netdev - Re: [RFC] virtio: orphan skbs if we're relying on timer to free them

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <200905211627.05814.rusty@rustcorp.com.au>
Date:	Thu, 21 May 2009 16:27:05 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [RFC] virtio: orphan skbs if we're relying on timer to free them

On Tue, 19 May 2009 12:10:13 pm David Miller wrote:
> From: Rusty Russell <rusty@...tcorp.com.au>
> Date: Mon, 18 May 2009 22:18:47 +0930
> > We check for finished xmit skbs on every xmit, or on a timer (unless
> > the host promises to force an interrupt when the xmit ring is empty).
> > This can penalize userspace tasks which fill their sockbuf.  Not much
> > difference with TSO, but measurable with large numbers of packets.
> >
> > There are a finite number of packets which can be in the transmission
> > queue.  We could fire the timer more than every 100ms, but that would
> > just hurt performance for a corner case.  This seems neatest.
...
> > Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
>
> If this is so great for virtio it would also be a great idea
> universally, but we don't do it.
>
> What you're doing by orphan'ing is creating a situation where a single
> UDP socket can loop doing sends and monopolize the TX queue of a
> device.  The only control we have over a sender for fairness in
> datagram protocols is that send buffer allocation.

Urgh, that hadn't even occurred to me.  Good point.

> I'm guilty of doing this too in the NIU driver, also because there I
> lack a "TX queue empty" interrupt and this can keep TCP sockets from
> getting stuck.
>
> I think we need a generic solution to this issue because it is getting
> quite common to see cases where the packets in the TX queue of a
> device can sit there indefinitely.

I haven't thought this through properly, but how about a hack where we don't 
orphan packets if the ring is over half full?

Then I guess we could overload the watchdog as a more general timer-after-no-
xmit?

Rusty.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html