netdev - Re: [PATCH] net: Add ndo_gso

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Date:	Tue, 30 Sep 2014 23:12:14 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Stephen Hemminger <stephen@...workplumber.org>,
	Or Gerlitz <gerlitz.or@...il.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Thomas Graf <tgraf@...g.ch>,
	Pravin Shelar <pshelar@...ira.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Andy Zhou <azhou@...ira.com>
Subject: Re: [PATCH] net: Add ndo_gso_check

On Tue, 2014-09-30 at 17:18 -0700, Eric Dumazet wrote:
> On Tue, 2014-09-30 at 17:05 -0700, Tom Herbert wrote:
> 
> > 
> > Will this still be an advantage with the burst TX implementation?
> 
> Segmenting a 64K packet into 45 sk_buff adds an additional 90
> allocations (45 sk_buff, 45 sk->head) and dirtying a _lot_ of memory.
> 
> We now have some helpers that might be used by drivers to perform TSO
> emulation.
> 
> commit e876f208af18b074f800656e4d1b99da75b2135f ("net: Add a software
> TSO helper API")
> 
> Check commit 3ae8f4e0b98b640aadf410c21185ccb6b5b02351 for reference
> ("net: mv643xx_eth: Implement software TSO")

BTW, even for NIC TSO capable, it seems TSO can be slower for small
packets (like 2 MSS packets) on some NICs at least...

In this case forcing segmentation can be a win (consumes more cpu
cycles, but reduce the stress on the NIC)

(Adding an extra check in netif_skb_features() would do as well, I
suppose)

+       if (skb_shinfo(skb)->gso_segs < skb->dev->gso_min_segs)
+               features &= ~NETIF_F_GSO_MASK;
 


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