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

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Date:   Fri, 14 Sep 2018 16:08:47 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Florian Fainelli <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14日 12:46, Willem de Bruijn wrote:
> On Thu, Sep 13, 2018 at 11:53 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>>
>> On 2018年09月14日 11:40, Willem de Bruijn wrote:
>>> On Thu, Sep 13, 2018 at 11:27 PM Jason Wang <jasowang@...hat.com> wrote:
>>>>
>>>> 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.
>>> Since you prefer coalesce, let's go with that (and a revision of your
>>> latest patch).
>> Good to know this.
>>
>>>>>>> +                     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?
>>> Right. It's a bit of a roundabout way to ensure that
>>> netif_tx_wake_queue and thus eventually free_old_xmit_skbs are called.
>>> It might make more sense to just wake the device without going through
>>> an interrupt.
>> I'm not sure I get this. If we don't enable tx napi, we tend to delay TX
>> interrupt if we found the ring is about to full to avoid interrupt
>> storm, so we're probably ok in this case.
> I'm only concerned about the transition state when converting from
> napi to no-napi when the queue is stopped and tx interrupt disabled.
>
> With napi mode the interrupt is only disabled if napi is scheduled,
> in which case it will eventually reenable the interrupt. But when
> switching to no-napi mode in this state no progress will be made.
>
> But it seems this cannot happen. When converting to no-napi
> mode, set_coalesce waits for napi to complete in napi_disable.
> So the interrupt should always start enabled when transitioning
> into no-napi mode.

Yes, I see.

Thanks