From: "Michael S. Tsirkin" <mst@redhat.com>
To: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Koichiro Den <den@klaipeden.com>,
Jason Wang <jasowang@redhat.com>,
virtualization@lists.linux-foundation.org,
Network Development <netdev@vger.kernel.org>
Subject: Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi
Date: Thu, 24 Aug 2017 01:57:06 +0300 [thread overview]
Message-ID: <20170824014553-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <CAF=yD-+U_aWxSmPPY8v8t=JO0MRZ+N+DzVJTAMKYQ5F=pE1PfA@mail.gmail.com>
On Wed, Aug 23, 2017 at 11:20:45AM -0400, Willem de Bruijn wrote:
> > Please let me make sure if I understand it correctly:
> > * always do copy with skb_orphan_frags_rx as Willem mentioned in the earlier
> > post, before the xmit_skb as opposed to my original patch, is safe but too
> > costly so cannot be adopted.
>
> One more point about msg_zerocopy in the guest. This does add new allocation
> limits on optmem and locked pages rlimit.
>
> Hitting these should be extremely rare. The tcp small queues limit normally
> throttles well before this.
>
> Virtio-net is an exception because it breaks the tsq signal by calling
> skb_orphan before transmission.
>
> As a result hitting these limits is more likely here. But, in this edge case the
> sendmsg call will not block, either, but fail with -ENOBUFS. The caller can
> send without zerocopy to make forward progress and
> trigger free_old_xmit_skbs from start_xmit.
>
> > * as a generic solution, if we were to somehow overcome the safety issue, track
> > the delay and do copy if some threshold is reached could be an answer, but it's
> > hard for now.> * so things like the current vhost-net implementation of deciding whether or not
> > to do zerocopy beforehand referring the zerocopy tx error ratio is a point of
> > practical compromise.
>
> The fragility of this mechanism is another argument for switching to tx napi
> as default.
>
> Is there any more data about the windows guest issues when completions
> are not queued within a reasonable timeframe? What is this timescale and
> do we really need to work around this.
I think it's pretty large, many milliseconds.
But I wonder what do you mean by "work around". Using buffers within
limited time frame sounds like a reasonable requirement to me. Neither
do I see why would using tx interrupts within guest be a work around -
AFAIK windows driver uses tx interrupts.
> That is the only thing keeping us from removing the HoL blocking in vhost-net zerocopy.
We don't enable network watchdog on virtio but we could and maybe
should.
--
MST
next prev parent reply other threads:[~2017-08-23 22:57 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-19 6:38 [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi Koichiro Den
2017-08-20 20:49 ` Willem de Bruijn
2017-08-21 12:40 ` Koichiro Den
2017-08-22 12:11 ` Willem de Bruijn
2017-08-22 14:04 ` Koichiro Den
2017-08-22 17:19 ` Willem de Bruijn
2017-08-23 14:26 ` Koichiro Den
2017-08-21 12:33 ` Jason Wang
2017-08-21 12:58 ` Koichiro Den
2017-08-21 15:41 ` Willem de Bruijn
2017-08-22 2:50 ` Jason Wang
2017-08-22 3:10 ` Willem de Bruijn
2017-08-22 11:47 ` Jason Wang
2017-08-22 13:42 ` Koichiro Den
2017-08-22 17:16 ` Willem de Bruijn
2017-08-23 14:24 ` Koichiro Den
2017-08-22 17:55 ` Michael S. Tsirkin
2017-08-22 18:01 ` David Miller
2017-08-22 18:28 ` Eric Dumazet
2017-08-22 18:39 ` Michael S. Tsirkin
2017-08-23 14:28 ` Koichiro Den
2017-08-23 14:47 ` Koichiro Den
2017-08-23 15:20 ` Willem de Bruijn
2017-08-23 22:57 ` Michael S. Tsirkin [this message]
2017-08-24 3:28 ` Willem de Bruijn
2017-08-24 4:34 ` Michael S. Tsirkin
2017-08-24 13:50 ` Michael S. Tsirkin
2017-08-24 20:20 ` Willem de Bruijn
2017-08-24 20:50 ` Michael S. Tsirkin
2017-08-25 22:44 ` Willem de Bruijn
2017-08-25 23:32 ` Michael S. Tsirkin
2017-08-26 1:03 ` Willem de Bruijn
2017-08-29 19:35 ` Willem de Bruijn
2017-08-29 19:42 ` Michael S. Tsirkin
2017-08-29 19:53 ` Willem de Bruijn
2017-08-29 20:40 ` Michael S. Tsirkin
2017-08-29 22:55 ` Willem de Bruijn
2017-08-30 1:45 ` Jason Wang
2017-08-30 3:11 ` Willem de Bruijn
2017-09-01 3:08 ` Jason Wang
2017-08-31 14:30 ` Willem de Bruijn
2017-09-01 3:25 ` Jason Wang
2017-09-01 16:15 ` Willem de Bruijn
2017-09-01 16:17 ` Willem de Bruijn
2017-09-04 3:03 ` Jason Wang
2017-09-05 14:09 ` Willem de Bruijn
2017-09-06 3:27 ` Michael S. Tsirkin
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170824014553-mutt-send-email-mst@kernel.org \
--to=mst@redhat.com \
--cc=den@klaipeden.com \
--cc=jasowang@redhat.com \
--cc=netdev@vger.kernel.org \
--cc=virtualization@lists.linux-foundation.org \
--cc=willemdebruijn.kernel@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).