Message-ID: <CAKhg4t+LpF=G0DBhbuRYtxKyTrMiR3pSc15sY42kc57iGQfPmw@mail.gmail.com>
Date: Mon, 11 Dec 2023 11:38:24 +0800
From: Liang Chen <liangchen.linux@...il.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, hawk@...nel.org, ilias.apalodimas@...aro.org, 
	linyunsheng@...wei.com, netdev@...r.kernel.org, linux-mm@...ck.org, 
	jasowang@...hat.com
Subject: Re: [PATCH net-next v7 4/4] skbuff: Optimization of SKB coalescing
 for page pool

On Sat, Dec 9, 2023 at 10:18 AM Mina Almasry <almasrymina@...gle.com> wrote:
>
> On Wed, Dec 6, 2023 at 2:54 AM Liang Chen <liangchen.linux@...il.com> wrote:
> >
> > In order to address the issues encountered with commit 1effe8ca4e34
> > ("skbuff: fix coalescing for page_pool fragment recycling"), the
> > combination of the following condition was excluded from skb coalescing:
> >
> > from->pp_recycle = 1
> > from->cloned = 1
> > to->pp_recycle = 1
> >
> > However, with page pool environments, the aforementioned combination can
> > be quite common(ex. NetworkMananger may lead to the additional
> > packet_type being registered, thus the cloning). In scenarios with a
> > higher number of small packets, it can significantly affect the success
> > rate of coalescing. For example, considering packets of 256 bytes size,
> > our comparison of coalescing success rate is as follows:
> >
> > Without page pool: 70%
> > With page pool: 13%
> >
> > Consequently, this has an impact on performance:
> >
> > Without page pool: 2.57 Gbits/sec
> > With page pool: 2.26 Gbits/sec
> >
> > Therefore, it seems worthwhile to optimize this scenario and enable
> > coalescing of this particular combination. To achieve this, we need to
> > ensure the correct increment of the "from" SKB page's page pool
> > reference count (pp_ref_count).
> >
> > Following this optimization, the success rate of coalescing measured in
> > our environment has improved as follows:
> >
> > With page pool: 60%
> >
> > This success rate is approaching the rate achieved without using page
> > pool, and the performance has also been improved:
> >
> > With page pool: 2.52 Gbits/sec
> >
> > Below is the performance comparison for small packets before and after
> > this optimization. We observe no impact to packets larger than 4K.
> >
> > packet size     before      after       improved
> > (bytes)         (Gbits/sec) (Gbits/sec)
> > 128             1.19        1.27        7.13%
> > 256             2.26        2.52        11.75%
> > 512             4.13        4.81        16.50%
> > 1024            6.17        6.73        9.05%
> > 2048            14.54       15.47       6.45%
> > 4096            25.44       27.87       9.52%
> >
> > Signed-off-by: Liang Chen <liangchen.linux@...il.com>
> > Reviewed-by: Yunsheng Lin <linyunsheng@...wei.com>
> > Suggested-by: Jason Wang <jasowang@...hat.com>
> > ---
> >  include/net/page_pool/helpers.h |  5 ++++
> >  net/core/skbuff.c               | 41 +++++++++++++++++++++++----------
> >  2 files changed, 34 insertions(+), 12 deletions(-)
> >
> > diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
> > index 9dc8eaf8a959..268bc9d9ffd3 100644
> > --- a/include/net/page_pool/helpers.h
> > +++ b/include/net/page_pool/helpers.h
> > @@ -278,6 +278,11 @@ static inline long page_pool_unref_page(struct page *page, long nr)
> >         return ret;
> >  }
> >
> > +static inline void page_pool_ref_page(struct page *page)
> > +{
> > +       atomic_long_inc(&page->pp_ref_count);
> > +}
> > +
> >  static inline bool page_pool_is_last_ref(struct page *page)
> >  {
> >         /* If page_pool_unref_page() returns 0, we were the last user */
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index 7e26b56cda38..3c2515a29376 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -947,6 +947,24 @@ static bool skb_pp_recycle(struct sk_buff *skb, void *data, bool napi_safe)
> >         return napi_pp_put_page(virt_to_page(data), napi_safe);
> >  }
> >
> > +/**
> > + * skb_pp_frag_ref() - Increase fragment reference count of a page
> > + * @page:      page of the fragment on which to increase a reference
> > + *
> > + * Increase fragment reference count (pp_ref_count) on a page, but if it is
> > + * not a page pool page, fallback to increase a reference(_refcount) on a
> > + * normal page.
> > + */
> > +static void skb_pp_frag_ref(struct page *page)
> > +{
> > +       struct page *head_page = compound_head(page);
> > +
> > +       if (likely(is_pp_page(head_page)))
> > +               page_pool_ref_page(head_page);
> > +       else
> > +               page_ref_inc(head_page);
> > +}
> > +
>
> I am confused by this, why add a new helper instead of modifying the
> existing helper, skb_frag_ref()?
>
> My mental model is that if the net stack wants to acquire a reference
> on a frag, it calls skb_frag_ref(), and if it wants to drop a
> reference on a frag, it should call skb_frag_unref(). Internally
> skb_frag_ref/unref() can do all sorts of checking to decide whether to
> increment page->refcount or page->pp_ref_count. I can't wrap my head
> around the introduction of skb_pp_frag_ref(), but no equivalent
> skb_pp_frag_unref().
>
> But even if skb_pp_frag_unref() was added, when should the net stack
> use skb_frag_ref/unref, and when should the stack use
> skb_pp_ref/unref? The docs currently describe what the function does,
> but when a program unfamiliar with the page pool should use it.
>
> >  static void skb_kfree_head(void *head, unsigned int end_offset)
> >  {
> >         if (end_offset == SKB_SMALL_HEAD_HEADROOM)
> > @@ -5769,17 +5787,12 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
> >                 return false;
> >
> >         /* In general, avoid mixing page_pool and non-page_pool allocated
> > -        * pages within the same SKB. Additionally avoid dealing with clones
> > -        * with page_pool pages, in case the SKB is using page_pool fragment
> > -        * references (page_pool_alloc_frag()). Since we only take full page
> > -        * references for cloned SKBs at the moment that would result in
> > -        * inconsistent reference counts.
> > -        * In theory we could take full references if @from is cloned and
> > -        * !@to->pp_recycle but its tricky (due to potential race with
> > -        * the clone disappearing) and rare, so not worth dealing with.
> > +        * pages within the same SKB. In theory we could take full
> > +        * references if @from is cloned and !@to->pp_recycle but its
> > +        * tricky (due to potential race with the clone disappearing) and
> > +        * rare, so not worth dealing with.
> >          */
> > -       if (to->pp_recycle != from->pp_recycle ||
> > -           (from->pp_recycle && skb_cloned(from)))
> > +       if (to->pp_recycle != from->pp_recycle)
> >                 return false;
> >
> >         if (len <= skb_tailroom(to)) {
> > @@ -5836,8 +5849,12 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
> >         /* if the skb is not cloned this does nothing
> >          * since we set nr_frags to 0.
> >          */
> > -       for (i = 0; i < from_shinfo->nr_frags; i++)
> > -               __skb_frag_ref(&from_shinfo->frags[i]);
> > +       if (from->pp_recycle)
> > +               for (i = 0; i < from_shinfo->nr_frags; i++)
> > +                       skb_pp_frag_ref(skb_frag_page(&from_shinfo->frags[i]));
> > +       else
> > +               for (i = 0; i < from_shinfo->nr_frags; i++)
> > +                       __skb_frag_ref(&from_shinfo->frags[i]);
>
> You added a check here to use skb_pp_frag_ref() instead of
> skb_frag_ref() here, but it's not clear to me why other callsites of
> skb_frag_ref() don't need to be modified in the same way after your
> patch.
>
> After your patch:
>
> skb_frag_ref() will always increment page->_refcount
> skb_frag_unref() will either decrement page->_refcount or decrement
> page->pp_ref_count (depending on the value of skb->pp_recycle).
> skb_pp_frag_ref() will either increment page->_refcount or increment
> page->pp_ref_count (depending on the value of is_pp_page(), not
> skb->pp_recycle).
> skb_pp_frag_unref() doesn't exist.
>
> Is this not confusing? Can we streamline things:
>
> skb_frag_ref() increments page->pp_ref_count for skb->pp_recycle,
> page->_refcount otherwise.
> skb_frag_unref() decrement page->pp_ref_count for skb->pp_recycle,
> page->_refcount otherwise.
>
> Or am I missing something that causes us to require this asymmetric
> reference counting?
>

This idea was previously implemented, as shown here:
https://lore.kernel.org/all/20211009093724.10539-5-linyunsheng@huawei.com/.
But implementing this would result in some unnecessary overhead, since
currently, 'skb_try_coalesce' is the only place where the page pool
reference count for skb frag might be increased. I would prefer to
move the logic to '__skb_frag_ref' when such a need becomes more
common. Thanks!

> >
> >         to->truesize += delta;
> >         to->len += len;
> > --
> > 2.31.1
> >
> >
>
>
> --
> Thanks,
> Mina

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