netdev - Re: [PATCH] Generalize socket rx gap / receive queue overflow cmsg

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Date:	Thu, 8 Oct 2009 13:20:29 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, socketcan@...tkopp.net
Subject: Re: [PATCH] Generalize socket rx gap / receive queue overflow cmsg

On Thu, Oct 08, 2009 at 04:45:48PM +0200, Eric Dumazet wrote:
> Neil Horman a écrit :
> >>> +	if (check_drops) {
> >>> +		skb = skb_recv_datagram(sk, flags|MSG_PEEK,
> >>> +				flags & MSG_DONTWAIT, &err);
> >> 	Ouch, this is too expensive, please find another way :)
> >>
> >>> +		if (skb) {
> >>> +			gap = skb->dropcount;
> >>> +			consume_skb(skb);
> >>> +		}
> >>> +	}
> >>> +
> > I'm not sure that I see the expense here, and what expense there is, I don't see
> > how it avoidable.  In order to do this reporting at the socket level, we need to
> > look at the skb at the head of the receive queue.  But we need to do so in a way
> > thats consistent with the flags being passed in (i.e. if this is a blocking
> > socket, we need to block here until something is available to read).  Then its
> > just an atomic_inc on skb->users, followed by a dec in the consume_skb.  I could
> > implement the logic for DONTWAIT myself, and skip the atomic_inc/dec, but I'm
> > not sure thats much of a savings.  If you have another thought, I'm certainly
> > open to it.
> 
> The expense is a lot of atomic ops. You forgot the lock, so thats four atomic ops.
> 
> You can do all this with no extra atomics.
> 
> All you need is some function with (struct msghdr *msg, struct sock *sk, struct sk_buff *skb)
> triplet.
> 
> hint : sock_recv_timestamp(struct msghdr *msg, struct sock *sk, struct sk_buff *skb)
> 
> Could be renamed to something else if you want...
> 
> sock_recv_ts_or_drops() or whatever
Ok, but that will require moving the flag that we're triggering this on down
into the sock structure, and not doing the check up in __sock_recvmsg, but I
suppose thats fine.  Ok, I'll repost soon.  Thanks!
Neil

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