[dpdk-dev] [PATCH] kni: fix wrong mbuf alloc count in kni_allocate_mbufs

[wangyunjian]

> -----Original Message-----
> From: Ferruh Yigit [mailto:ferruh.yigit at intel.com]
> Sent: Monday, June 21, 2021 7:26 PM
> To: wangyunjian <wangyunjian at huawei.com>; dev at dpdk.org
> Cc: liucheng (J) <liucheng11 at huawei.com>; dingxiaoxiong
> <dingxiaoxiong at huawei.com>
> Subject: Re: [dpdk-dev] [PATCH] kni: fix wrong mbuf alloc count in
> kni_allocate_mbufs
> 
> On 6/21/2021 4:27 AM, wangyunjian wrote:
> >> -----Original Message-----
> >> From: Ferruh Yigit [mailto:ferruh.yigit at intel.com]
> >> Sent: Friday, June 18, 2021 9:37 PM
> >> To: wangyunjian <wangyunjian at huawei.com>; dev at dpdk.org
> >> Cc: liucheng (J) <liucheng11 at huawei.com>; dingxiaoxiong
> >> <dingxiaoxiong at huawei.com>
> >> Subject: Re: [dpdk-dev] [PATCH] kni: fix wrong mbuf alloc count in
> >> kni_allocate_mbufs
> >>
> >> On 5/31/2021 1:09 PM, wangyunjian wrote:
> >>> 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 (e.g 32 & (32 - 1) = 0), and it
> >>> will not fill the alloc_q. When the alloc_q's free count is zero, it
> >>> will drop the packet in kernel kni.
> >>>
> >>
> >> nack
> >>
> >> Both 'read' & 'write' pointers can be max 'len-1', so 'read - write -
> >> 1' can't be 'len'.
> >> For above example first part can't be '32'.
> >>
> >> But if you are observing a problem, can you please describe it a
> >> little more, it may be because of something else.
> >
> > 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 recieve 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 recieve 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.
> >
> 
> I see now, yes it is technically possible to have above scenario and it can cause
> glitch in the datapath.
> 
> Below fix looks good, +1 to use 'kni_fifo_free_count()' instead of calculation
> within the function which may be wrong for the 'RTE_USE_C11_MEM_MODEL'
> case.

I compiled them on the ARM and x86 platforms with the 'RTE_USE_C11_MEM_MODEL'
case, and no error is reported.

> 
> Can you please add fixes line too?

OK, will include it in next version.

Thanks

> 
> > Thanks
> >
> >>
> >>> In this patch, we set the allocq_free as the min between
> >>> MAX_MBUF_BURST_NUM and the free count of the alloc_q.
> >>>
> >>> Signed-off-by: Cheng Liu <liucheng11 at huawei.com>
> >>> Signed-off-by: Yunjian Wang <wangyunjian at huawei.com>
> >>> ---
> >>>  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..20d8f20cef 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);
> >>> +	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)) {
> >>>
> >