Date:   Fri, 17 Sep 2021 18:01:03 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Jesper Dangaard Brouer <jbrouer@...hat.com>, brouer@...hat.com,
        Alexander Duyck <alexander.duyck@...il.com>,
        davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxarm@...neuler.org,
        hawk@...nel.org, jonathan.lemon@...il.com, alobakin@...me,
        willemb@...gle.com, cong.wang@...edance.com, pabeni@...hat.com,
        haokexin@...il.com, nogikh@...gle.com, elver@...gle.com,
        memxor@...il.com, edumazet@...gle.com, dsahern@...il.com
Subject: Re: [PATCH net-next v2 3/3] skbuff: keep track of pp page when
 __skb_frag_ref() is called

> >>>>> 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.

Can we check this somehow?  I can send a patch for this,  but my 
testing is limited to 1Gbit for the recycling.  I can find
25/100Gbit interfaces for the 'normal' path.

> 
> > 
> >>
> >>>
> >>> 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?
>  

I am not sure I understand this.

> > 
> >>
> >>
> >>>
> >>>> 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".
> >>>
> >>> 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?

Yea that's another thing we gain,  but I am not sure how useful how this is
going to turn out.  

> 
> > 
> > - Keep all of them(?) and use frag->bv_page bit0 similarly to pp_recycle
> >   bit?  I don't see much value on this one,  I am just keeping it here for
> >   completeness.
> 
> 
> 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.

What's the safety or performance we gain out of this?  The only performance
I can think of is the dirty cache line of the recycle bit we set to 0.
If we do move it to skb_shared)info we'll have to make sure it's on the
same cacheline as the ones we already change.
> 
> 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?

Yea that's essentially identical to (2) but we move the pp_recycle on the
skb_shared_info.  I'd really prefer getting rid of the pp_recycle entirely,
since it's the cleanest thing we can do in my head.  If we ever need an
extra 4/8 bytes in the future,  we can always go back and implement this.

Alexander/Jesper any additional thoughts?

Regards
/Ilias
> 
> > 
> > Thanks
> > /Ilias
> 

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