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

[Jason Wang <jasowang@redhat.com>]

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?

>
> 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

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