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

[Jason Wang <jasowang@redhat.com>]

On 2018年02月28日 23:43, Michael S. Tsirkin wrote:
> 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?

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