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

On 2021/9/17 14:38, Ilias Apalodimas wrote:

Hi Yunsheng,

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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.

I was mostly referring to the skb head here and how would you trigger the
recycling path. 

I think we are talking about different things here.  
One idea is to use the last bit of frag->bv_page to identify fragments
allocated from page_pool, which is done today with the signature.

The signature however exists in the head page so my question was, can we rid
of that without having a performance penalty?

As both skb frag and head page is eventually operated on the head page
of a compound page(if it is a compound page) for normal case too, maybe
we can refactor the code to get the head page of a compound page before
the signature checking without doing a second virt_to_head_page() or
compound_head() call?

Yea that's doable, but my concern is different here.  If we do that the
standard network stack, even for drivers that don't use page_pool,  will
have to do a virt_to_head_page() -> check signature, to decide if it has to
try recycling the packet.  That's the performance part I am worried about,
since it happens for every packet. 

Yes, there is theoretically performance penalty for virt_to_head_page() or
compound_head(), will do more test if we decide to go with the signature
checking.

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IOW in skb_free_head() an we replace:

if (skb_pp_recycle(skb, head)) 
with
if (page->pp_magic & ~0x3UL) == PP_SIGNATURE)
and get rid of the 'bool recycle' argument in __skb_frag_unref()?

For the frag page of a skb, it seems ok to get rid of the 'bool recycle'
argument in __skb_frag_unref(), as __skb_frag_unref() and __skb_frag_ref()
is symmetrically called to put/get a page.

For the head page of a skb, we might need to make sure the head page
passed to __build_skb_around() meet below condition:
do pp_frag_count incrementing instead of _refcount incrementing when
the head page is not newly allocated and it is from page pool.
It seems hard to audit that?

Yea that seems a bit weird at least to me and I am not sure, it's the only
place we'll have to go and do that.

Yes, That is why I avoid changing the behavior of a head page for a skb.
In other word, maybe we should not track if head page for a skb is pp page
or not when the page'_refcount is incremented during network stack journey,
just treat it as normal page?

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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.

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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.

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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".

But we'll have to keep it for the skb head in this case.

As above, I am not really look into skb head case:)

Let me take a step back here, because I think we drifted a bit. 
The page signature was introduced in order to be able to identify skb
fragments. The problem was that you couldn't rely on the pp_recycle bit of
the skb head,  since fragments could come from anywhere.  So you use the
skb bit as a hint for skb frags, and you eventually decide using the page
signature.

So we got 3 options (Anything I've missed ?)
- try to remove pp_recycle bit, since the page signature is enough for the
  skb head and fragments.  That in my opinion is the cleanest option,  as
  long as we can prove there's no performance hit on the standard network
  path.

- Replace the page signature with frag->bv_page bit0.  In that case we
  still have to keep the pp_recycle bit,  but we do have an 'easier'
  indication that a skb frag comes from page_pool.  That's still pretty
  safe, since you now have unique identifiers for the skb and page
  fragments and you can be sure of their origin (page pool or not).
  What I am missing here, is what do we get out of this?  I think the
  advantage is not having to call virt_to_head_page() for frags ?

Not using the signature will free a word space in struct page for future
feature?

For safty and performance reason:
1. maybe we should move the pp_recycle bit from "struct sk_buff" to
   "struct skb_shared_info", and use it to only indicate if the head page of
   a skb is from page pool.

2. The frag->bv_page bit0 is used to indicate if the frag page of a skb is
   from page pool, and modify __skb_frag_unref() and __skb_frag_ref() to keep
   track of it.

3. For safty or debugging reason, keep the page signature for now, and put a
   page signature WARN_ON checking in page pool to catch any misbehaviour?

If there is not bug showing up later, maybe we can free the page signature space
for other usage?

Thanks
/Ilias