[dpdk-dev] [PATCH] ring: guarantee ordering of cons/prod loading when doing enqueue/dequeue

[Jia He]

On 10/20/2017 4:02 AM, Ananyev, Konstantin Wrote:
>
>> -----Original Message-----
>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Ananyev, Konstantin
>> Sent: Thursday, October 19, 2017 3:15 PM
>> To: Zhao, Bing <ilovethull at 163.com>; Jia He <hejianet at gmail.com>; Jerin Jacob <jerin.jacob at caviumnetworks.com>
>> Cc: Olivier MATZ <olivier.matz at 6wind.com>; dev at dpdk.org; jia.he at hxt-semitech.com; jie2.liu at hxt-semitech.com; bing.zhao at hxt-
>> semitech.com
>> Subject: Re: [dpdk-dev] [PATCH] ring: guarantee ordering of cons/prod loading when doing enqueue/dequeue
>>
>>> Hi,
>>>
>>> On 2017/10/19 18:02, Ananyev, Konstantin wrote:
>>>> Hi Jia,
>>>>
>>>>> Hi
>>>>>
>>>>>
>>>>> On 10/13/2017 9:02 AM, Jia He Wrote:
>>>>>> Hi Jerin
>>>>>>
>>>>>>
>>>>>> On 10/13/2017 1:23 AM, Jerin Jacob Wrote:
>>>>>>> -----Original Message-----
>>>>>>>> Date: Thu, 12 Oct 2017 17:05:50 +0000
>>>>>>>>
>>>>> [...]
>>>>>>> On the same lines,
>>>>>>>
>>>>>>> Jia He, jie2.liu, bing.zhao,
>>>>>>>
>>>>>>> Is this patch based on code review or do you saw this issue on any of
>>>>>>> the
>>>>>>> arm/ppc target? arm64 will have performance impact with this change.
>>>>> sorry, miss one important information
>>>>> Our platform is an aarch64 server with 46 cpus.
>>>>> If we reduced the involved cpu numbers, the bug occurred less frequently.
>>>>>
>>>>> Yes, mb barrier impact the performance, but correctness is more
>>>>> important, isn't it ;-)
>>>>> Maybe we can  find any other lightweight barrier here?
>>>>>
>>>>> Cheers,
>>>>> Jia
>>>>>> Based on mbuf_autotest, the rte_panic will be invoked in seconds.
>>>>>>
>>>>>> PANIC in test_refcnt_iter():
>>>>>> (lcore=0, iter=0): after 10s only 61 of 64 mbufs left free
>>>>>> 1: [./test(rte_dump_stack+0x38) [0x58d868]]
>>>>>> Aborted (core dumped)
>>>>>>
>>>> So is it only reproducible with mbuf refcnt test?
>>>> Could it be reproduced with some 'pure' ring test
>>>> (no mempools/mbufs refcnt, etc.)?
>>>> The reason I am asking - in that test we also have mbuf refcnt updates
>>>> (that's what for that test was created) and we are doing some optimizations here too
>>>> to avoid excessive atomic updates.
>>>> BTW, if the problem is not reproducible without mbuf refcnt,
>>>> can I suggest to extend the test  with:
>>>>     - add a check that enqueue() operation was successful
>>>>     - walk through the pool and check/printf refcnt of each mbuf.
>>>> Hopefully that would give us some extra information what is going wrong here.
>>>> Konstantin
>>>>
>>>>
>>> Currently, the issue is only found in this case here on the ARM
>>> platform, not sure how it is going with the X86_64 platform
>> I understand that it is only reproducible on arm so far.
>> What I am asking - with dpdk is there any other way to reproduce it (on arm)
>> except then running mbuf_autotest?
>> Something really simple that not using mbuf/mempool etc?
>> Just do dequeue/enqueue from multiple threads and check data integrity at the end?
>> If not  - what makes you think that the problem is precisely in rte_ring code?
>> Why not in rte_mbuf let say?
> Actually I reread your original mail and finally get your point.
> If I understand you correctly the problem with read reordering here is that
> after we read prot.tail but before we read cons.head
> both cons.head and prod.tail might be updated,
> but for us prod.tail change might be unnoticed.
> As an example:
> time 0 (cons.head == 0, prod.tail == 0):
> prod_tail = r->prod.tail; /* due read reordering */
> /* prod_tail == 0 */
>
>   time 1 (cons.head ==5, prod.tail == 5):
> *old_head = r->cons.head;
> /* cons.head == 5 */
> *entries = (prod_tail - *old_head);
> /* *entries == (0 - 5) == 0xFFFFFFFB */
>
> And we will move our cons.head forward, even though there are no filled entries in the ring.
> Is that right?
Yes
> As I side notice, I wonder why we have here:
> *entries = (prod_tail - *old_head);
> instead of:
> *entries = r->size + prod_tail - *old_head;
> ?
Yes, I agree with you at this code line.
But reordering will still mess up things even after this change(I have 
tested, still the same as before)
I thought the *entries is a door to prevent consumer from moving forward 
too fast than the producer.
But in some cases, it is correct that prod_tail is smaller than 
*old_head due to  the cirular queue.
In other cases, it is incorrect because of read/read reordering.

AFAICT, the root cause here is the producer tail and cosumer head are 
dependant on each other.
Thus a memory barrier is neccessary.

Cheers,
Jia

>
> Konstantin
>
>>> . In another
>>> mail of this thread, we've made a simple test based on this and captured
>>> some information and I pasted there.(I pasted the patch there :-))
>> Are you talking about that one:
>> http://dpdk.org/dev/patchwork/patch/30405/
>> ?
>> It still uses test/test/test_mbuf.c...,
>> but anyway I don't really understand how mbuf_autotest supposed
>> to work with these changes:
>> @@ -730,7 +739,7 @@ test_refcnt_iter(unsigned int lcore, unsigned int iter,
>> rte_ring_enqueue(refcnt_mbuf_ring, m);
>>                           }
>>                   }
>> -               rte_pktmbuf_free(m);
>> +               // rte_pktmbuf_free(m);
>>           }
>> @@ -741,6 +750,12 @@ test_refcnt_iter(unsigned int lcore, unsigned int iter,
>>           while (!rte_ring_empty(refcnt_mbuf_ring))
>>                   ;
>>
>> +       if (NULL != m) {
>> +               if (1 != rte_mbuf_refcnt_read(m))
>> +                       printf("m ref is %u\n", rte_mbuf_refcnt_read(m));
>> +               rte_pktmbuf_free(m);
>> +       }
>> +
>>           /* check that all mbufs are back into mempool by now */
>>           for (wn = 0; wn != REFCNT_MAX_TIMEOUT; wn++) {
>>                   if ((i = rte_mempool_avail_count(refcnt_pool)) == n) {
>>
>> That means all your mbufs (except the last one) will still be allocated.
>> So the test would fail - as it should, I think.
>>
>>> And
>>> it seems that Juhamatti & Jacod found some reverting action several
>>> months ago.
>> Didn't get that one either.
>> Konstantin