netdev - Re: How to best consolidate list of skbs (msdu) for receive?

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Date:	Thu, 27 Feb 2014 14:31:45 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	Florian Fainelli <f.fainelli@...il.com>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>
Subject: Re: How to best consolidate list of skbs (msdu) for receive?

On Thu, 2014-02-27 at 13:51 -0800, Ben Greear wrote:

> That said, the code below appears to work, even if it is
> not as efficient as it might be?
> 
> 			if (msdu_chaining) {
> 				struct sk_buff *next = msdu_head->next;
> 				struct sk_buff *to_free = next;
> 				int space;
> 				static int do_once = 1;
> 				msdu_head->next = NULL;
> 
> 				if (unlikely(do_once)) {
> 					ath10k_warn("htt rx msdu_chaining detected %d\n",
> 						    msdu_chaining);
> 					do_once = 0;
> 				}
> 
> 				while (next) {
> 					space = next->len - skb_tailroom(msdu_head);
> 
> 					if ((space > 0) &&
> 					    (pskb_expand_head(msdu_head, 0, space, GFP_ATOMIC) < 0)) {
> 						/* TODO:  bump some rx-oom error stat */
> 						goto outside_continue;
> 					}
> 					skb_copy_from_linear_data(next, skb_put(msdu_head, next->len),
> 								  next->len);
> 					next = next->next;
> 				}

Yep, this is very inefficient , you might copy data very often in
pskb_expand_head(), then in skb_copy_from_linear_data()

Also, if total length is big, you end up doing high order allocations,
since you generate a linear skb. This might fail under memory pressure.

What is the maximal skb->len of resulting skb ?


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