Re: [dpdk-dev] [PATCH v3 1/2] mbuf: support attaching external buffer to mbuf

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> 
> On Mon, Apr 23, 2018 at 11:53:04AM +0000, Ananyev, Konstantin wrote:
> [...]
> > > @@ -693,9 +711,14 @@ rte_mbuf_to_baddr(struct rte_mbuf *md)
> > >  #define RTE_MBUF_INDIRECT(mb)   ((mb)->ol_flags & IND_ATTACHED_MBUF)
> > >
> > >  /**
> > > + * Returns TRUE if given mbuf has external buffer, or FALSE otherwise.
> > > + */
> > > +#define RTE_MBUF_HAS_EXTBUF(mb) ((mb)->ol_flags & EXT_ATTACHED_MBUF)
> > > +
> > > +/**
> > >   * Returns TRUE if given mbuf is direct, or FALSE otherwise.
> > >   */
> > > -#define RTE_MBUF_DIRECT(mb)     (!RTE_MBUF_INDIRECT(mb))
> > > +#define RTE_MBUF_DIRECT(mb) (!RTE_MBUF_INDIRECT(mb) && !RTE_MBUF_HAS_EXTBUF(mb))
> >
> > As a nit:
> > RTE_MBUF_DIRECT(mb)  (((mb)->ol_flags & (IND_ATTACHED_MBUF | EXT_ATTACHED_MBUF)) == 0)
> 
> It was for better readability and I expected compiler did the same.
> But, if you still want this way, I can change it.

I know compilers are quite smart these days, but you never know for sure,
so yes, I think better to do that explicitly.

> 
> [...]
> > >  /**
> > > - * Detach an indirect packet mbuf.
> > > + * @internal used by rte_pktmbuf_detach().
> > > + *
> > > + * Decrement the reference counter of the external buffer. When the
> > > + * reference counter becomes 0, the buffer is freed by pre-registered
> > > + * callback.
> > > + */
> > > +static inline void
> > > +__rte_pktmbuf_free_extbuf(struct rte_mbuf *m)
> > > +{
> > > +	struct rte_mbuf_ext_shared_info *shinfo;
> > > +
> > > +	RTE_ASSERT(RTE_MBUF_HAS_EXTBUF(m));
> > > +
> > > +	shinfo = rte_mbuf_ext_shinfo(m);
> > > +
> > > +	if (rte_extbuf_refcnt_update(shinfo, -1) == 0)
> > > +		shinfo->free_cb(m->buf_addr, shinfo->fcb_opaque);
> >
> >
> > I understand the reason but extra function call for each external mbuf - seems quite expensive.
> > Wonder is it possible to group them somehow and amortize the cost?
> 
> Good point. I thought about it today.
> 
> Comparing to the regular mbuf, maybe three differences. a) free function isn't
> inlined but a real branch. b) no help from core local cache like mempool's c) no
> free_bulk func like rte_mempool_put_bulk(). But these look quite costly and
> complicated for the external buffer attachment.
> 
> For example, to free it in bulk, external buffers should be grouped as the
> buffers would have different callback functions. To do that, I have to make an
> API to pre-register an external buffer group to prepare resources for the bulk
> free. Then, buffers can't be anonymous anymore but have to be registered in
> advance. If so, it would be better to use existing APIs, especially when a user
> wants high throughput...
> 
> Let me know if you have better idea to implement it. Then, I'll gladly take
> that. Or, we can push any improvement patch in the next releases.

I don't have any extra-smart thoughts here.
One option I thought about - was to introduce group of external buffers with
common free routine (I think o mentioned it already).
Second - hide all that external buffer management inside mempool,
i.e. if user wants to use external buffers he create a mempool
(with rte_mbuf_ext_shared_info as elements?), then attach external buffer to shinfo
and call mbuf_attach_external(mbuf, shinfo).
Though for free we can just call mempool_put(shinfo) and let particular implementation
decide when/how call free_cb(), etc. 

Konstantin