netdev - Re: [PATCH net] r8169: disable default rx interrupt coalescing on RTL8168

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Date:   Mon, 1 Apr 2019 09:50:36 +0100
From:   Mike Crowe <mac@...owe.com>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        David Miller <davem@...emloft.net>
Cc:     Realtek linux nic maintainers <nic_swsd@...ltek.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] r8169: disable default rx interrupt coalescing on
 RTL8168

On Saturday 30 March 2019 at 17:13:24 +0100, Heiner Kallweit wrote:
> It was reported that re-introducing ASPM, in combination with RX
> interrupt coalescing, results in significantly increased packet
> latency, see [0]. Disabling ASPM or RX interrupt coalescing fixes
> the issue. Therefore change the driver's default to disable RX
> interrupt coalescing. Users still have the option to enable RX
> coalescing via ethtool.
> 
> [0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=925496
> 
> Fixes: a99790bf5c7f ("r8169: Reinstate ASPM Support")
> Reported-by: Mike Crowe <mac@...owe.com>
> Signed-off-by: Heiner Kallweit <hkallweit1@...il.com>
> ---
>  drivers/net/ethernet/realtek/r8169.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/realtek/r8169.c b/drivers/net/ethernet/realtek/r8169.c
> index c9ee1c8eb..8a8a4f70d 100644
> --- a/drivers/net/ethernet/realtek/r8169.c
> +++ b/drivers/net/ethernet/realtek/r8169.c
> @@ -5460,7 +5460,7 @@ static void rtl_hw_start_8168(struct rtl8169_private *tp)
>  	tp->cp_cmd |= PktCntrDisable | INTT_1;
>  	RTL_W16(tp, CPlusCmd, tp->cp_cmd);
>  
> -	RTL_W16(tp, IntrMitigate, 0x5151);
> +	RTL_W16(tp, IntrMitigate, 0x5100);
>  
>  	/* Work around for RxFIFO overflow. */
>  	if (tp->mac_version == RTL_GIGA_MAC_VER_11) {
> -- 
> 2.21.0

Tested-by: Mike Crowe <mac@...owe.com>

I configured my ASUS BIOS to reenable "EPA Power Saving Mode" and tried
running the current state of linux-5.0.y with the above patch applied on
top.

As expected, packet latency was good immediately after booting. Running
"ethtool -c" showed rx-usecs=0, rx-frames=1. Also as expected, manually
increasing these to rx-usecs=200, rx-frames=4 caused the latency problem to
return.

Thanks for finding a way to address this problem without causing both users
of ageing motherboards and modern notebooks to scream.

Mike.