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

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Message-ID: <543E5706.10305@redhat.com>
Date:	Wed, 15 Oct 2014 19:14:14 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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 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.
>
>> - 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.
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