Re: [PATCH 1/3] tuntap: rx batching

[Jason Wang]

On 2016å11æ15æ 11:41, Michael S. Tsirkin wrote:
> On Tue, Nov 15, 2016 at 11:14:48AM +0800, Jason Wang wrote:
> > >
> > >
> > >On 2016å11æ12æ 00:20, Michael S. Tsirkin wrote:
> > > > >On Fri, Nov 11, 2016 at 12:28:38PM +0800, Jason Wang wrote:
> > > > > > >
> > > > > > >On 2016å11æ11æ 12:17, John Fastabend wrote:
> > > > > > > > >On 16-11-10 07:31 PM, Michael S. Tsirkin wrote:
> > > > > > > > > > > > >On Fri, Nov 11, 2016 at 10:07:44AM +0800, Jason Wang wrote:
> > > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > >On 2016å11æ10æ 00:38, Michael S. Tsirkin wrote:
> > > > > > > > > > > > > > > > > > > > >On Wed, Nov 09, 2016 at 03:38:31PM +0800, Jason Wang wrote:
> > > > > > > > > > > > > > > > > > > > > > > > >Backlog were used for tuntap rx, but it can only process 1 packet at
> > > > > > > > > > > > > > > > > > > > > > > > >one time since it was scheduled during sendmsg() synchronously in
> > > > > > > > > > > > > > > > > > > > > > > > >process context. This lead bad cache utilization so this patch tries
> > > > > > > > > > > > > > > > > > > > > > > > >to do some batching before call rx NAPI. This is done through:
> > > > > > > > > > > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > > > > > > > > > >- accept MSG_MORE as a hint from sendmsg() caller, if it was set,
> > > > > > > > > > > > > > > > > > > > > > > > >     batch the packet temporarily in a linked list and submit them all
> > > > > > > > > > > > > > > > > > > > > > > > >     once MSG_MORE were cleared.
> > > > > > > > > > > > > > > > > > > > > > > > >- implement a tuntap specific NAPI handler for processing this kind of
> > > > > > > > > > > > > > > > > > > > > > > > >     possible batching. (This could be done by extending backlog to
> > > > > > > > > > > > > > > > > > > > > > > > >     support skb like, but using a tun specific one looks cleaner and
> > > > > > > > > > > > > > > > > > > > > > > > >     easier for future extension).
> > > > > > > > > > > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > > > > > > > > > >Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
> > > > > > > > > > > > > > > > > > > > >So why do we need an extra queue?
> > > > > > > > > > > > > > > > >The idea was borrowed from backlog to allow some kind of bulking and avoid
> > > > > > > > > > > > > > > > >spinlock on each dequeuing.
> > > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > > > > > >    This is not what hardware devices do.
> > > > > > > > > > > > > > > > > > > > >How about adding the packet to queue unconditionally, deferring
> > > > > > > > > > > > > > > > > > > > >signalling until we get sendmsg without MSG_MORE?
> > > > > > > > > > > > > > > > >Then you need touch spinlock when dequeuing each packet.
> > > > > > > > >Random thought, I have a cmpxchg ring I am using for the qdisc work that
> > > > > > > > >could possibly replace the spinlock implementation. I haven't figured
> > > > > > > > >out the resizing API yet because I did not need it but I assume it could
> > > > > > > > >help here and let you dequeue multiple skbs in one operation.
> > > > > > > > >
> > > > > > > > >I can post the latest version if useful or an older version is
> > > > > > > > >somewhere on patchworks as well.
> > > > > > > > >
> > > > > > > > >.John
> > > > > > > > >
> > > > > > > > >
> > > > > > >Look useful here, and I can compare the performance if you post.
> > > > > > >
> > > > > > >A question is can we extend the skb_array to support that?
> > > > > > >
> > > > > > >Thanks
> > > > >I'd like to start with simple patch adding napi with one queue, then add
> > > > >optimization patches on top.
> > >
> > >The point is tun is using backlog who uses two queues (process_queue and
> > >input_pkt_queue).
> > >
> > >How about something like:
> > >
> > >1) NAPI support with skb_array
> I would start with just write queue linked list. It all runs on a single
> CPU normally,

True for virt but I'm not sure the others. If we have multiple senders 
at the same time, current code scales very well.

>   so the nice reductions of cache line bounces due to skb
> array should never materialize.
>
> While we are at it, limiting the size of the queue might
> be a good idea. Kind of like TUNSETSNDBUF but 1. actually
> working where instead of tracking packets within net stack
> we make sndbuf track the internal buffer

Get your point, will start from simple skb list.

Thanks