Re: [PATCH net-next v4 4/5] page_pool: remove PP_FLAG_PAGE_FRAG flag

[Yunsheng Lin]

On 2023/6/16 2:26, Alexander Duyck wrote:
> On Thu, Jun 15, 2023 at 9:51 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>>
>> On Thu, 15 Jun 2023 15:17:39 +0800 Yunsheng Lin wrote:
>>>> Does hns3_page_order() set a good example for the users?
>>>>
>>>> static inline unsigned int hns3_page_order(struct hns3_enet_ring *ring)
>>>> {
>>>> #if (PAGE_SIZE < 8192)
>>>>     if (ring->buf_size > (PAGE_SIZE / 2))
>>>>             return 1;
>>>> #endif
>>>>     return 0;
>>>> }
>>>>
>>>> Why allocate order 1 pages for buffers which would fit in a single page?
>>>> I feel like this soft of heuristic should be built into the API itself.
>>>
>>> hns3 only support fixed buf size per desc by 512 byte, 1024 bytes, 2048 bytes
>>> 4096 bytes, see hns3_buf_size2type(), I think the order 1 pages is for buf size
>>> with 4096 bytes and system page size with 4K, as hns3 driver still support the
>>> per-desc ping-pong way of page splitting when page_pool_enabled is false.
>>>
>>> With page pool enabled, you are right that order 0 pages is enough, and I am not
>>> sure about the exact reason we use the some order as the ping-pong way of page
>>> splitting now.
>>> As 2048 bytes buf size seems to be the default one, and I has not heard any one
>>> changing it. Also, it caculates the pool_size using something as below, so the
>>> memory usage is almost the same for order 0 and order 1:
>>>
>>> .pool_size = ring->desc_num * hns3_buf_size(ring) /
>>>               (PAGE_SIZE << hns3_page_order(ring)),
>>>
>>> I am not sure it worth changing it, maybe just change it to set good example for
>>> the users:) anyway I need to discuss this with other colleague internally and do
>>> some testing before doing the change.
>>
>> Right, I think this may be a leftover from the page flipping mode of
>> operation. But AFAIU we should leave the recycling fully to the page
>> pool now. If we make any improvements try to make them at the page pool
>> level.

I checked, the per-desc buf with 4096 bytes for hnse does not seem to
be used mainly because of the larger memory usage you mentioned below.

>>
>> I like your patches as they isolate the drivers from having to make the
>> fragmentation decisions based on the system page size (4k vs 64k but
>> we're hearing more and more about ARM w/ 16k pages). For that use case
>> this is great.

Yes, That is my point. For hw case, the page splitting in page pool is
mainly to enble multi-descs to use the same page as my understanding.

>>
>> What we don't want is drivers to start requesting larger page sizes
>> because it looks good in iperf on a freshly booted, idle system :(
> 
> Actually that would be a really good direction for this patch set to
> look at going into. Rather than having us always allocate a "page" it
> would make sense for most drivers to allocate a 4K fragment or the
> like in the case that the base page size is larger than 4K. That might
> be a good use case to justify doing away with the standard page pool
> page and look at making them all fragmented.

I am not sure if I understand the above, isn't the frag API able to
support allocating a 4K fragment when base page size is larger than
4K before or after this patch? what more do we need to do?

> 
> In the case of the standard page size being 4K a standard page would
> just have to take on the CPU overhead of the atomic_set and
> atomic_read for pp_ref_count (new name) which should be minimal as on
> most sane systems those just end up being a memory write and read.

If I understand you correctly, I think what you are trying to do
may break some of Jesper' benchmarking:)

[1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/bench_page_pool_simple.c

> .
>