Re: [Intel-wired-lan] [PATCH net-next v5 03/14] page_pool: avoid calling no-op externals when possible

[Alexander Lobakin]

From: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
Date: Wed, 29 Nov 2023 11:17:50 +0800

> On 2023/11/27 22:32, Alexander Lobakin wrote:
>>
>> Chris, any thoughts on a global flag for skipping DMA syncs ladder?
> 
> It seems there was one already in the past:
> 
> https://lore.kernel.org/netdev/7c55a4d7-b4aa-25d4-1917-f6f355bd722e@xxxxxxx/T/

It addresses a different problem, meaningless indirect calls, while this
one addresses meaningless direct calls :>
When the above gets merged, we could combine these two into one global,
but Eric wasn't active with his patch and I remember there were some
problems, so I wouldn't count on that it will arrive soon.

> 
>>
>>>
>>>
> 
>>>> +static inline bool page_pool_set_dma_addr(const struct page_pool *pool,
>>>> +					  struct page *page,
>>>> +					  dma_addr_t addr)
>>>>  {
>>>> +	unsigned long val = addr;
>>>> +
>>>> +	if (unlikely(!addr)) {
>>>> +		page->dma_addr = 0;
>>>> +		return true;
>>>> +	}
>>>
>>> The above seems unrelated change?
>>
>> Related. We use page_put_set_dma_addr() to clear ::dma_addr as well
>> (grep for it in page_pool.c). In this case, we don't want
>> dma_need_sync() to be called as we explicitly pass zero. This check
>> zeroes the field and exits as quickly as possible.
> 
> The question seems to be about if we need to ensure the LSB of
> page->dma_addr is not set when page_pool releases a page back to page
> allocator?

But why do we need to call dma_need_sync(0) when freeing a page wasting
CPU cycles on relatively hot path?

> 
>> In case with the call mentioned above, zero is a compile-time constant
>> there, so that this little branch will be inlined with the rest dropped.

Thanks,
Olek