linux-kernel - Re: [PATCH net-next] macvtap/macvlan: use IFF_NO

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Date:	Wed, 26 Aug 2015 13:45:30 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	vyasevic@...hat.com, "Michael S. Tsirkin" <mst@...hat.com>
CC:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH net-next] macvtap/macvlan: use IFF_NO_QUEUE



On 08/26/2015 12:32 AM, Vlad Yasevich wrote:
> On 08/25/2015 07:30 AM, Jason Wang wrote:
>>
>> On 08/25/2015 06:17 PM, Michael S. Tsirkin wrote:
>>> On Mon, Aug 24, 2015 at 04:33:12PM +0800, Jason Wang wrote:
>>>>> For macvlan, switch to use IFF_NO_QUEUE instead of tx_queue_len = 0.
>>>>>
>>>>> For macvtap, after commit 6acf54f1cf0a6747bac9fea26f34cfc5a9029523
>>>>> ("macvtap: Add support of packet capture on macvtap
>>>>> device."). Multiqueue macvtap suffers from single qdisc lock
>>>>> contention. This is because macvtap claims a non zero tx_queue_len and
>>>>> it reuses this value as it socket receive queue size.Thanks to
>>>>> IFF_NO_QUEUE, we can remove the lock contention without breaking
>>>>> existing socket receive queue length logic.
>>>>>
>>>>> Cc: Patrick McHardy <kaber@...sh.net>
>>>>> Cc: Vladislav Yasevich <vyasevic@...hat.com>
>>>>> Cc: Michael S. Tsirkin <mst@...hat.com>
>>>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>>> Seems to make sense. Give me a day or two to get over the jet lag
>>> (and get out from under the pile of mail accumulated while I was traveling),
>>> I'll review properly and ack.
>>>
>> A note on this patch: only default qdisc were removed but we don't lose
>> the ability to attach a qdisc to macvtap (though it may cause lock
>> contention on multiqueue case).
>>
> Wouldn't that lock contention be solved if we really had multiple queues
> for multi-queue macvtaps?
>
> -vlad

Yes, but this introduce another layer of txq locks contention? And it
also needs macvlan multiqueue support. We used to do something like this
but switch to NETIF_F_LLTX finally. You may refer:

2c11455321f37da6fe6cc36353149f9ac9183334 macvlan: add multiqueue capability
8ffab51b3dfc54876f145f15b351c41f3f703195 macvlan: lockless tx path

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