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

From: Michael S. Tsirkin
Date: Wed Oct 15 2014 - 06:22:29 EST

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

> 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.

> 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?

> - 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?

> Smoking test is done, and will do complete netperf test. Send the
> seires for early review.
> Thanks
> Jason Wang (6):
> virtio: make sure used event never go backwards
> virtio: introduce virtio_enable_cb_avail()
> virtio-net: small optimization on free_old_xmit_skbs()
> virtio-net: return the number of packets sent in free_old_xmit_skbs()
> virtio-net: enable tx interrupt
> virtio-net: enable tx interrupt only for the final skb in the chain
> drivers/net/virtio_net.c | 125 ++++++++++++++++++++++++++++++------------
> drivers/virtio/virtio_ring.c | 25 ++++++++-
> include/linux/virtio.h | 2 +
> 3 files changed, 115 insertions(+), 37 deletions(-)
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