netdev - Re: [PATCH net-next V3 3/3] tun: rx batching

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Message-Id: <20161231.123147.1116351209865273335.davem@davemloft.net>
Date:   Sat, 31 Dec 2016 12:31:47 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     jasowang@...hat.com
Cc:     mst@...hat.com, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        wexu@...hat.com
Subject: Re: [PATCH net-next V3 3/3] tun: rx batching

From: Jason Wang <jasowang@...hat.com>
Date: Fri, 30 Dec 2016 13:20:51 +0800

> @@ -1283,10 +1314,15 @@ 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();
> +	if (!rx_batched) {
> +		local_bh_disable();
> +		netif_receive_skb(skb);
> +		local_bh_enable();
> +	} else {
> +		tun_rx_batched(tfile, skb, more);
> +	}
>  #else
>  	netif_rx_ni(skb);
>  #endif

If rx_batched has been set, and we are talking to clients not using
this new MSG_MORE facility (or such clients don't have multiple TX
packets to send to you, thus MSG_MORE is often clear), you are doing a
lot more work per-packet than the existing code.

You take the queue lock, you test state, you splice into a local queue
on the stack, then you walk that local stack queue to submit just one
SKB to netif_receive_skb().

I think you want to streamline this sequence in such cases so that the
cost before and after is similar if not equivalent.