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

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Date:   Thu, 1 Dec 2016 15:05:24 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Jason Wang <jasowang@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Peter Klausler <pmk@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Markus Elfring <elfring@...rs.sourceforge.net>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>, syzkaller@...glegroups.com
Subject: Re: [PATCH v2] tun: Use netif_receive_skb instead of netif_rx

On Thu, Dec 01, 2016 at 10:34:40AM +0100, Andrey Konovalov wrote:
> 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>

Acked-by: Michael S. Tsirkin <mst@...hat.com>

> ---
> 
> Changes since v1:
> - incorporate Eric's note about speed improvements in commit description
> - use netif_receive_skb CONFIG_4KSTACKS is not enabled
> 
>  drivers/net/tun.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 8093e39..d310b13 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1304,7 +1304,13 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>  	skb_probe_transport_header(skb, 0);
>  
>  	rxhash = skb_get_hash(skb);
> +#ifndef CONFIG_4KSTACKS
> +	local_bh_disable();
> +	netif_receive_skb(skb);
> +	local_bh_enable();
> +#else
>  	netif_rx_ni(skb);
> +#endif
>  
>  	stats = get_cpu_ptr(tun->pcpu_stats);
>  	u64_stats_update_begin(&stats->syncp);
> -- 
> 2.8.0.rc3.226.g39d4020