netdev - Re: [PATCH net-next] net: generalize skb freeing deferral to per-cpu lists

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Message-ID: <20220422094014.1bcf78d5@kernel.org>
Date:   Fri, 22 Apr 2022 09:40:14 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] net: generalize skb freeing deferral to
 per-cpu lists

On Thu, 21 Apr 2022 08:39:20 -0700 Eric Dumazet wrote:
> 10 runs of one TCP_STREAM flow

Was the test within a NUMA node or cross-node?

For my learning - could this change cause more cache line bouncing 
than individual per-socket lists for non-RFS setups. Multiple CPUs 
may try to queue skbs for freeing on one remove node.

> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 7dccbfd1bf5635c27514c70b4a06d3e6f74395dd..0162a9bdc9291e7aae967a044788d09bd2ef2423 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3081,6 +3081,9 @@ struct softnet_data {
>  	struct sk_buff_head	input_pkt_queue;
>  	struct napi_struct	backlog;
>  
> +	/* Another possibly contended cache line */
> +	struct sk_buff_head	skb_defer_list ____cacheline_aligned_in_smp;

If so maybe we can avoid some dirtying and use a single-linked list? 
No point modifying the cache line of the skb already on the list.

> +	call_single_data_t  csd_defer;
>  };