Re: [dpdk-dev] [PATCH] app/testpmd: fix doubling of 'total TX dropped'

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 7/16/2019 4:00 PM, Andrew Rybchenko wrote:
> On 7/16/19 5:28 PM, Bruce Richardson wrote:
>> On Tue, Jul 16, 2019 at 03:23:03PM +0100, Ferruh Yigit wrote:
>>> On 7/16/2019 1:23 PM, Andrew Rybchenko wrote:
>>>> On 7/15/19 5:53 PM, Ferruh Yigit wrote:
>>>>> On 7/12/2019 9:32 AM, A.McLoughlin wrote:
>>>>>> The 'Accumulated forward statistics for all ports' incorrectly displayed
>>>>>> double the actual value for 'total_tx_dropped'. This was because 2
>>>>>> lines in the same function both incremented total_tx_dropped every time
>>>>>> a packet was dropped.  I removed one of these lines to fix this issue.
>>>>>>
>>>>>> Fixes: 53324971a14e ("app/testpmd: display/clear forwarding stats on demand")
>>>>>> Cc: david.marchand@redhat.com
>>>>>> Cc: stable@dpdk.org
>>>>>>
>>>>>> Signed-off-by: A.McLoughlin <aideen.mcloughlin@intel.com>
>>>>>> ---
>>>>>>    app/test-pmd/testpmd.c | 1 -
>>>>>>    1 file changed, 1 deletion(-)
>>>>>>
>>>>>> diff --git a/app/test-pmd/testpmd.c b/app/test-pmd/testpmd.c
>>>>>> index 3ed3523b7..c41bada50 100644
>>>>>> --- a/app/test-pmd/testpmd.c
>>>>>> +++ b/app/test-pmd/testpmd.c
>>>>>> @@ -1555,7 +1555,6 @@ fwd_stats_display(void)
>>>>>>    		total_recv += stats.ipackets;
>>>>>>    		total_xmit += stats.opackets;
>>>>>>    		total_rx_dropped += stats.imissed;
>>>>>> -		total_tx_dropped += ports_stats[pt_id].tx_dropped;
>>>>>>    		total_tx_dropped += stats.oerrors;
>>>>>>    		total_rx_nombuf  += stats.rx_nombuf;
>>>>>>    
>>>>>>
>>>>>
>>>>> Hi Aideen,
>>>>>
>>>>> Indeed 'rte_eth_stats->oerrors' and 'tx_dropped' are different values,
>>>>>
>>>>> in testpmd, 'TX-total' is taken as "total_xmit + total_tx_dropped", from this
>>>>> description it may be fair to say
>>>>> "total_tx_dropped = oerrors + tx_dropped"
>>>>>
>>>>> This is easier to see in HW devices, 'oerrors' is the packets sent to HW but HW
>>>>> reported failure for some reason, so these packets not transmitted to the medium.
>>>>> 'tx_dropped' is mostly calculated by application, driver returns packets that
>>>>> can't able to sent to HW, so application can re-try to send or free them and
>>>>> increase 'tx_dropped' counter.
>>>>>
>>>>>
>>>>> The problem is in the virtual drivers, the packets not able to sent are
>>>>> calculated as 'oerrors' and tx_burst functions returns the number of the
>>>>> successfully sent packets which cause application calculate remaining ones as
>>>>> 'tx_dropped' which cause the duplication.
>>>>
>>>> I don't understand how it is. Tx burst returns a number of owned packets
>>>> (either successfully transmitted or internally dropped/freed). If it is
>>>> smaller than number of packets in request, other packets are either
>>>> retried or calculated as tx_dropped.
>>>
>>> Virtual PMDs, at least the ones I checked, calculating not sent packets as
>>> error, also application calculates them as tx_dropped.
>>>
>>> Like:
>>> https://git.dpdk.org/dpdk/tree/drivers/net/ring/rte_eth_ring.c?h=v19.08-rc1#n97
>>>
>> That is probably incorrect to do. Virtual PMDs should behave as real ones
>> do as far as possible. I think we should change them to not count as errors
>> any that are not handled by the driver, provided those are returned to the
>> app.
> 
> I agree with Bruce. It is a bug in ring PMD. If so, too fast attempts to 
> transmit packets will blow up err_pkts counter.
> 

+1, as far as I can see following PMDs requires fixing:
ring
kni
pcap
tap