netdev - Re: [PATCH 0/3]: net: dsa: mt7530: support MT7530 in the MT7621 SoC

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Date:   Sun, 16 Dec 2018 14:14:58 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     neil@...wn.name
Cc:     bjorn@...k.no, gerg@...nel.org, sean.wang@...iatek.com,
        andrew@...n.ch, vivien.didelot@...oirfairelinux.com,
        f.fainelli@...il.com, netdev@...r.kernel.org, blogic@...nwrt.org,
        opensource@...rst.com
Subject: Re: [PATCH 0/3]: net: dsa: mt7530: support MT7530 in the MT7621 SoC

From: NeilBrown <neil@...wn.name>
Date: Mon, 17 Dec 2018 09:08:54 +1100

> In my 4.4 kernel, the build_skb() call in (the equivalent of)
> mtk_poll_rx() takes about 1.2usec and the call to napi_gro_receive()
> takes about 3usec.
> 
> In my 4.20 kernel, these calls take about 30 and 24 usec respectively.
> This easily explains the slowdown.

That's a huge difference.

Nothing jumps out as a possible cause except perhaps retpoline or
something like that.