[ovs-dev] [PATCH v9 2/2] netdev-dpdk:Detailed packet drop statistics

Ilya Maximets i.maximets at ovn.org
Fri Oct 18 12:59:08 UTC 2019

On 16.10.2019 19:48, Kevin Traynor wrote:
> On 16/10/2019 13:16, Ilya Maximets wrote:
>> Hi Kevin,
>> Thanks for review. Some comments inline.
>> On 16.10.2019 12:15, Kevin Traynor wrote:
>>> Hi Sriram,
>>> Thanks for working on making more fine grained stats for debugging. As
>>> mentioned yesterday, this patch needs rebase so I just reviewed docs and
>>> netdev-dpdk.c which applied. Comments below.
>>> On 21/09/2019 03:40, Sriram Vatala wrote:
>>>> OVS may be unable to transmit packets for multiple reasons and
>>>> today there is a single counter to track packets dropped due to
>>>> any of those reasons. The most common reason is that a VM is
>>>> unable to read packets fast enough causing the vhostuser port
>>>> transmit queue on the OVS side to become full. This manifests
>>>> as a problem with VNFs not receiving all packets. Having a
>>>> separate drop counter to track packets dropped because the
>>>> transmit queue is full will clearly indicate that the problem
>>>> is on the VM side and not in OVS. Similarly maintaining separate
>>> It reads like a bit of a contradiction. On one hand the "The *most
>>> common* reason is that a VM is unable to read packets fast enough".
>>> While having a stat "*will clearly indicate* that the problem is on the
>>> VM side".
>>>> counters for all possible drops helps in indicating sensible
>>>> cause for packet drops.
>>>> This patch adds custom software stats counters to track packets
>>>> dropped at port level and these counters are displayed along with
>>>> other stats in "ovs-vsctl get interface <iface> statistics"
>>>> command. The detailed stats will be available for both dpdk and
>>>> vhostuser ports.
>>> I think the commit msg could be a bit clearer, suggest something like
>>> below - feel free to modify (or reject):
>>> OVS may be unable to transmit packets for multiple reasons on the DPDK
>>> datapath and today there is a single counter to track packets dropped
>>> due to any of those reasons.
>>> This patch adds custom software stats for the different reasons packets
>>> may be dropped during tx on the DPDK datapath in OVS.
>>> - MTU drops : drops that occur due to a too large packet size
>>> - Qos drops: drops that occur due to egress QOS
>>> - Tx failures: drops as returned by the DPDK PMD send function
>>> Note that the reason for tx failures is not specificied in OVS. In
>>> practice for vhost ports it is most common that tx failures are because
>>> there are not enough available descriptors, which is usually caused by
>>> misconfiguration of the guest queues and/or because the guest is not
>>> consuming packets fast enough from the queues.
>>> These counters are displayed along with other stats in "ovs-vsctl get
>>> interface <iface> statistics" command and are available for dpdk and
>>> vhostuser/vhostuserclient ports.
>>> Signed-off-by: Sriram Vatala <sriram.v at altencalsoftlabs.com>
>>> ---
>>> v9:...
>>> v8:...
>>> Note that signed-off, --- and version info should be like this ^^^.
>>> otherwise the review version comments will be in the git commit log when
>>> it is applied.
>>>> --
>>>> Changes since v8:
>>>> Addressed comments given by Ilya.
>>>> Signed-off-by: Sriram Vatala <sriram.v at altencalsoftlabs.com>
>>>> ---
>>>>    Documentation/topics/dpdk/vhost-user.rst      | 13 ++-
>>>>    lib/netdev-dpdk.c                             | 81 +++++++++++++++----
>>>>    utilities/bugtool/automake.mk                 |  3 +-
>>>>    utilities/bugtool/ovs-bugtool-get-port-stats  | 25 ++++++
>>>>    .../plugins/network-status/openvswitch.xml    |  1 +
>>>>    5 files changed, 105 insertions(+), 18 deletions(-)
>>>>    create mode 100755 utilities/bugtool/ovs-bugtool-get-port-stats
>>>> diff --git a/Documentation/topics/dpdk/vhost-user.rst b/Documentation/topics/dpdk/vhost-user.rst
>>>> index 724aa62f6..89388a2bf 100644
>>>> --- a/Documentation/topics/dpdk/vhost-user.rst
>>>> +++ b/Documentation/topics/dpdk/vhost-user.rst
>>>> @@ -551,7 +551,18 @@ processing packets at the required rate.
>>>>    The amount of Tx retries on a vhost-user or vhost-user-client interface can be
>>>>    shown with::
>>>> -  $ ovs-vsctl get Interface dpdkvhostclient0 statistics:tx_retries
>>>> +  $ ovs-vsctl get Interface dpdkvhostclient0 statistics:netdev_dpdk_tx_retries
>>> I think the "netdev_dpdk_" prefixes should be removed for a few reasons,
>>> - These are custom stats that will only be displayed for the dpdk ports
>>> so they don't need any extra prefix to say they are for dpdk ports
>>> - The 'tx_retries' stat is part of OVS 2.12, I don't like to change its
>>> name to a different one in OVS 2.13 unless there is a very good reason
>>> - The existing stats don't have this type of prefixes (granted most of
>>> them are general stats):
>> The main reason for the prefix for me is to distinguish those stats
>> from the stats that comest from the driver/HW.  Our new stats has
>> very generic names that could be named a lot like or even equal to
>> HW stats.  This might be not an issue for vhostuser, but for HW
>> NICs this may be really confusing for users.
>> I'm not insisting on 'netdev_dpdk_' prefix, but, IMHO, we need a
>> way to clearly distinguish those stats.  We may use different prefixes
>> like 'sw_' or just 'ovs_'.
> ok, I can see that we might want to distinguish custom stats to indicate
> that they are specific for that device but that also has the side effect
> of making them less self-descriptive and the user will have to be told
> somewhere what the prefix means.
> 'sw_' and 'ovs_' seem better than 'netdev_dpdk_' but it might be
> confusing too, as for example in DPDK case, Tx drops are calculated in
> sw/ovs the same as qos/mtu/txfails but won't have that prefix. If we
> really need a prefix 'cstat_' is the best I can think of.

What we're trying to solve here is to distinguish stats that counted
internally in OVS from the stats provided by the third parties.
Just to not have same/similar names

Usual stats from 'netdev_stats' are combined from various sources.
For example, 'rx_dropped' in netdev-dpdk is a combination of our
QoS dorps from OVS and HW counters like rte_stats.rx_nombuf.
For me it looks like for these stats we have kind of best-effort
policy to provide as full info as possible.  Trying to describe
how each of these counters calculated doesn't make sense because
we do not know the origin of many of the components that comes
from DPDK or HW.

>>> # ovs-vsctl get Interface dpdkvhost0 statistics
>>> {"rx_1024_to_1522_packets"=0, "rx_128_to_255_packets"=0,
>>> "rx_1523_to_max_packets"=0, "rx_1_to_64_packets"=25622176,
>>> "rx_256_to_511_packets"=0, "rx_512_to_1023_packets"=0,
>>> "rx_65_to_127_packets"=0, rx_bytes=1537330560, rx_dropped=0,
>>> rx_errors=0, rx_packets=25622176, tx_bytes=3829825920, tx_dropped=0,
>>> tx_packets=63830432, tx_retries=0}
>>> Also, just to note that if there are changes to existing
>>> interfaces/behaviour it should really mention that in the commit message
>>> so it is highlighted.
>>>> +
>>>> +When the guest is not able to consume the packets fast enough, the transmit
>>>> +queue of the port gets filled up i.e queue runs out of free descriptors.
>>>> +This is the most likely reason why dpdk transmit API will fail to send packets
>>>> +besides other reasons.
>>>> +
>>>> +The amount of tx failure drops on a dpdk vhost/physical interface can be
>>>> +shown with::
>>>> +
>>>> +  $ ovs-vsctl get Interface dpdkvhostclient0 \
>>>> +                            statistics:netdev_dpdk_tx_failure_drops
>>> The commit msg/docs are only focusing on one stat for vhost ports, but
>>> there are other stats and dpdk ports, so they should all get some mention.
>> I don't feel comfortable for documenting custom stats.  This is just because
>> we can't describe them all.  These are things to be changed over time.
>> And we don't really know what some types of stats means and if they means the
>> same for different types of HW NICs (they are definitely not) or OVS netdevs
>> (even within netdev-dpdk).
>> And that is one more reason to have a prefix for OVS internal statistics on
>> which we have at least partial control.
>> I think, that user documentation could mention some special statistics while
>> describing the troubleshooting for some hard special cases, but this should
>> not be a glossary of all the possible custom stats.  From my point of view,
>> names of the stats should be as possible self-descriptive and should not
>> require additional documentation.
> For sure any prefix would need to be explained because that part would
> not be intuitive.

We could add a note to the common DPDK guide about that in a single sentence:
"There are custom statistics that OVS accumulates itself and these stats
has 'your_prefix_here_' as a prefix."

'cstat_' or 'sw_' might need this, but 'netdev_dpdk_' or 'ovs_' looks
self-descriptive enough to not do that.

> thanks,
> Kevin.
>> BTW, you will not find any description for statistics provided by the linux
>> or DPDK drivers.  You could only look at the driver code or device spec.
>> Best regards, Ilya Maximets.

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