netdev - Re: [PATCH 0/3] Receive Side Coalescing for macb driver

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Date:   Mon, 16 Apr 2018 11:08:55 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     rafalo@...ence.com
Cc:     nicolas.ferre@...rochip.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] Receive Side Coalescing for macb driver

From: Rafal Ozieblo <rafalo@...ence.com>
Date: Sat, 14 Apr 2018 21:53:07 +0100

> This patch series adds support for receive side coalescing
> for Cadence GEM driver. Receive segmentation coalescing
> is a mechanism to reduce CPU overhead. This is done by
> coalescing received TCP message segments together into
> a single large message. This means that when the message
> is complete the CPU only has to process the single header
> and act upon the one data payload.

You're going to have to think more deeply about enabling this
feature.

If you can't adjust the receive buffer offset, then the various
packet header fields will be unaligned.

On certain architectures this will result in unaligned traps
all over the networking stack as the packet is being processed.

So enabling this by default will hurt performance on such
systems a lot.

The whole "skb_reserve(skb, NET_IP_ALIGN)" is not just for fun,
it is absolutely essential.