linux-kernel - Re: [PATCH 1/3] tuntap: rx batching

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Date:   Tue, 15 Nov 2016 05:41:08 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] tuntap: rx batching

On Tue, Nov 15, 2016 at 11:14:48AM +0800, Jason Wang wrote:
> 
> 
> On 2016年11月12日 00:20, Michael S. Tsirkin wrote:
> > On Fri, Nov 11, 2016 at 12:28:38PM +0800, Jason Wang wrote:
> > > 
> > > On 2016年11月11日 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 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<jasowang@...hat.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.
> > > > 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, 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


> 2) MSG_MORE support
> 3) other optimizations on top
> 
> ?
> 
> > 
> > One issue that comes to mind is that write queue limits
> > are byte based, they do not count packets unlike tun rx queue.
> 
> I'm not sure I get the issue, write queue is not exported and only used for
> batching. We probably need an internal limit in tun to avoid OOM attacker
> from guest.
> 
> Thanks