Re: [dpdk-dev] rte_eth_tx_burst improperly freeing mbufs from KNI mbuf pool

[Paras Jha <dreadiscool@gmail.com>]

Sorry, I meant that "the mbufs will not be freed as the threshold for
freeing seems to not be based on the pool the mbuf originated from, but
based on the pool the PMD is configured to use"

On Tue, Apr 30, 2019 at 11:37 AM Paras Jha <dreadiscool@gmail.com> wrote:

> Hi,
>
> I think this issue seems to be due to how the PMD frees mbufs. If the PMD
> is configured with pool X, and KNI configured with pool Y, and pool Y has
> far fewer mbufs available than pool X, when an application calls tx_burst
> on the PMD, the mbufs will not be freed as the threshold for freeing seems
> to be based on how many mbufs are in the pool the mbuf originated from.
> Setting the amount of mbufs in pool X and Y to be equal resolved the issue,
> even with arbitrarily small or large counts. This kind of seems like a
> "gotcha" where it isn't immediately clear without experimentation.
>
> On Tue, Apr 30, 2019 at 10:59 AM Ferruh Yigit <ferruh.yigit@intel.com>
> wrote:
>
>> On 4/10/2019 2:10 PM, Paras Jha wrote:
>> > Hi all,
>> >
>> > I've been chasing down a strange issue related to rte_kni_tx_burst.
>> >
>> > My application calls rte_kni_rx_burst, which allocates from a discrete
>> mbuf
>> > pool using kni_allocate_mbufs. That traffic is immediately sent to
>> > rte_eth_tx_burst which does not seem to be freeing mbufs even upon
>> > succesful completion.
>> >
>> > My application follows the standard model of freeing mbufs only if the
>> > number of tx mbufs is less than the rx mbufs - however, after sending as
>> > many mbufs as there are in the pool, I get KNI: Out of memory soon after
>> > when calling rte_kni_rx_burst
>> >
>> > My concern is that if I free all mbufs allocated by the KNI during
>> > rte_kni_rx_burst the application seems to work as intended without
>> memory
>> > leaks, even though this goes against how the actual PMDs work. Is this a
>> > bug, or intended behavior? The documented examples on the DPDK website
>> seem
>> > to only free mbufs if it fails to send, even with the KNI example.
>>
>> The behavior in the KNI sample application is correct thing to do, free
>> only the
>> mbufs failed to Tx. As far as I understand you are doing same thing as
>> the kni
>> sample app, so it should be OK, sample app works fine.
>>
>> 'rte_eth_tx_burst()' sends packets to the kernel, so it shouldn't free the
>> mbufs, right, userspace can't know when kernel side will be done with
>> them.
>> When kernel side is done, it puts the mbufs into 'free_q' fifo,
>> 'kni_free_mbufs()' pulls the mbuf from 'free_q' fifo and frees them.
>> So the mbufs sent via 'rte_eth_tx_burst()' freed asynchronously.
>> I hope this helps.
>>
>>