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

From: Jason Wang
Date: Thu Mar 01 2018 - 01:41:22 EST




On 2018å02æ28æ 23:43, Michael S. Tsirkin wrote:
On Wed, Feb 28, 2018 at 10:20:33PM +0800, Jason Wang wrote:

On 2018å02æ28æ 22:01, Michael S. Tsirkin wrote:
On Wed, Feb 28, 2018 at 02:28:21PM +0800, Jason Wang wrote:
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
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?

Yes, so I believe we won't use this for XDP.

Thanks

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