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From: Rusty Russell <rusty@rustcorp•com.au>
To: "Michael S. Tsirkin" <mst@redhat•com>
Cc: Krishna Kumar2 <krkumar2@in•ibm.com>,
	davem@davemloft•net, netdev@vger•kernel.org, yvugenfi@redhat•com
Subject: Re: [PATCH] virtio_net: Fix queue full check
Date: Mon, 8 Nov 2010 09:38:47 +1030	[thread overview]
Message-ID: <201011080938.47938.rusty@rustcorp.com.au> (raw)
In-Reply-To: <20101104122424.GA29830@redhat.com>

On Thu, 4 Nov 2010 10:54:24 pm Michael S. Tsirkin wrote:
> I thought about this some more.  I think the original
> code is actually correct in returning ENOSPC: indirect
> buffers are nice, but it's a mistake
> to rely on them as a memory allocation might fail.
> 
> And if you look at virtio-net, it is dropping packets
> under memory pressure which is not really a happy outcome:
> the packet will get freed, reallocated and we get another one,
> adding pressure on the allocator instead of releasing it
> until we free up some buffers.
> 
> So I now think we should calculate the capacity
> assuming non-indirect entries, and if we manage to
> use indirect, all the better.

I've long said it's a weakness in the network stack that it insists
drivers stop the tx queue before they *might* run out of room, leading to
worst-case assumptions and underutilization of the tx ring.

However, I lost that debate, and so your patch is the way it's supposed to
work.  The other main indirect user (block) doesn't care as its queue
allows for post-attempt blocking.

I enhanced your commentry a little:

Subject: virtio: return correct capacity to users
Date: Thu, 4 Nov 2010 14:24:24 +0200
From: "Michael S. Tsirkin" <mst@redhat•com>

We can't rely on indirect buffers for capacity
calculations because they need a memory allocation
which might fail.  In particular, virtio_net can get
into this situation under stress, and it drops packets
and performs badly.

So return the number of buffers we can guarantee users.

Signed-off-by: Michael S. Tsirkin <mst@redhat•com>
Signed-off-by: Rusty Russell <rusty@rustcorp•com.au>
Reported-By: Krishna Kumar2 <krkumar2@in•ibm.com>

Thanks!
Rusty.

next prev parent reply	other threads:[~2010-11-07 23:08 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-10-28  5:10 [PATCH] virtio_net: Fix queue full check Krishna Kumar
2010-10-29  9:47 ` Rusty Russell
2010-10-29 10:55   ` Krishna Kumar2
2010-10-29 11:28     ` Rusty Russell
2010-11-02 16:17       ` Michael S. Tsirkin
2010-11-04 12:24         ` Michael S. Tsirkin
2010-11-04 16:17           ` Krishna Kumar2
2010-11-04 16:45             ` Michael S. Tsirkin
2010-11-07 23:08           ` Rusty Russell [this message]
2010-11-09  4:26             ` Krishna Kumar2
2010-11-09 13:15               ` Michael S. Tsirkin
2010-11-09 15:30                 ` Krishna Kumar2
2010-11-09 15:30                   ` Michael S. Tsirkin

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