Re: NET_MPSAFE [was Re: CVS commit: src/sys]

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

On Sat, Jan 21, 2017 at 7:13 PM, Nick Hudson <skrll%netbsd.org@localhost> wrote:
> On 01/20/17 03:34, Ryota Ozaki wrote:
>>
>> On Mon, Dec 12, 2016 at 5:24 PM, Nick Hudson <skrll%netbsd.org@localhost> wrote:
>>>
>>> On 11/28/16 09:43, Ryota Ozaki wrote:
>>>>
>>>> On Mon, Nov 28, 2016 at 4:29 PM, Nick Hudson <skrll%netbsd.org@localhost> wrote:
>>>
>>>
>>>> This is hurting me again.
>>>
>>>
>>> FYI
>>>
>>>
>>> https://nxr.netbsd.org/xref/src/sys/dev/usb/if_axe.c#527
>>>
>>> I think we need NET_MPSAFE finished for netbsd-8
>>
>> Further investigations show that even if we enable NET_MPSAFE by default,
>> there are cases that calling driver Tx with holding softnet_lock. For
>> example, packet transmission via a unconnected socket and TCP syn ack
>> (tcp_input still needs softnet_lock).
>>
>> So I have brought another idea to address the issue: Tx softint; it
>> defers if_start to softint, which is implemented with deferred if_start.
>> By doing so, driver's if_start is called without holding softnet_lock.
>> Note that of course the feature is only enabled for drivers that request
>> it.
>>
>> This is a patch: http://www.netbsd.org/~ozaki-r/tx_softint.diff
>> The patch includes changes to if_axe for an example.
>>
>> The drawback of the approach is of course performance degradation.
>> I tried it with vioif and saw 3% down on iperf. I haven't tested
>> it using USB devices, so I don't know the performance impact on USB Tx.
>>
>> Nick, how do you think the approach? And can you try the patch and
>> benchmark if the approach satisfies you?
>
>
> I'll look into this, but...
>
>
>>
>> Note that this is a tentative solution for netbsd-8. The ideal solution
>> is still to get rid of softnet_lock completely.
>
>
> How hard is it to remove the need to hold softnet_lock for the case you
> mention above? It
> would be a shame to rototill all the drivers if it isn't required.

I don't know because I'm not familiar with Layer 4 and the socket layer.
Someone wizard on the areas may be able to address the issue easier than
I expect, but I'm not sure...

  ozaki-r