Re: [dpdk-dev] [PATCH 2/7] rte_sched: use reserved field to allow more VLAN's

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 2 Feb 2015 14:21:58 +0000
"Ananyev, Konstantin" <konstantin.ananyev@intel.com> wrote:

> Hi Stephen,
> 
> > -----Original Message-----
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Stephen Hemminger
> > Sent: Sunday, February 01, 2015 10:04 AM
> > To: dev@dpdk.org
> > Cc: Stephen Hemminger
> > Subject: [dpdk-dev] [PATCH 2/7] rte_sched: use reserved field to allow more VLAN's
> > 
> > From: Stephen Hemminger <shemming@brocade.com>
> > 
> > The QoS subport is limited to 8 bits in original code.
> > But customers demanded ability to support full number of VLAN's (4096)
> > therefore use reserved field of mbuf for this field instead
> > of packing inside other classify portions.
> > 
> > Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> > ---
> >  lib/librte_mbuf/rte_mbuf.h   |  2 +-
> >  lib/librte_sched/rte_sched.h | 31 ++++++++++++++++++++-----------
> >  2 files changed, 21 insertions(+), 12 deletions(-)
> > 
> > diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> > index 16059c6..b6b08f4 100644
> > --- a/lib/librte_mbuf/rte_mbuf.h
> > +++ b/lib/librte_mbuf/rte_mbuf.h
> > @@ -242,7 +242,7 @@ struct rte_mbuf {
> >  	uint16_t data_len;        /**< Amount of data in segment buffer. */
> >  	uint32_t pkt_len;         /**< Total pkt len: sum of all segments. */
> >  	uint16_t vlan_tci;        /**< VLAN Tag Control Identifier (CPU order) */
> > -	uint16_t reserved;
> > +	uint16_t subport;	  /**< SCHED Subport ID */
> 
> As I remember, we keep these reserved 2 bytes for RX 2 double vlan tag offload.
> So probably not a good idea to use it for something that is rte_sched specific.
> If you really need extra space fo rte_sched fields inside mbuf, can't you move it into second cache line?
> Or might be you can use userdata, to either store sched information directly, or as a pointer to some external memory  location? 
> Another possibility - union mbuf.hash is 64bit now, while sched uses only 32bits.
> So might be you can rearrange it to make sched 64bits too?
> Something like:
> 
> union {
>                 uint32_t rss;     /**< RSS hash result if RSS enabled */
>                 struct {
>                         union {
>                                 struct {
>                                         uint16_t hash;
>                                         uint16_t id;
>                                 };
>                                 uint32_t lo;
>                                 /**< Second 4 flexible bytes */
>                         };
>                         uint32_t hi;
>                         /**< First 4 flexible bytes or FD ID, dependent on
>                              PKT_RX_FDIR_* flag in ol_flags. */
>                 } fdir;           /**< Filter identifier if FDIR enabled */
> -                uint32_t sched;   /**< Hierarchical scheduler */
> +               uint64_t sched;   /**< Hierarchical scheduler */
>                 uint32_t usr;     /**< User defined tags. See @rte_distributor_p
> rocess */
> } hash;                   /**< hash information */

Increasing the size of that union totally breaks other alignment and is a not starter.

The reserved field is not use upstream merged code and therefore is fair game.
First to claim it wins.