netdev - Re: [RFC PATCH net-next 0/6] Always use tx interrupt for virtio-net

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Date:	Wed, 15 Oct 2014 14:58:32 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	davem@...emloft.net, eric.dumazet@...il.com
Subject: Re: [RFC PATCH net-next 0/6] Always use tx interrupt for virtio-net

On Wed, Oct 15, 2014 at 07:14:14PM +0800, Jason Wang wrote:
> On 10/15/2014 06:25 PM, Michael S. Tsirkin wrote:
> > On Wed, Oct 15, 2014 at 03:25:24PM +0800, Jason Wang wrote:
> >> According to David, proper accounting and queueing (at all levels, not
> >> just TCP sockets) is more important than trying to skim a bunch of
> >> cycles by avoiding TX interrupts.
> > He also mentioned we should find other ways to batch
> >
> 
> Right.
> >> Having an event to free the SKB is
> >> absolutely essential for the stack to operate correctly.
> >>
> >> This series tries to enable tx interrupt for virtio-net. The idea is
> >> simple: enable tx interrupt and schedule a tx napi to free old xmit
> >> skbs.
> >>
> >> Several notes:
> >> - Tx interrupt storm avoidance when queue is about to be full is
> >>   kept.
> > But queue is typically *not* full. More important to avoid interrupt
> > storms in that case IMO.
> 
> Yes.
> >> Since we may enable callbacks in both ndo_start_xmit() and tx
> >>   napi, patch 1 adds a check to make sure used event never go
> >>   back. This will let the napi not enable the callbacks wrongly after
> >>   delayed callbacks was used.
> > So why not just use delayed callbacks?
> 
> This means the tx interrupt are coalesced in a somewhat adaptive way.
> Need benchmark to see its effect.

I think it's a minimal change, and does not need new APIs.
If that's not optimal, we can do smarter things on top.

> >
> >> - For bulk dequeuing, there's no need to enable tx interrupt for each
> >>   packet. The last patch only enable tx interrupt for the final skb in
> >>   the chain through xmit_more and a new helper to publish current avail
> >>   idx as used event.
> >>
> >> This series fixes several issues of original rfc pointed out by Michael.
> > Could you list the issues, for ease of review?
> 
> Probably just one:
> 
> - Move the virtqueue_disable_cb() from skb_xmit_done() into
> virtnet_poll_tx() under tx lock.

I think I did this already, I'll recheck.

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