From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: Re: [PATCH RFC 2/3] virtio_net: fix tx capacity checks
	using new API
Date: Thu, 2 Jun 2011 16:28:52 +0300
Message-ID: <20110602132852.GH7141@redhat.com>
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Cc: Krishna Kumar <krkumar2-xthvdsQ13ZrQT0dZR+AlfA@public.gmane.org>, Carsten Otte <cotte-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>,
	lguest-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org, Shirley Ma <xma-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>,
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	Heiko Carstens <heiko.carstens-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
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	Christian Borntraeger <borntraeger-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>,
	Tom Lendacky <tahm-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>,
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To: Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>
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On Thu, Jun 02, 2011 at 11:40:26AM +0930, Rusty Russell wrote:
> On Wed, 1 Jun 2011 12:49:54 +0300, "Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> > In the (rare) case where new descriptors are used
> > while virtio_net enables vq callback for the TX vq,
> > virtio_net uses the number of sg entries in the skb it frees to
> > calculate how many descriptors in the ring have just been made
> > available.  But this value is an overestimate: with indirect buffers
> > each skb only uses one descriptor entry, meaning we may wake the queue
> > only to find we still can't transmit anything.
> 
> This is a bit misleading.
> 
> The value is an overestimate, but so is the requirement for
> 2+MAX_SKB_FRAGS, *unless* we suddenly drop into direct mode due to OOM.
> 
> Thanks,
> Rusty.

I agree, it's unlikely.

s/still can't transmit anything/are still out of space and need to stop
the ring almost at once/

Better?