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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 28 Feb 2018 17:43:06 +0200

On Wed, Feb 28, 2018 at 10:20:33PM +0800, Jason Wang wrote:
> 
> 
> On 2018年02月28日 22:01, Michael S. Tsirkin wrote:
> > On Wed, Feb 28, 2018 at 02:28:21PM +0800, Jason Wang wrote:
> > > 
> > > On 2018年02月28日 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
> > This is not true in my experiments.  In my experiments, ring size of 4k
> > bytes is enough to see packet drops in single %s of cases.
> > 
> > To you have workloads where rings are full most of the time?
> 
> E.g using xdp_redirect to redirect packets from ixgbe to tap. In my test,
> ixgeb can produce ~8Mpps. But vhost can only consume ~3.5Mpps.

Then you are better off just using a small ring and dropping
packets early, right?

> > 
> > One other nice side effect of this patch is that instead of dropping
> > packets quickly it slows down producer to match consumer speeds.
> 
> In some case, producer may not want to be slowed down, e.g in devmap which
> can redirect packets into several different interfaces.
> > IOW, it can go either way in theory, we will need to test and see the effect.
> > 
> 
> Yes.
> 
> Thanks