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

[Jason Wang]

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