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

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

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@dpdk.org
> 
> Signed-off-by: Yangchao Zhou <zhouyates@gmail.com>
> ---
>  lib/kni/rte_kni.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/kni/rte_kni.c b/lib/kni/rte_kni.c
> index 8ab6c47153..bfa6a001ff 100644
> --- a/lib/kni/rte_kni.c
> +++ b/lib/kni/rte_kni.c
> @@ -634,8 +634,8 @@ rte_kni_rx_burst(struct rte_kni *kni, struct rte_mbuf **mbufs, unsigned int num)
>  {
>  	unsigned int ret = kni_fifo_get(kni->tx_q, (void **)mbufs, num);
>  
> -	/* If buffers removed, allocate mbufs and then put them into alloc_q */
> -	if (ret)
> +	/* If buffers removed or alloc_q is empty, allocate mbufs and then put them into alloc_q */
> +	if (ret || (kni_fifo_count(kni->alloc_q) == 0))
>  		kni_allocate_mbufs(kni);
>  
>  	return ret;