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

[Jason Wang]

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.

> > ---
> >   drivers/net/tun.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++-----
> >   1 file changed, 65 insertions(+), 6 deletions(-)

[...]

> >   	rxhash = skb_get_hash(skb);
> > -	netif_rx_ni(skb);
> > +	skb_queue_tail(&tfile->socket.sk->sk_write_queue, skb);
> > +
> > +	if (!more) {
> > +		local_bh_disable();
> > +		napi_schedule(&tfile->napi);
> > +		local_bh_enable();
> Why do we need to disable bh here? I thought napi_schedule can
> be called from any context.

Yes, it's unnecessary. Will remove.

Thanks