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

[Jason Wang]

On 2018å02æ27æ 04:34, Michael S. Tsirkin wrote:
> On Mon, Feb 26, 2018 at 11:15:42AM +0800, Jason Wang wrote:
> > On 2018å02æ26æ 09:17, Michael S. Tsirkin wrote:
> > > So pointer rings work fine, but they have a problem: make them too small
> > > and not enough entries fit.  Make them too large and you start flushing
> > > your cache and running out of memory.
> > >
> > > This is a new idea of mine: a ring backed by a linked list. Once you run
> > > out of ring entries, instead of a drop you fall back on a list with a
> > > common lock.
> > >
> > > Should work well for the case where the ring is typically sized
> > > correctly, but will help address the fact that some user try to set e.g.
> > > tx queue length to 1000000.
> > >
> > > In other words, the idea is that if a user sets a really huge TX queue
> > > length, we allocate a ptr_ring which is smaller, and use the backup
> > > linked list when necessary to provide the requested TX queue length
> > > legitimately.
> > >
> > > My hope this will move us closer to direction where e.g. fw codel can
> > > use ptr rings without locking at all.  The API is still very rough, and
> > > I really need to take a hard look at lock nesting.
> > >
> > > Compiled only, sending for early feedback/flames.
> > >
> > > Signed-off-by: Michael S. Tsirkin<mst@xxxxxxxxxx>
> > > ---
> > >
> > > changes from v1:
> > > - added clarifications by DaveM in the commit log
> > > - build fixes
> > >
> > >    include/linux/ptr_ring.h | 64 +++++++++++++++++++++++++++++++++++++++++++++---
> > >    1 file changed, 61 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> > > index d72b2e7..8aa8882 100644
> > > --- a/include/linux/ptr_ring.h
> > > +++ b/include/linux/ptr_ring.h
> > > @@ -31,11 +31,18 @@
> > >    #include <asm/errno.h>
> > >    #endif
> > > +/* entries must start with the following structure */
> > > +struct plist {
> > > +	struct plist *next;
> > > +	struct plist *last; /* only valid in the 1st entry */
> > > +};
> > So I wonder whether or not it's better to do this in e.g skb_array
> > implementation. Then it can use its own prev/next field.
> XDP uses ptr ring directly, doesn't it?

Well I believe the main user for this is qdisc, which use skb array. And 
we can not use what implemented in this patch directly for sk_buff 
without some changes on the data structure.

For XDP, we need to embed plist in struct xdp_buff too, so it looks to 
me that the better approach is to have separated function for ptr ring 
and skb array.

Thanks