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    SubjectRe: [PATCH net-next] tun/macvtap: limit the packets queued through rcvbuf
    On 01/15/2014 03:21 PM, Michael S. Tsirkin wrote:
    > On Wed, Jan 15, 2014 at 11:36:01AM +0800, Jason Wang wrote:
    >> On 01/14/2014 05:52 PM, Michael S. Tsirkin wrote:
    >>> On Tue, Jan 14, 2014 at 04:45:24PM +0800, Jason Wang wrote:
    >>>>> On 01/14/2014 04:25 PM, Michael S. Tsirkin wrote:
    >>>>>>> On Tue, Jan 14, 2014 at 02:53:07PM +0800, Jason Wang wrote:
    >>>>>>>>> We used to limit the number of packets queued through tx_queue_length. This
    >>>>>>>>> has several issues:
    >>>>>>>>>
    >>>>>>>>> - tx_queue_length is the control of qdisc queue length, simply reusing it
    >>>>>>>>> to control the packets queued by device may cause confusion.
    >>>>>>>>> - After commit 6acf54f1cf0a6747bac9fea26f34cfc5a9029523 ("macvtap: Add
    >>>>>>>>> support of packet capture on macvtap device."), an unexpected qdisc
    >>>>>>>>> caused by non-zero tx_queue_length will lead qdisc lock contention for
    >>>>>>>>> multiqueue deivce.
    >>>>>>>>> - What we really want is to limit the total amount of memory occupied not
    >>>>>>>>> the number of packets.
    >>>>>>>>>
    >>>>>>>>> So this patch tries to solve the above issues by using socket rcvbuf to
    >>>>>>>>> limit the packets could be queued for tun/macvtap. This was done by using
    >>>>>>>>> sock_queue_rcv_skb() instead of a direct call to skb_queue_tail(). Also two
    >>>>>>>>> new ioctl() were introduced for userspace to change the rcvbuf like what we
    >>>>>>>>> have done for sndbuf.
    >>>>>>>>>
    >>>>>>>>> With this fix, we can safely change the tx_queue_len of macvtap to
    >>>>>>>>> zero. This will make multiqueue works without extra lock contention.
    >>>>>>>>>
    >>>>>>>>> Cc: Vlad Yasevich<vyasevic@redhat.com>
    >>>>>>>>> Cc: Michael S. Tsirkin<mst@redhat.com>
    >>>>>>>>> Cc: John Fastabend<john.r.fastabend@intel.com>
    >>>>>>>>> Cc: Stephen Hemminger<stephen@networkplumber.org>
    >>>>>>>>> Cc: Herbert Xu<herbert@gondor.apana.org.au>
    >>>>>>>>> Signed-off-by: Jason Wang<jasowang@redhat.com>
    >>>>>>> No, I don't think we can change userspace-visible behaviour like that.
    >>>>>>>
    >>>>>>> This will break any existing user that tries to control
    >>>>>>> queue length through sysfs,netlink or device ioctl.
    >>>>> But it looks like a buggy API, since tx_queue_len should be for qdisc
    >>>>> queue length instead of device itself.
    >>> Probably, but it's been like this since 2.6.x time.
    >>> Also, qdisc queue is unused for tun so it seemed kind of
    >>> reasonable to override tx_queue_len.
    >>>
    >>>>> If we really want to preserve the
    >>>>> behaviour, how about using a new feature flag and change the behaviour
    >>>>> only when the device is created (TUNSETIFF) with the new flag?
    >>> OK this addresses the issue partially, but there's also an issue
    >>> of permissions: tx_queue_len can only be changed if
    >>> capable(CAP_NET_ADMIN). OTOH in your patch a regular user
    >>> can change the amount of memory consumed per queue
    >>> by calling TUNSETRCVBUF.
    >> Yes, but we have the same issue for TUNSETSNDBUF.
    > To an extent, but TUNSETSNDBUF is different. It limits how much device can queue
    > *in the networking stack* but each queue in the stack is also
    > limited, when we exceed that we star dropping packets.
    > So while with infinite value (which is the default btw)
    > you can keep host pretty busy, you will not be able to run
    > it out of memory.
    >
    > The proposed TUNSETRCVBUF would keep configured amount
    > of memory around indefinitely so you can run host out of memory.
    >
    > So assuming all this
    > How about an ethtool or netlink command to configure this
    > instead?
    >

    Ok, so we can add net admin check for before trying to set rcvbuf. I
    think it's better to use ioctl since we've already use it for sndbuf.
    Using ethool means you need a dedicated new ethtool method just for
    tuntap which seems sub-optimal. Netlink looks better, but we should also
    implement other ioctl also.
    >>>>>>> Take a look at my patch in msg ID 20140109071721.GD19559@redhat.com
    >>>>>>> which gives one way to set tx_queue_len to zero without
    >>>>>>> breaking userspace.
    >>>>> If I read the patch correctly, it will make no way for the user who
    >>>>> really want to change the qdisc queue length for tun.
    >>> Why would this matter? As far as I can see qdisc queue is currently unused.
    >>>
    >> User may use qdisc to do port mirroring, bandwidth limitation, traffic
    >> prioritization or more for a VM. So we do have users and maybe more
    >> consider the case of vpn.
    > Well it's not used by default at least.
    > I remember that we discussed this previously actually.
    >
    > If all we want to do actually is utilize no_qdisc by default,
    > we can simply use Eric's patch:
    >
    > http://article.gmane.org/gmane.linux.kernel/1279597
    >
    > and a similar patch for macvtap.
    > I tried it at the time and it didn't seem to help performance
    > at all, but a lot has changed since, in particular I didn't
    > test mq.
    >
    > If you now have results showing how it's beneficial, pls post them.
    >

    I will have a test to see the difference.


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