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

[Bruce Richardson <bruce.richardson@intel.com>]

On Sun, Sep 28, 2014 at 11:17:34PM +0000, Wiles, Roger Keith wrote:
> 
> On Sep 28, 2014, at 5:41 PM, Ananyev, Konstantin <konstantin.ananyev@intel.com> wrote:
> 
> > 
> > 
> >> -----Original Message-----
> >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Wiles, Roger Keith
> >> Sent: Sunday, September 28, 2014 6:52 PM
> >> To: <dev@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.

If you want to have such a function, i think it should align with the 
functions on the rings. In this case, this would mean having a get_burst 
function, which returns less than or equal to the number of elements 
requested.  I would not change the behaviour of the existing function 
without also changing the rings "bulk" function to match.

/Bruce

> > 
> > NACK in summary.
> > Konstantin
> 
> Keith Wiles, Principal Technologist with CTO office, Wind River mobile 972-213-5533
>