Re: [PATCH RFC net-next v4 5/9] libie: add Rx buffer management (via Page Pool)

[Yunsheng Lin <yunshenglin0825@gmail.com>]

On 2023/7/7 0:28, Alexander Lobakin wrote:
> From: Yunsheng Lin <linyunsheng@huawei.com>
> Date: Thu, 6 Jul 2023 20:47:28 +0800
> 
>> On 2023/7/5 23:55, Alexander Lobakin wrote:
>>
>>> +/**
>>> + * libie_rx_page_pool_create - create a PP with the default libie settings
>>> + * @napi: &napi_struct covering this PP (no usage outside its poll loops)
>>> + * @size: size of the PP, usually simply Rx queue len
>>> + *
>>> + * Returns &page_pool on success, casted -errno on failure.
>>> + */
>>> +struct page_pool *libie_rx_page_pool_create(struct napi_struct *napi,
>>> +					    u32 size)
>>> +{
>>> +	struct page_pool_params pp = {
>>> +		.flags		= PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV,
>>> +		.order		= LIBIE_RX_PAGE_ORDER,
>>> +		.pool_size	= size,
>>> +		.nid		= NUMA_NO_NODE,
>>> +		.dev		= napi->dev->dev.parent,
>>> +		.napi		= napi,
>>> +		.dma_dir	= DMA_FROM_DEVICE,
>>> +		.offset		= LIBIE_SKB_HEADROOM,
>>
>> I think it worth mentioning that the '.offset' is not really accurate
>> when the page is split, as we do not really know what is the offset of
>> the frag of a page except for the first frag.
> 
> Yeah, this is read as "offset from the start of the page or frag to the
> actual frame start, i.e. its Ethernet header" or "this is just
> xdp->data - xdp->data_hard_start".

So the problem seems to be if most of drivers have a similar reading as
libie does here, as .offset seems to have a clear semantics which is used
to skip dma sync operation for buffer range that is not touched by the
dma operation. Even if it happens to have the same value of "offset from
the start of the page or frag to the actual frame start", I am not sure
it is future-proofing to reuse it.

When page frag is added, I didn't really give much thought about that as
we use it in a cache coherent system.
It seems we might need to extend or update that semantics if we really want
to skip dma sync operation for all the buffer ranges that are not touched
by the dma operation for page split case.
Or Skipping dma sync operation for all untouched ranges might not be worth
the effort, because it might need a per frag dma sync operation, which is
more costly than a batched per page dma sync operation. If it is true, page
pool already support that currently as my understanding, because the dma
sync operation is only done when the last frag is released/freed.

> 
>>
>>> +	};
>>> +	size_t truesize;
>>> +
>>> +	pp.max_len = libie_rx_sync_len(napi->dev, pp.offset);

As mentioned above, if we depend on the last released/freed frag to do the
dma sync, the pp.max_len might need to cover all the frag.

>>> +
>>> +	/* "Wanted" truesize, passed to page_pool_dev_alloc() */
>>> +	truesize = roundup_pow_of_two(SKB_HEAD_ALIGN(pp.offset + pp.max_len));
>>> +	pp.init_arg = (void *)truesize;
>>
>> I am not sure if it is correct to use pp.init_arg here, as it is supposed to
>> be used along with init_callback. And if we want to change the implemetation
> 
> I know. I abused it to save 1 function argument :p It's safe since I
> don't use init_callback (not an argument).
> I was thinking also of having a union in PP params or even a new field
> like "wanted true size", so that your function could even take values
> from there in certain cases (e.g. if I pass 0 as parameter).
> 
>> of init_callback, we may stuck with it as the driver is using it very
>> differently here.
>>
>> Is it possible to pass the 'wanted true size' by adding a parameter for
>> libie_rx_alloc()?
> 
> Yes, or I could store it somewhere on the ring, but looks uglier =\ This
> one does as well to some degree, but at least hidden in the library and
> doesn't show up in the drivers :D

It seems most hw driver know the size of memory it needs when creating
the ring/queue, setting the frag size and deciding how many is a page
split into before allocation seems like a possible future optimization.

For now, it would be better to add helper to acess pp.init_arg at least
instead of acess pp.init_arg directly to make it more obvious and make
the future optimization more easier.

> 
>>
>>> +
>>> +	return page_pool_create(&pp);
>>> +}
>>> +EXPORT_SYMBOL_NS_GPL(libie_rx_page_pool_create, LIBIE);
> 
> Thanks,
> Olek