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

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Date:   Mon, 17 Dec 2018 10:19:22 +1100
From:   NeilBrown <neil@...wn.name>
To:     David Miller <davem@...emloft.net>
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

On Sun, Dec 16 2018, David Miller wrote:

> 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.

I'll keep that in mind - thanks.

My guess was CPU-cache invalidation.
I just checked and the other CPU core (there are two - each
hyper-threaded - "other" meaning not the one that handles ethernet
interrupts) gets several thousand "IPI resched" interrupts while
running a 10 second (226MByte) iperf3 receive test.
About 17KB transferred per IPI.
I cannot see where build_skb() would do cache invalidation though.

Thanks,
NeilBrown

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