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

[Andrew Rybchenko <arybchenko@solarflare.com>]

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.