[dpdk-dev] [RFC] More changes for rte_mempool.h:__mempool_get_bulk()

[Wiles, Roger Keith]

On Sep 28, 2014, at 5:41 PM, Ananyev, Konstantin <konstantin.ananyev at intel.com> wrote:

> 
> 
>> -----Original Message-----
>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Wiles, Roger Keith
>> Sent: Sunday, September 28, 2014 6:52 PM
>> To: <dev at dpdk.org>
>> Subject: [dpdk-dev] [RFC] More changes for rte_mempool.h:__mempool_get_bulk()
>> 
>> Here is a Request for Comment on __mempool_get_bulk() routine. I believe I am seeing a few more issues in this routine, please look
>> at the code below and see if these seem to fix some concerns in how the ring is handled.
>> 
>> The first issue I believe is cache->len is increased by ret and not req as we do not know if ret == req. This also means the cache->len
>> may still not satisfy the request from the cache.
>> 
>> The second issue is if you believe the above code then we have to account for that issue in the stats.
>> 
>> Let me know what you think?
>> ++Keith
>> ---
>> 
>> diff --git a/lib/librte_mempool/rte_mempool.h b/lib/librte_mempool/rte_mempool.h
>> index 199a493..b1b1f7a 100644
>> --- a/lib/librte_mempool/rte_mempool.h
>> +++ b/lib/librte_mempool/rte_mempool.h
>> @@ -945,9 +945,7 @@ __mempool_get_bulk(struct rte_mempool *mp, void **obj_table,
>>                   unsigned n, int is_mc)
>> {
>>        int ret;
>> -#ifdef RTE_LIBRTE_MEMPOOL_DEBUG
>> -       unsigned n_orig = n;
>> -#endif
> 
> Yep, as I said in my previous mail n_orig could be removed in total.
> Though from other side - it is harmless.
> 
>> +
>> #if RTE_MEMPOOL_CACHE_MAX_SIZE > 0
>>        struct rte_mempool_cache *cache;
>>        uint32_t index, len;
>> @@ -979,7 +977,21 @@ __mempool_get_bulk(struct rte_mempool *mp, void **obj_table,
>>                        goto ring_dequeue;
>>                }
>> 
>> -               cache->len += req;
>> +               cache->len += ret;      // Need to adjust len by ret not req, as (ret != req)
>> +
> 
> rte_ring_mc_dequeue_bulk(.., req) at line 971, would either get all req objects from the ring and return 0 (success),
> or wouldn't get any entry from the ring and return negative value (failure).
> So  this change is erroneous.

Sorry, I combined my thoughts on changing the get_bulk behavior and you would be correct for the current design. This is why I decided to make it an RFC :-)
> 
>> +               if ( cache->len < n ) {
> 
> If n > cache_size, then we will go straight to  'ring_dequeue' see line 959.
> So no need for that check here.

My thinking (at the time) was get_bulk should return ’n’ instead of zero, which I feel is the better coding. You are correct it does not make sense unless you factor in my thinking at time :-(
> 
>> +                       /*
>> +                        * Number (ret + cache->len) may not be >= n. As
>> +                        * the 'ret' value maybe zero or less then 'req'.
>> +                        *
>> +                        * Note:
>> +                        * An issue of order from the cache and common pool could
>> +                        * be an issue if (cache->len != 0 and less then n), but the
>> +                        * normal case it should be OK. If the user needs to preserve
>> +                        * the order of packets then he must set cache_size == 0.
>> +                        */
>> +                       goto ring_dequeue;
>> +               }
>>        }
>> 
>>        /* Now fill in the response ... */
>> @@ -1002,9 +1014,12 @@ ring_dequeue:
>>                ret = rte_ring_sc_dequeue_bulk(mp->ring, obj_table, n);
>> 
>>        if (ret < 0)
>> -               __MEMPOOL_STAT_ADD(mp, get_fail, n_orig);
>> -       else
>> +               __MEMPOOL_STAT_ADD(mp, get_fail, n);
>> +       else {
>>                __MEMPOOL_STAT_ADD(mp, get_success, ret);
>> +               // Catch the case when ret != n, adding zero should not be a problem.
>> +               __MEMPOOL_STAT_ADD(mp, get_fail, n - ret);
> 
> As I said above, ret == 0 on success, so need for that change.
> Just n (or n_orig) is ok here.
> 
>> +       }
>> 
>>        return ret;
>> }
>> 
>> Keith Wiles, Principal Technologist with CTO office, Wind River mobile 972-213-5533

Do we think it is worth it to change the behavior of get_bulk returning ’n’ instead of zero on success? It would remove a few test IMO in a couple of places. We could also return <0 on the zero case as well, just to make sure code did not try to follow the success case by mistake.
> 
> NACK in summary.
> Konstantin

Keith Wiles, Principal Technologist with CTO office, Wind River mobile 972-213-5533