netdev - Re: [PATCH v2 net-next] net: core: use listified Rx for GRO_NORMAL in napi_gro

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Message-ID: <cc08834c-ccb3-263a-2967-f72a9d72535a@solarflare.com>
Date:   Mon, 25 Nov 2019 10:31:12 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Nicholas Johnson <nicholas.johnson-opensource@...look.com.au>,
        "Alexander Lobakin" <alobakin@...nk.ru>
CC:     David Miller <davem@...emloft.net>,
        "jiri@...lanox.com" <jiri@...lanox.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "idosch@...lanox.com" <idosch@...lanox.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "petrm@...lanox.com" <petrm@...lanox.com>,
        "sd@...asysnail.net" <sd@...asysnail.net>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "jaswinder.singh@...aro.org" <jaswinder.singh@...aro.org>,
        "ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "johannes.berg@...el.com" <johannes.berg@...el.com>,
        "emmanuel.grumbach@...el.com" <emmanuel.grumbach@...el.com>,
        "luciano.coelho@...el.com" <luciano.coelho@...el.com>,
        "linuxwifi@...el.com" <linuxwifi@...el.com>,
        "kvalo@...eaurora.org" <kvalo@...eaurora.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
Subject: Re: [PATCH v2 net-next] net: core: use listified Rx for GRO_NORMAL in
 napi_gro_receive()

On 25/11/2019 09:09, Nicholas Johnson wrote:
> The default value of /proc/sys/net/core/gro_normal_batch was 8.
> Setting it to 1 allowed it to connect to Wi-Fi network.
>
> Setting it back to 8 did not kill the connection.
>
> But when I disconnected and tried to reconnect, it did not re-connect.
>
> Hence, it appears that the problem only affects the initial handshake 
> when associating with a network, and not normal packet flow.
That sounds like the GRO batch isn't getting flushed at the endof the
 NAPI — maybe the driver isn't calling napi_complete_done() at the
 appropriate time?
Indeed, from digging through the layers of iwlwifi I eventually get to
 iwl_pcie_rx_handle() which doesn't really have a NAPI poll (the
 napi->poll function is iwl_pcie_dummy_napi_poll() { WARN_ON(1);
 return 0; }) and instead calls napi_gro_flush() at the end of its RX
 handling.  Unfortunately, napi_gro_flush() is no longer enough,
 because it doesn't call gro_normal_list() so the packets on the
 GRO_NORMAL list just sit there indefinitely.

It was seeing drivers calling napi_gro_flush() directly that had me
 worried in the first place about whether listifying napi_gro_receive()
 was safe and where the gro_normal_list() should go.
I wondered if other drivers that show up in [1] needed fixing with a
 gro_normal_list() next to their napi_gro_flush() call.  From a cursory
 check:
brocade/bna: has a real poller, calls napi_complete_done() so is OK.
cortina/gemini: calls napi_complete_done() straight after
 napi_gro_flush(), so is OK.
hisilicon/hns3: calls napi_complete(), so is _probably_ OK.
But it's far from clear to me why *any* of those drivers are calling
 napi_gro_flush() themselves...

-Ed

[1]: https://elixir.bootlin.com/linux/latest/ident/napi_gro_flush