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SubjectRe: [RFC PATCH v2] ptr_ring: linked list fallback
On Wed, Feb 28, 2018 at 11:28:57AM +0800, Jason Wang wrote:
>
>
> On 2018年02月28日 01:12, Michael S. Tsirkin wrote:
> > On Tue, Feb 27, 2018 at 10:29:26AM +0800, Jason Wang wrote:
> > >
> > > On 2018年02月27日 04:34, Michael S. Tsirkin wrote:
> > > > On Mon, Feb 26, 2018 at 11:15:42AM +0800, Jason Wang wrote:
> > > > > On 2018年02月26日 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@redhat.com>
> > > > > > ---
> > > > > >
> > > > > > 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.
> > Why not? skb has next and prev pointers at 1st two fields:
> >
> > struct sk_buff {
> > union {
> > struct {
> > /* These two members must be first. */
> > struct sk_buff *next;
> > struct sk_buff *prev;
> > ...
> > }
> >
> > so it's just a question of casting to struct plist.
>
> Well, then the casting can only be done in skb_array implementation?

why not?

> >
> > 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.

> > > so it looks to me
> > > that the better approach is to have separated function for ptr ring and skb
> > > array.
> > >
> > > Thanks

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