[PATCH v3] kni: fix possible alloc_q starvation when mbufs are exhausted

[Ferruh Yigit]

On 1/4/2023 11:57 AM, Matt wrote:
> Hi Ferruh,
> 
> In my case, the traffic is not large, so I can't see the impact.
> I also tested under high load(>2Mpps with 2 DPDK cores and 2 kernel threads)
> and found no significant difference in performance either.
> I think the reason should be that it will not
> run to 'kni_fifo_count(kni->alloc_q) == 0' under high load.
> 

I agree, additional check most likely hit on the low bandwidth,
thanks for checking for performance impact.

> On Tue, Jan 3, 2023 at 8:47 PM Ferruh Yigit <ferruh.yigit at amd.com
> <mailto:ferruh.yigit at amd.com>> wrote:
> 
>     On 12/30/2022 4:23 AM, Yangchao Zhou wrote:
>     > In some scenarios, mbufs returned by rte_kni_rx_burst are not freed
>     > immediately. So kni_allocate_mbufs may be failed, but we don't know.
>     >
>     > Even worse, when alloc_q is completely exhausted, kni_net_tx in
>     > rte_kni.ko will drop all tx packets. kni_allocate_mbufs is never
>     > called again, even if the mbufs are eventually freed.
>     >
>     > In this patch, we try to allocate mbufs for alloc_q when it is empty.
>     >
>     > According to historical experience, the performance bottleneck of KNI
>     > is offen the usleep_range of kni thread in rte_kni.ko.
>     > The check of kni_fifo_count is trivial and the cost should be
>     acceptable.
>     >
> 
>     Hi Yangchao,
> 
>     Are you observing any performance impact with this change in you use
>     case?
> 
> 
>     > Fixes: 3e12a98fe397 ("kni: optimize Rx burst")
>     > Cc: stable at dpdk.org <mailto:stable at dpdk.org>
>     >
>     > Signed-off-by: Yangchao Zhou <zhouyates at gmail.com
>     <mailto:zhouyates at gmail.com>>

Acked-by: Ferruh Yigit <ferruh.yigit at amd.com>