netdev - Re: [PATCH net-next] virtio_net: Disable interrupts if napi_complete

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Date:   Fri, 08 Dec 2017 13:19:23 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     jasowang@...hat.com
Cc:     mst@...hat.com, makita.toshiaki@....ntt.co.jp,
        netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        edumazet@...gle.com
Subject: Re: [PATCH net-next] virtio_net: Disable interrupts if
 napi_complete_done rescheduled napi

From: Jason Wang <jasowang@...hat.com>
Date: Thu, 7 Dec 2017 15:08:45 +0800

> 
> 
> On 2017年12月07日 13:09, Michael S. Tsirkin wrote:
>> On Thu, Dec 07, 2017 at 01:15:15PM +0900, Toshiaki Makita wrote:
>>> Since commit 39e6c8208d7b ("net: solve a NAPI race") napi has been
>>> able
>>> to be rescheduled within napi_complete_done() even in non-busypoll
>>> case,
>>> but virtnet_poll() always enabled interrupts before complete, and when
>>> napi was rescheduled within napi_complete_done() it did not disable
>>> interrupts.
>>> This caused more interrupts when event idx is disabled.
>>>
>>> According to commit cbdadbbf0c79 ("virtio_net: fix race in RX VQ
>>> processing") we cannot place virtqueue_enable_cb_prepare() after
>>> NAPI_STATE_SCHED is cleared, so disable interrupts again if
>>> napi_complete_done() returned false.
>>>
>>> Tested with vhost-user of OVS 2.7 on host, which does not have the
>>> event
>>> idx feature.
...
>>> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
>> Acked-by: Michael S. Tsirkin <mst@...hat.com>
>>
>> it might make sense in net and not -next since tx napi regressed
>> performance
>> in some configs, this might bring it back at least partially.
>> Jason - what do you think?
> 
> No sure, the regression I saw was tested with event idx on. And
> virtqueue_disable_cb() does almost nothing for event idx (or even a
> little bit slower).
> 
> The patch looks good.
> 
> Acked-by: Jason Wang <jasowang@...hat.com>

I'm going to put this into net-next for now.