Re: [Linuxarm] Re: [PATCH net-next v2 3/3] skbuff: keep track of pp page when __skb_frag_ref() is called

[Yunsheng Lin]

On 2021/9/16 18:38, Ilias Apalodimas wrote:
> On Thu, Sep 16, 2021 at 05:33:39PM +0800, Yunsheng Lin wrote:
>> On 2021/9/16 16:44, Ilias Apalodimas wrote:
>>>>>> appear if we try to pull in your patches on using page pool and recycling
>>>
>>> [...]
>>>
>>>>>> for Tx where TSO and skb_split are used?
>>>>
>>>> As my understanding, the problem might exists without tx recycling, because a
>>>> skb from wire would be passed down to the tcp stack and retransmited back to
>>>> the wire theoretically. As I am not able to setup a configuration to verify
>>>> and test it and the handling seems tricky, so I am targetting net-next branch
>>>> instead of net branch.
>>>>
>>>>>>
>>>>>> I'll be honest, when I came up with the recycling idea for page pool, I
>>>>>> never intended to support Tx.  I agree with Alexander here,  If people want
>>>>>> to use it on Tx and think there's value,  we might need to go back to the
>>>>>> drawing board and see what I've missed.  It's still early and there's a
>>>>>> handful of drivers using it,  so it will less painful now.
>>>>
>>>> Yes, we also need to prototype it to see if there is something missing in the
>>>> drawing board and how much improvement we get from that:)
>>>>
>>>>>
>>>>> I agree, page_pool is NOT designed or intended for TX support.
>>>>> E.g. it doesn't make sense to allocate a page_pool instance per socket, as the backing memory structures for page_pool are too much.
>>>>> As the number RX-queues are more limited it was deemed okay that we use page_pool per RX-queue, which sacrifice some memory to gain speed.
>>>>
>>>> As memtioned before, Tx recycling is based on page_pool instance per socket.
>>>> it shares the page_pool instance with rx.
>>>>
>>>> Anyway, based on feedback from edumazet and dsahern, I am still trying to
>>>> see if the page pool is meaningful for tx.
>>>>
>>>>>
>>>>>
>>>>>> The pp_recycle_bit was introduced to make the checking faster, instead of
>>>>>> getting stuff into cache and check the page signature.  If that ends up
>>>>>> being counterproductive, we could just replace the entire logic with the
>>>>>> frag count and the page signature, couldn't we?  In that case we should be
>>>>>> very cautious and measure potential regression on the standard path.
>>>>>
>>>>> +1
>>>>
>>>> I am not sure "pp_recycle_bit was introduced to make the checking faster" is a
>>>> valid. The size of "struct page" is only about 9 words(36/72 bytes), which is
>>>> mostly to be in the same cache line, and both standard path and recycle path have
>>>> been touching the "struct page", so it seems the overhead for checking signature
>>>> seems minimal.
>>>>
>>>> I agree that we need to be cautious and measure potential regression on the
>>>> standard path.
>>>
>>> well pp_recycle is on the same cache line boundary with the head_frag we
>>> need to decide on recycling. After that we start checking page signatures
>>> etc,  which means the default release path remains mostly unaffected.  
>>>
>>> I guess what you are saying here, is that 'struct page' is going to be
>>> accessed eventually by the default network path,  so there won't be any 
>>> noticeable performance hit?  What about the other usecases we have
>>
>> Yes.
> 
> In that case you'd need to call virt_to_head_page() early though, get it
> and then compare the signature.   I guess that's avoidable by using 
> frag->bv_page for the fragments?

If a page of a skb frag is from page pool, It seems frag->bv_page is
always point to head_page of a compound page, so the calling of
virt_to_head_page() does not seems necessary.

bit 0 of frag->bv_page is different way of indicatior for a pp page,
it is better we do not confuse with the page signature way. Using
a bit 0 may give us a free word in 'struct page' if we manage to
use skb->pp_recycle to indicate a head page of the skb uniquely, meaning
page->pp_magic can be used for future feature.


> 
>>
>>> for pp_recycle right now?  __skb_frag_unref() in skb_shift() or
>>> skb_try_coalesce() (the latter can probably be removed tbh).
>>
>> If we decide to go with accurate indicator of a pp page, we just need
>> to make sure network stack use __skb_frag_unref() and __skb_frag_ref()
>> to put and get a page frag, the indicator checking need only done in
>> __skb_frag_unref() and __skb_frag_ref(), so the skb_shift() and
>> skb_try_coalesce() should be fine too.
>>
>>>
>>>>
>>>> Another way is to use the bit 0 of frag->bv_page ptr to indicate if a frag
>>>> page is from page pool.
>>>
>>> Instead of the 'struct page' signature?  And the pp_recycle bit will
>>> continue to exist?  
>>
>> pp_recycle bit might only exist or is only used for the head page for the skb.
>> The bit 0 of frag->bv_page ptr can be used to indicate a frag page uniquely.
>> Doing a memcpying of shinfo or "*fragto = *fragfrom" automatically pass the
>> indicator to the new shinfo before doing a __skb_frag_ref(), and __skb_frag_ref()
>> will increment the _refcount or pp_frag_count according to the bit 0 of
>> frag->bv_page.
>>
>> By the way, I also prototype the above idea, and it seems to work well too.
>>
> 
> As long as no one else touches this, it's just another way of identifying a
> page_pool allocated page.  But are we gaining by that?  Not using
> virt_to_head_page() as stated above? But in that case you still need to
> keep pp_recycle around. 

No, we do not need the pp_recycle, as long as the we make sure __skb_frag_ref()
is called after memcpying the shinfo or doing "*fragto = *fragfrom".

> 
>>> .
>>> Right now the 'naive' explanation on the recycling decision is something like:
>>>
>>> if (pp_recycle) <--- recycling bit is set
>>>     (check page signature) <--- signature matches page pool
>>> 		(check fragment refcnt) <--- If frags are enabled and is the last consumer
>>> 			recycle
>>>
>>> If we can proove the performance is unaffected when we eliminate the first if,
>>> then obviously we should remove it.  I'll try running that test here and see,
>>> but keep in mind I am only testing on an 1GB interface.  Any chance we can get 
>>> measurements on a beefier hardware using hns3 ?
>>
>> Sure, I will try it.
>> As the kind of performance overhead is small, any performance testcase in mind?
>>
> 
> 'eliminate the first if' wasn't accurate.  I meant switch the first if and
> check the struct page signature instead.  That would be the best solution
> imho.  We effectively have a single rule to check if a packet comes from
> page_pool or not.

I am not sure what does "switch " means here, if the page signature can
indicate a pp page uniquely, the "if (pp_recycle)" checking can be removed.

> 
> You can start by sending a lot of packets and dropping those immediately.
> That should put enough stress on the receive path and the allocators and it
> should give us a rough idea. 
> 
>>>
>>>>
>>>>>
>>>>>> But in general,  I'd be happier if we only had a simple logic in our
>>>>>> testing for the pages we have to recycle.  Debugging and understanding this
>>>>>> otherwise will end up being a mess.
>>>>>
>>>>>
>>>
>>> [...]
>>>
>>> Regards
>>> /Ilias
>>> .
>>>
> 
> Regards
> /Ilias
> .
>