netdev - Re: [PATCH net-next 4/5] net/socket: add helpers for recvmmsg

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Date:   Fri, 25 Nov 2016 14:30:22 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Sabrina Dubroca <sd@...asysnail.net>
Subject: Re: [PATCH net-next 4/5] net/socket: add helpers for recvmmsg

On Fri, 2016-11-25 at 16:39 +0100, Paolo Abeni wrote:
> _skb_try_recv_datagram_batch dequeues multiple skb's from the
> socket's receive queue, and runs the bulk_destructor callback under
> the receive queue lock.

...

> +	last = (struct sk_buff *)queue;
> +	first = (struct sk_buff *)queue->next;
> +	skb_queue_walk(queue, skb) {
> +		last = skb;
> +		totalsize += skb->truesize;
> +		if (++datagrams == batch)
> +			break;
> +	}

This is absolutely not good.

Walking through a list, bringing 2 cache lines per skb, is not the
proper way to deal with bulking.

And I do not see where 'batch' value coming from user space is capped ?

Is it really vlen argument coming from recvmmsg() system call ???

This code runs with BH masked, so you do not want to give user a way to
make you loop there 1000 times 

Bulking is nice, only if you do not compromise with system stability and
latency requirements from other users/applications.