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

[Jason Wang <jasowang@redhat.com>]

On 2018年09月13日 03:11, Willem de Bruijn wrote:
> On Wed, Sep 12, 2018 at 2:16 PM Florian Fainelli <f.fainelli@gmail.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@gmail.com> wrote:
>>>>
>>>>
>>>> On 9/9/2018 3:44 PM, Willem de Bruijn wrote:
>>>>> From: Willem de Bruijn <willemb@google.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.
>
>>> Configurable napi-tx is not a common feature across devices. We really
>>> want virtio-net to also just convert to napi-tx as default, but need a
>>> way to gradually convert with application opt-out if some workloads
>>> see regressions.
>> The rationale makes sense, no questions about it.
>>
>>> There is prior art in interpreting coalesce values as
>>> more than a direct mapping to usec. The e1000 is one example.
>>>
>> Looked at both e1000 and e1000e and they both have a similar programming
>> of the HW's interrupt target rate register, which is relevant to
>> interrupt coalescing, what part of these drivers do you see as doing
>> something not quite coalescing related?
> It's all coalescing related, for sure. e1000_set_coalesce just does not
> translate the tx-usecs values into microsecond latency directly.
>
> It modifies both the interrupt throttle rate adapter->itr and interrupt mode
> adapter->itr_setting, which are initially set in e1000_check_options from
> module param InterruptThrottleRate.
>
> Value 0 disables interrupt moderation. 1 and 3 program a dynamic mode.
> 2 is an illegal value as is 5..9. 10..10000 converts from usec to interrupt
> rate/sec.
>
> I took tx-napi to be a similar interrupt related option as, say, dynamic
> conservative mode interrupt moderation.

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

Thanks