netdev - Re: [PATCH v2] tun: Use netif_receive_skb instead of netif

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Message-Id: <20161201.144357.33718747165573960.davem@davemloft.net>
Date:   Thu, 01 Dec 2016 14:43:57 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     andreyknvl@...gle.com
Cc:     herbert@...dor.apana.org.au, jasowang@...hat.com,
        edumazet@...gle.com, pmk@...gle.com, pabeni@...hat.com,
        mst@...hat.com, soheil@...gle.com, elfring@...rs.sourceforge.net,
        rppt@...ux.vnet.ibm.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, dvyukov@...gle.com, kcc@...gle.com,
        syzkaller@...glegroups.com
Subject: Re: [PATCH v2] tun: Use netif_receive_skb instead of netif_rx

From: Andrey Konovalov <andreyknvl@...gle.com>
Date: Thu,  1 Dec 2016 10:34:40 +0100

> This patch changes tun.c to call netif_receive_skb instead of netif_rx
> when a packet is received (if CONFIG_4KSTACKS is not enabled to avoid
> stack exhaustion). The difference between the two is that netif_rx queues
> the packet into the backlog, and netif_receive_skb proccesses the packet
> in the current context.
> 
> This patch is required for syzkaller [1] to collect coverage from packet
> receive paths, when a packet being received through tun (syzkaller collects
> coverage per process in the process context).
> 
> As mentioned by Eric this change also speeds up tun/tap. As measured by
> Peter it speeds up his closed-loop single-stream tap/OVS benchmark by
> about 23%, from 700k packets/second to 867k packets/second.
> 
> A similar patch was introduced back in 2010 [2, 3], but the author found
> out that the patch doesn't help with the task he had in mind (for cgroups
> to shape network traffic based on the original process) and decided not to
> go further with it. The main concern back then was about possible stack
> exhaustion with 4K stacks.
> 
> [1] https://github.com/google/syzkaller
> 
> [2] https://www.spinics.net/lists/netdev/thrd440.html#130570
> 
> [3] https://www.spinics.net/lists/netdev/msg130570.html
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> ---
> 
> Changes since v1:
> - incorporate Eric's note about speed improvements in commit description
> - use netif_receive_skb CONFIG_4KSTACKS is not enabled

Applied to net-next, thanks!