netdev - Re: [PATCH net-next 0/2] pskb_extract() helper function.

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Message-ID: <20160422232301.GA1594@localhost.localdomain>
Date:	Fri, 22 Apr 2016 20:23:01 -0300
From:	marcelo.leitner@...il.com
To:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:	netdev@...r.kernel.org, rds-devel@....oracle.com,
	santosh.shilimkar@...cle.com, davem@...emloft.net,
	eric.dumazet@...il.com
Subject: Re: [PATCH net-next 0/2] pskb_extract() helper function.

On Wed, Apr 20, 2016 at 03:17:40AM -0700, Sowmini Varadhan wrote:
> This patchset follows up on the discussion in
>  https://www.mail-archive.com/netdev@vger.kernel.org/msg105090.html
> 
> For RDS-TCP, we have to deal with the full gamut of
> nonlinear sk_buffs, including all the frag_list variants.
> Also, the parent skb has to remain unchanged, while the clone
> is queued for Rx on the PF_RDS socket. 
> 
> Patch 1 of this patchset adds a pskb_extract() function that 
> does all this without the redundant memcpy's in pskb_expand_head() 
> and __pskb_pull_tail().

I applied this patchset and updated SCTP to also use it for data chunks.

My tests results were very similar to what I had without it. Varying to
better or worse, tending worse.  Thing is, SCTP always works on
linearized skbs as it can't crawl on fragments, so those clone/trim
operations are just offset adjusts regarding the data, and it's shared.
With pskb_extract, it implies in a new memory allocation and a copy,
even in this best case, so for SCTP, for now, it's actually a drawback
I'm afraid.

  Marcelo