On Fri, Nov 11, 2016 at 10:07:44AM +0800, Jason Wang wrote:
>
>
> On 2016年11月10日 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 <jasow...@redhat.com>
> > 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.
It runs on the same CPU, right? Otherwise we should use skb_array...
> >
> >
> > > ---
> > > 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
