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 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.

> 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.

> .
> 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?

> 
>>
>>>
>>>> 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
> .
>