linux-kernel - Re: [PATCH v5 0/2] skb_array: array based FIFO for skbs

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Date:	Mon, 23 May 2016 06:31:46 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
	davem@...emloft.net, netdev@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>, brouer@...hat.com
Subject: Re: [PATCH v5 0/2] skb_array: array based FIFO for skbs

On Mon, 2016-05-23 at 13:43 +0300, Michael S. Tsirkin wrote:
> This is in response to the proposal by Jason to make tun
> rx packet queue lockless using a circular buffer.
> My testing seems to show that at least for the common usecase
> in networking, which isn't lockless, circular buffer
> with indices does not perform that well, because
> each index access causes a cache line to bounce between
> CPUs, and index access causes stalls due to the dependency.
> 
> By comparison, an array of pointers where NULL means invalid
> and !NULL means valid, can be updated without messing up barriers
> at all and does not have this issue.

Note that both consumers and producers write in the array, so in light
load (like TCP_RR), there are 2 cache line used byt the producers, and 2
cache line used for consumers, with potential bouncing.

In the other hand, the traditional sk_buff_head has one cache line,
holding the spinlock and list head/tail.

We might use the 'shared cache line' :

+       /* Shared consumer/producer data */
+       int size ____cacheline_aligned_in_smp; /* max entries in queue
*/
+       struct sk_buff **queue;


To put here some fast path involving a single cache line access when
queue has 0 or 1 item.