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

[wangyunjian]

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas at monjalon.net]
> Sent: Wednesday, June 23, 2021 4:46 AM
> To: wangyunjian <wangyunjian at huawei.com>; liucheng (J)
> <liucheng11 at huawei.com>
> Cc: dev at dpdk.org; stable at dpdk.org; ferruh.yigit at intel.com;
> gowrishankar.m at linux.vnet.ibm.com; dingxiaoxiong
> <dingxiaoxiong at huawei.com>; wangyunjian <wangyunjian at huawei.com>
> Subject: Re: [dpdk-stable] [dpdk-dev] [PATCH v3] kni: fix mbuf allocation for alloc
> FIFO
> 
> 22/06/2021 14:44, wangyunjian:
> > From: Yunjian Wang <wangyunjian at 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 at dpdk.org
> >
> > Signed-off-by: Cheng Liu <liucheng11 at huawei.com>
> > Signed-off-by: Yunjian Wang <wangyunjian at huawei.com>
> > Acked-by: Ferruh Yigit <ferruh.yigit at 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?

OK, how about like this?

/* Because 'read/write' maybe not volatile, so use kni_fifo_free_count()
 * to get the num of available elements in the fifo
 */

> 
> > +	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?

The title is "kni: fix mbuf allocation for FIFO queue"?