On 6/22/2021 8:32 AM, wangyunjian wrote:

quoted

-----Original Message-----
From: Ferruh Yigit [mailto:ferruh.yigit@intel.com]
Sent: Monday, June 21, 2021 7:26 PM
To: wangyunjian <redacted>; dev@dpdk.org
Cc: liucheng (J) <redacted>; dingxiaoxiong
[off-list ref]
Subject: Re: [dpdk-dev] [PATCH] kni: fix wrong mbuf alloc count in
kni_allocate_mbufs

On 6/21/2021 4:27 AM, wangyunjian wrote:

quoted

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-----Original Message-----
From: Ferruh Yigit [mailto:ferruh.yigit@intel.com]
Sent: Friday, June 18, 2021 9:37 PM
To: wangyunjian <redacted>; dev@dpdk.org
Cc: liucheng (J) <redacted>; dingxiaoxiong
[off-list ref]
Subject: Re: [dpdk-dev] [PATCH] kni: fix wrong mbuf alloc count in
kni_allocate_mbufs

On 5/31/2021 1:09 PM, wangyunjian wrote:

quoted

From: Yunjian Wang <redacted>

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.

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Then the kernel send 32 packets to userspace. At this time, write = 1023,

read = 32.

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And then the userspace recieve this 32 packets. Then fill the kni->alloc_q, (32

- 1023 - 1)&31 = 0, fill nothing.

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...
Then the kernel send 32 packets to userspace. At this time, write = 1023,

read = 992.

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And then the userspace recieve this 32 packets. Then fill the kni->alloc_q,

(992 - 1023 - 1)&31 = 0, fill nothing.

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Then the kernel send 32 packets to userspace. The kni->alloc_q only has 31

mbufs and will drop one packet.

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Absolutely, this is a special scene. Normally, it will fill some mbufs everytime,

but may not enough for the kernel to use.

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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()'.

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Can you please add fixes line too?

OK, will include it in next version.

Thanks.

Thanks

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Thanks

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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 <redacted>
Signed-off-by: Yunjian Wang <redacted>
---
 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)) {