linux-kernel - Re: [RFC PATCH v2] ptr_ring: linked list fallback

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Date:   Tue, 27 Feb 2018 10:29:26 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH v2] ptr_ring: linked list fallback



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@...hat.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.

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