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

[Ilya Maximets]

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_'.

> # 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.

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.