST_NOCOUNT in ip6_rt_pcpu_alloc()