Re: [dpdk-dev] [PATCH v2] rte_mbuf: mbuf bulk alloc/free functions added + unittest

[Neil Horman <nhorman@tuxdriver.com>]

On Thu, Mar 19, 2015 at 09:41:25AM +0100, Olivier MATZ wrote:
> Hi Neil,
> 
> On 03/18/2015 09:58 PM, Neil Horman wrote:
> >>+/**
> >>+ * Free a bulk of mbufs into its original mempool.
> >>+ * This function assumes:
> >>+ * - refcnt equals 1
> >>+ * - mbufs are direct
> >>+ * - all mbufs must belong to the same mempool
> >>+ *
> >>+ * @param mbufs
> >>+ *    Array of pointers to mbuf
> >>+ * @param count
> >>+ *    Array size
> >>+ */
> >>+static inline void rte_pktmbuf_bulk_free(struct rte_mbuf **mbufs,
> >>+					 unsigned count)
> >>+{
> >>+	unsigned idx;
> >>+
> >>+	RTE_MBUF_ASSERT(count > 0);
> >>+
> >>+	for (idx = 0; idx < count; idx++) {
> >>+		RTE_MBUF_ASSERT(mbufs[idx]->pool == mbufs[0]->pool);
> >>+		RTE_MBUF_ASSERT(rte_mbuf_refcnt_read(mbufs[idx]) == 1);
> >>+		rte_mbuf_refcnt_set(mbufs[idx], 0);
> >This is really a misuse of the API.  The entire point of reference counting is
> >to know when an mbuf has no more references and can be freed.  By forcing all
> >the reference counts to zero here, you allow the refcnt infrastructure to be
> >circumvented, causing memory leaks.
> >
> >I think what you need to do here is enhance the underlying pktmbuf interface
> >such that an rte_mbuf structure has a destructor method association with it
> >which is called when its refcnt reaches zero.  That way the
> >rte_pktmbuf_bulk_free function can just decrement the refcnt on each
> >mbuf_structure, and the pool as a whole can be returned when the destructor
> >function discovers that all mbufs in that bulk pool are freed.
> 
> I don't really understand what's the problem here. The API explicitly
> describes the conditions for calling this functions: the segments are
> directs, they belong to the same mempool, and their refcnt is 1.
> 
> This function could be useful in a driver which knows that the mbuf
> it allocated matches this conditions. I think an application that
> only uses direct mbufs and one mempool could also use this function.
> 


That last condition is my issue with this patch, that the user has to know what
refcnts are.  It makes this api useful for little more than the test case that
is provided with it.  Its irritating enough that for singly allocated mbufs the
user has to know what the refcount of a buffer is before freeing, but at least
they can macrotize a {rte_pktmbuf_refcnt_update; if(rte_pktmbuf_refct_read) then
free} operation.

With this, you've placed the user in charge of not only doing that, but also of
managing the relationship between pktmbufs and the pool they came from.  while
that makes sense for the test case, it really doesn't in any general use case in
which packet processing is ever deferred or queued, because it means that the
application is now responsible for holding a pointer to every packet it
allocates and checking its refcount periodically until it completes.

There is never any reason that an application won't need to do this management,
so making it the purview of the application to handle rather than properly
integrating that functionality in the library is really a false savings.

Regards
Neil

> Regards,
> Olivier
> 
>