netdev - Re: [PATCH net-next v8 4/4] skbuff: Optimization of SKB coalescing for page pool

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Message-ID: <CAKhg4tJDgaVeMp437q1BHuE3aZo2NU4JnOhaQEXepJuQhPnTZQ@mail.gmail.com>
Date: Thu, 14 Dec 2023 10:26:47 +0800
From: Liang Chen <liangchen.linux@...il.com>
To: Ilias Apalodimas <ilias.apalodimas@...aro.org>
Cc: Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net, edumazet@...gle.com, 
	pabeni@...hat.com, hawk@...nel.org, linyunsheng@...wei.com, 
	netdev@...r.kernel.org, linux-mm@...ck.org, jasowang@...hat.com, 
	almasrymina@...gle.com
Subject: Re: [PATCH net-next v8 4/4] skbuff: Optimization of SKB coalescing
 for page pool

On Wed, Dec 13, 2023 at 3:10 PM Ilias Apalodimas
<ilias.apalodimas@...aro.org> wrote:
>
> Hi Jakub,
>
> On Mon, 11 Dec 2023 at 22:14, Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Mon, 11 Dec 2023 09:46:55 +0200 Ilias Apalodimas wrote:
> > > As I said in the past the patch look correct. I don't like the fact
> > > that more pp internals creep into the default network stack, but
> > > perhaps this is fine with the bigger adoption?
> > > Jakub any thoughts/objections?
> >
> > Now that you asked... the helper does seem to be in sort of
> > a in-between state of being skb specific.
> >
> > What worries me is that this:
> >
> > +/**
> > + * 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);
> > +}
> >
> > doesn't even document that the caller must make sure that the skb
> > which owns this page is marked for pp recycling. The caller added
> > by this patch does that, but we should indicate somewhere that doing
> > skb_pp_frag_ref() for frag in a non-pp-recycling skb is not correct.
>
> Correct
>
> >
> > We can either lean in the direction of making it less skb specific,
> > put the code in page_pool.c / helpers.h and make it clear that the
> > caller has to be careful.
> > Or we make it more skb specific, take a skb pointer as arg, and also
> > look at its recycling marking..
> > or just improve the kdoc.
>
> I've mentioned this in the past, but I generally try to prevent people
> from shooting themselves in the foot when creating APIs. Unless
> there's a proven performance hit, I'd move the pp_recycle checking in
> skb_pp_frag_ref().
>

/**
 * skb_pp_frag_ref() - Increase fragment references of a page pool aware skb
 * @skb:    page pool aware skb
 *
 * Increase the fragment reference count (pp_ref_count) of a skb. This is
 * intended to gain fragment references only for page pool aware skbs,
 * i.e. when skb->pp_recycle is true, and not for fragments in a
 * non-pp-recycling skb. It has a fallback to increase references on normal
 * pages, as page pool aware skbs may also have normal page fragments.
 */

Sure. Below is a snippet of the implementation for skb_pp_frag_ref,
which takes an skb as its argument. The loop that iterates through the
frags has been moved inside the function to avoid checking
skb->pp_recycle each time a frag reference is taken(though there would
be some optimization from the compiler). If there is no objection, it
will be included in v10. Thanks!

static int skb_pp_frag_ref(struct sk_buff *skb)
{
    struct skb_shared_info *shinfo;
    struct page *head_page;
    int i;

    if (!skb->pp_recycle)
        return -EINVAL;

    shinfo = skb_shinfo(skb);

    for (i = 0; i < shinfo->nr_frags; i++){
        head_page = compound_head(skb_frag_page(&shinfo->frags[i]));
        if (likely(is_pp_page(head_page)))
            page_pool_ref_page(head_page);
        else
            page_ref_inc(head_page);
    }
    return 0;
}

/* if the skb is not cloned this does nothing
 * since we set nr_frags to 0.
 */
if (skb_pp_frag_ref(from)) {
    for (i = 0; i < from_shinfo->nr_frags; i++)
        __skb_frag_ref(&from_shinfo->frags[i]);
}

> Thanks
> /Ilias
>
> /Ilias