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

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

On 6/22/2021 8:32 AM, wangyunjian wrote:
>> -----Original Message-----
>> From: Ferruh Yigit [mailto:ferruh.yigit@intel.com]
>> Sent: Monday, June 21, 2021 7:26 PM
>> To: wangyunjian <wangyunjian@huawei.com>; dev@dpdk.org
>> Cc: liucheng (J) <liucheng11@huawei.com>; dingxiaoxiong
>> <dingxiaoxiong@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@intel.com]
>>>> Sent: Friday, June 18, 2021 9:37 PM
>>>> To: wangyunjian <wangyunjian@huawei.com>; dev@dpdk.org
>>>> Cc: liucheng (J) <liucheng11@huawei.com>; dingxiaoxiong
>>>> <dingxiaoxiong@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@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.
> 

May not be build error, but in 'RTE_USE_C11_MEM_MODEL' case 'read'/'write' are
not volatile and need to read them via C11 atomic instructions. 'allocq_free'
calculation in the 'kni_allocate_mbufs()' doesn't do that, that is why better to
replace calculation with 'kni_fifo_free_count()'.

>>
>> Can you please add fixes line too?
> 
> OK, will include it in next version.
> 

Thanks.

> 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@huawei.com>
>>>>> Signed-off-by: Yunjian Wang <wangyunjian@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)) {
>>>>>
>>>
>