netdev - Re: [PATCH net] net: wireless: intel: iwlwifi: fix GRO

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Date:   Wed, 27 Nov 2019 19:31:08 +0300
From:   Alexander Lobakin <alobakin@...nk.ru>
To:     Edward Cree <ecree@...arflare.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jiri Pirko <jiri@...lanox.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Ido Schimmel <idosch@...lanox.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Petr Machata <petrm@...lanox.com>,
        Sabrina Dubroca <sd@...asysnail.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Manish Chopra <manishc@...vell.com>,
        GR-Linux-NIC-Dev@...vell.com,
        Johannes Berg <johannes.berg@...el.com>,
        Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        Luca Coelho <luciano.coelho@...el.com>,
        Intel Linux Wireless <linuxwifi@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Nicholas Johnson <nicholas.johnson-opensource@...look.com.au>,
        "Kenneth R. Crudup" <kenny@...ix.com>, netdev@...r.kernel.org,
        linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: wireless: intel: iwlwifi: fix GRO_NORMAL packet
 stalling

Edward Cree wrote 27.11.2019 19:05:
> On 27/11/2019 09:41, Alexander Lobakin wrote:
>> Commit 6570bc79c0df ("net: core: use listified Rx for GRO_NORMAL in
>> napi_gro_receive()") has applied batched GRO_NORMAL packets processing
>> to all napi_gro_receive() users, including mac80211-based drivers.
>> 
>> However, this change has led to a regression in iwlwifi driver [1][2] 
>> as
>> it is required for NAPI users to call napi_complete_done() or
>> napi_complete() and the end of every polling iteration, whilst iwlwifi
>> doesn't use NAPI scheduling at all and just calls napi_gro_flush().
>> In that particular case, packets which have not been already flushed
>> from napi->rx_list stall in it until at least next Rx cycle.
>> 
>> Fix this by adding a manual flushing of the list to iwlwifi driver 
>> right
>> before napi_gro_flush() call to mimic napi_complete() logics.
>> 
>> I prefer to open-code gro_normal_list() rather than exporting it for 2
>> reasons:
>> * to prevent from using it and napi_gro_flush() in any new drivers,
>>   as it is the *really* bad way to use NAPI that should be avoided;
>> * to keep gro_normal_list() static and don't lose any CC 
>> optimizations.
>> 
>> I also don't add the "Fixes:" tag as the mentioned commit was only a
>> trigger that only exposed an improper usage of NAPI in this particular
>> driver.
>> 
>> [1] 
>> https://lore.kernel.org/netdev/PSXP216MB04388962C411CD0B17A86F47804A0@PSXP216MB0438.KORP216.PROD.OUTLOOK.COM
>> [2] https://bugzilla.kernel.org/show_bug.cgi?id=205647
>> 
>> Signed-off-by: Alexander Lobakin <alobakin@...nk.ru>
> Reviewed-by: Edward Cree <ecree@...arflare.com>

Thanks! And you were the first who's found the root of the issue.

Regards,
ᚷ ᛖ ᚢ ᚦ ᚠ ᚱ