netdev - Re: [PATCH net-next] virtio_net: ethtool tx napi configuration

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Date:   Fri, 14 Sep 2018 11:27:18 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     f.fainelli@...il.com, Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, caleb.raitto@...il.com,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "Jon Olson (Google Drive)" <jonolson@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] virtio_net: ethtool tx napi configuration



On 2018年09月13日 22:58, Willem de Bruijn wrote:
> On Thu, Sep 13, 2018 at 5:02 AM Jason Wang <jasowang@...hat.com> wrote:
>>
>>
>> On 2018年09月13日 07:27, Willem de Bruijn wrote:
>>> On Wed, Sep 12, 2018 at 3:11 PM Willem de Bruijn
>>> <willemdebruijn.kernel@...il.com> wrote:
>>>> On Wed, Sep 12, 2018 at 2:16 PM Florian Fainelli <f.fainelli@...il.com> wrote:
>>>>>
>>>>> On 9/12/2018 11:07 AM, Willem de Bruijn wrote:
>>>>>> On Wed, Sep 12, 2018 at 1:42 PM Florian Fainelli <f.fainelli@...il.com> wrote:
>>>>>>>
>>>>>>> On 9/9/2018 3:44 PM, Willem de Bruijn wrote:
>>>>>>>> From: Willem de Bruijn <willemb@...gle.com>
>>>>>>>>
>>>>>>>> Implement ethtool .set_coalesce (-C) and .get_coalesce (-c) handlers.
>>>>>>>> Interrupt moderation is currently not supported, so these accept and
>>>>>>>> display the default settings of 0 usec and 1 frame.
>>>>>>>>
>>>>>>>> Toggle tx napi through a bit in tx-frames. So as to not interfere
>>>>>>>> with possible future interrupt moderation, use bit 10, well outside
>>>>>>>> the reasonable range of real interrupt moderation values.
>>>>>>>>
>>>>>>>> Changes are not atomic. The tx IRQ, napi BH and transmit path must
>>>>>>>> be quiesced when switching modes. Only allow changing this setting
>>>>>>>> when the device is down.
>>>>>>> Humm, would not a private ethtool flag to switch TX NAPI on/off be more
>>>>>>> appropriate rather than use the coalescing configuration API here?
>>>>>> What do you mean by private ethtool flag? A new field in ethtool
>>>>>> --features (-k)?
>>>>> I meant using ethtool_drvinfo::n_priv_flags, ETH_SS_PRIV_FLAGS and then
>>>>> ETHTOOL_GFPFLAGS and ETHTOOL_SPFLAGS to control the toggling of that
>>>>> private flag. mlx5 has a number of privates flags for instance.
>>>> Interesting, thanks! I was not at all aware of those ethtool flags.
>>>> Am having a look. It definitely looks promising.
>>> Okay, I made that change. That is indeed much cleaner, thanks.
>>> Let me send the patch, initially as RFC.
>>>
>>> I've observed one issue where if we toggle the flag before bringing
>>> up the device, it hits a kernel BUG at include/linux/netdevice.h:515
>>>
>>>           BUG_ON(!test_bit(NAPI_STATE_SCHED, &n->state));
>> This reminds me that we need to check netif_running() before trying to
>> enable and disable tx napi in ethtool_set_coalesce().
> The first iteration of my patch checked IFF_UP and effectively
> only allowed the change when not running. What do you mean
> by need to check?

I mean if device is not up, there's no need to toggle napi state and tx 
lock.

>
> And to respond to the other follow-up notes at once:
>
>> Consider we may have interrupt moderation in the future, I tend to use
>> set_coalesce. Otherwise we may need two steps to enable moderation:
>>
>> - tx-napi on
>> - set_coalesce
> FWIW, I don't care strongly whether we do this through coalesce or priv_flags.

Ok.

>>> +                     if (!napi_weight)
>>> +                             virtqueue_enable_cb(vi->sq[i].vq);
>> I don't get why we need to disable enable cb here.
> To avoid entering no-napi mode with too few descriptors to
> make progress and no way to get out of that state. This is a
> pretty crude attempt at handling that, admittedly.

But in this case, we will call enable_cb_delayed() and we will finally 
get a interrupt?

Thanks