netdev - Re: [PATCH net-next 2/3] net: skbuff: cache one skb

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Date:   Wed, 15 Feb 2023 10:20:15 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Alexander Lobakin <aleksander.lobakin@...el.com>
Cc:     Edward Cree <ecree.xilinx@...il.com>, <davem@...emloft.net>,
        <netdev@...r.kernel.org>, <edumazet@...gle.com>,
        <pabeni@...hat.com>, <willemb@...gle.com>, <fw@...len.de>
Subject: Re: [PATCH net-next 2/3] net: skbuff: cache one skb_ext for use by
 GRO

On Wed, 15 Feb 2023 19:01:19 +0100 Alexander Lobakin wrote:
> > I was hoping to leave sizing of the cache until we have some data from
> > a production network (or at least representative packet traces).
> > 
> > NAPI_SKB_CACHE_SIZE kinda assumes we're not doing much GRO, right?  
> 
> It assumes we GRO a lot :D
> 
> Imagine that you have 64 frames during one poll and the GRO layer
> decides to coalesce them by batches of 16. Then only 4 skbs will be
> used, the rest will go as frags (with "stolen heads") -> 60 of 64 skbs
> will return to that skb cache and will then be reused by napi_build_skb().

Let's say 5 - for 4 resulting skbs GRO will need the 4 resulting and
one extra to shuttle between the driver and GRO (worst case).
With a cache of 1 I'm guaranteed to save 59 alloc calls, 92%, right?

That's why I'm saying - the larger cache would help workloads which
don't GRO as much. Am I missing the point or how GRO works?