Re: [dpdk-dev] [dpdk-stable] [PATCH v3] kni: fix mbuf allocation for alloc FIFO

[Thomas Monjalon <thomas@monjalon.net>]

22/06/2021 14:44, wangyunjian:
> From: Yunjian Wang <wangyunjian@huawei.com>
> 
> In kni_allocate_mbufs(), we alloc mbuf for alloc_q as this code.
> allocq_free = (kni->alloc_q->read - kni->alloc_q->write - 1) \
> 		& (MAX_MBUF_BURST_NUM - 1);
> The value of allocq_free maybe zero, for example :
> The ring size is 1024. After init, write = read = 0. Then we fill
> kni->alloc_q to full. At this time, write = 1023, read = 0.
> 
> Then the kernel send 32 packets to userspace. At this time, write
> = 1023, read = 32. And then the userspace receive this 32 packets.
> Then fill the kni->alloc_q, (32 - 1023 - 1) & 31 = 0, fill nothing.
> ...
> Then the kernel send 32 packets to userspace. At this time, write
> = 1023, read = 992. And then the userspace receive this 32 packets.
> Then fill the kni->alloc_q, (992 - 1023 - 1) & 31 = 0, fill nothing.
> 
> Then the kernel send 32 packets to userspace. The kni->alloc_q only
> has 31 mbufs and will drop one packet.
> 
> Absolutely, this is a special scene. Normally, it will fill some
> mbufs everytime, but may not enough for the kernel to use.
> 
> In this patch, we always keep the kni->alloc_q to full for the kernel
> to use.
> 
> Fixes: 49da4e82cf94 ("kni: allocate no more mbuf than empty slots in queue")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Cheng Liu <liucheng11@huawei.com>
> Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
> Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
> ---
> v3:
>    update patch title
> v2:
>    add fixes tag and update commit log
> ---
>  lib/kni/rte_kni.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/kni/rte_kni.c b/lib/kni/rte_kni.c
> index 9dae6a8d7c..eb24b0d0ae 100644
> --- a/lib/kni/rte_kni.c
> +++ b/lib/kni/rte_kni.c
> @@ -677,8 +677,9 @@ kni_allocate_mbufs(struct rte_kni *kni)
>  		return;
>  	}
>  
> -	allocq_free = (kni->alloc_q->read - kni->alloc_q->write - 1)
> -			& (MAX_MBUF_BURST_NUM - 1);
> +	allocq_free = kni_fifo_free_count(kni->alloc_q);

Can we insert a comment here to explain the logic?

> +	allocq_free = (allocq_free > MAX_MBUF_BURST_NUM) ?
> +		MAX_MBUF_BURST_NUM : allocq_free;
>  	for (i = 0; i < allocq_free; i++) {
>  		pkts[i] = rte_pktmbuf_alloc(kni->pktmbuf_pool);
>  		if (unlikely(pkts[i] == NULL)) {

About the title, I don't understand the part "for alloc FIFO",
given all mbufs are in a FIFO queue in KNI, right?