netdev - Re: [net-next 1/9] net/mlx5e: Switch to using napi_build

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Message-ID: <20230216095324.4fa4f6fb@kernel.org>
Date:   Thu, 16 Feb 2023 09:53:24 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Alexander Lobakin <aleksander.lobakin@...el.com>
Cc:     Saeed Mahameed <saeed@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "Saeed Mahameed" <saeedm@...dia.com>, <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...dia.com>, Gal Pressman <gal@...dia.com>
Subject: Re: [net-next 1/9] net/mlx5e: Switch to using napi_build_skb()

On Thu, 16 Feb 2023 18:26:19 +0100 Alexander Lobakin wrote:
> > Before: 26.5 Gbits/sec
> > After:  30.1 Gbits/sec (+13.6%)  
> 
> +14%, gosh! Happy to see more and more vendors switching to it, someone
> told me back then we have so fast RAM nowadays that it won't make any
> sense to directly recycle kmem-cached objects. Maybe it's fast, but
> seems like not *so* fast :D

Interestingly I had a similar patch in my tree when testing the skb_ext
cache and enabling slow_gro kills this gain.

IOW without adding an skb_ext using napi_build_skb() gives me ~12%
boost. If I start adding skb_ext (with the cache and perfect reuse) 
I'm back to the baseline (26.5Gbps in this case).

But without using napi_build_skb() adding skb_ext (with the cache)
doesn't change anything, skb_ext or not, I'll get 26.5Gbps.

Very finicky. Not sure why this happens. Perhaps napi_build_skb() 
let's us fit under some CPU resource constraint and additional
functionality knocks us back over the line?