netdev - Re: [PATCH] reduce the spinlock conflict during massive connect

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Date:   Sun, 05 Nov 2017 21:27:15 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Liu Yu <liuyu924@...il.com>
Cc:     netdev@...r.kernel.org,
        "\"David S. Miller\" <davem@...emloft.net>;Alexey Kuznetsov "
         "<kuznet@....inr.ac.ru>;Hideaki YOSHIFUJI" <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH] reduce the spinlock conflict during massive connect

On Mon, 2017-11-06 at 10:28 +0800, Liu Yu wrote:
> From: Liu Yu <allanyuliu@...cent.com>
> 
> When a mount of processes connect to the same port at the same address
> simultaneously, they are likely getting the same bhash and therefore
> conflict with each other.
> 
> The more the cpu number, the worse in this case.
> 
> Use spin_trylock instead for this scene, which seems doesn't matter
> for common case.
> 
> Signed-off-by: Liu Yu <allanyuliu@...cent.com>
> ---
>  net/ipv4/inet_hashtables.c |    6 +++++-
>  1 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index e7d15fb..cc11ec7 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -581,13 +581,17 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
>  other_parity_scan:
>  	port = low + offset;
>  	for (i = 0; i < remaining; i += 2, port += 2) {
> +		int ret;
> +
>  		if (unlikely(port >= high))
>  			port -= remaining;
>  		if (inet_is_local_reserved_port(net, port))
>  			continue;
>  		head = &hinfo->bhash[inet_bhashfn(net, port,
>  						  hinfo->bhash_size)];
> -		spin_lock_bh(&head->lock);
> +		ret = spin_trylock(&head->lock);
> +		if (unlikely(!ret))
> +			continue;
>  
>  		/* Does not bother with rcv_saddr checks, because
>  		 * the established check is already unique enough.

This is broken.

I am pretty sure you have not really tested this patch properly.

Chances are very high that a connect() will miss slots and wont succeed,
when table is almost full.

Performance is nice, but we actually need to allocate a 4-tuple in a
more deterministic fashion.