Re: [RFC PATCH v2] ptr_ring: linked list fallback

[Jason Wang]

On 2018å02æ28æ 12:09, Michael S. Tsirkin wrote:
> > > Or we can add plist to a union:
> > >
> > >
> > > struct sk_buff {
> > >           union {
> > >                   struct {
> > >                           /* These two members must be first. */
> > >                           struct sk_buff          *next;
> > >                           struct sk_buff          *prev;
> > >                           union {
> > >                                   struct net_device       *dev;
> > >                                   /* Some protocols might use this space to store information,
> > >                                    * while device pointer would be NULL.
> > >                                    * UDP receive path is one user.
> > >                                    */
> > >                                   unsigned long           dev_scratch;
> > >                           };
> > >                   };
> > >                   struct rb_node  rbnode; /* used in netem & tcp stack */
> > > +		struct plist plist; /* For use with ptr_ring */
> > >           };
> > This look ok.
> >
> > > > For XDP, we need to embed plist in struct xdp_buff too,
> > > Right - that's pretty straightforward, isn't it?
> > Yes, it's not clear to me this is really needed for XDP consider the lock
> > contention it brings.
> >
> > Thanks
> The contention is only when the ring overflows into the list though.

Right, but there's usually a mismatch of speed between producer and 
consumer. In case of a fast producer, we may get this contention very 
frequently.

Thanks