Re: [PATCH bpf-next v3 3/3] xsk: build skb by page

[Alexander Lobakin <alobakin@pm.me>]

From: Magnus Karlsson <magnus.karlsson@gmail.com>
Date: Fri, 22 Jan 2021 13:18:47 +0100

> On Fri, Jan 22, 2021 at 12:57 PM Alexander Lobakin <alobakin@pm.me> wrote:
> >
> > From: Alexander Lobakin <alobakin@pm.me>
> > Date: Fri, 22 Jan 2021 11:47:45 +0000
> >
> > > From: Eric Dumazet <eric.dumazet@gmail.com>
> > > Date: Thu, 21 Jan 2021 16:41:33 +0100
> > >
> > > > On 1/21/21 2:47 PM, Xuan Zhuo wrote:
> > > > > This patch is used to construct skb based on page to save memory copy
> > > > > overhead.
> > > > >
> > > > > This function is implemented based on IFF_TX_SKB_NO_LINEAR. Only the
> > > > > network card priv_flags supports IFF_TX_SKB_NO_LINEAR will use page to
> > > > > directly construct skb. If this feature is not supported, it is still
> > > > > necessary to copy data to construct skb.
> > > > >
> > > > > ---------------- Performance Testing ------------
> > > > >
> > > > > The test environment is Aliyun ECS server.
> > > > > Test cmd:
> > > > > ```
> > > > > xdpsock -i eth0 -t  -S -s <msg size>
> > > > > ```
> > > > >
> > > > > Test result data:
> > > > >
> > > > > size    64      512     1024    1500
> > > > > copy    1916747 1775988 1600203 1440054
> > > > > page    1974058 1953655 1945463 1904478
> > > > > percent 3.0%    10.0%   21.58%  32.3%
> > > > >
> > > > > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > > > Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
> > > > > ---
> > > > >  net/xdp/xsk.c | 104 ++++++++++++++++++++++++++++++++++++++++++++++++----------
> > > > >  1 file changed, 86 insertions(+), 18 deletions(-)
> > > > >
> > > > > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > > > > index 4a83117..38af7f1 100644
> > > > > --- a/net/xdp/xsk.c
> > > > > +++ b/net/xdp/xsk.c
> > > > > @@ -430,6 +430,87 @@ static void xsk_destruct_skb(struct sk_buff *skb)
> > > > >   sock_wfree(skb);
> > > > >  }
> > > > >
> > > > > +static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
> > > > > +                                       struct xdp_desc *desc)
> > > > > +{
> > > > > + u32 len, offset, copy, copied;
> > > > > + struct sk_buff *skb;
> > > > > + struct page *page;
> > > > > + void *buffer;
> > > > > + int err, i;
> > > > > + u64 addr;
> > > > > +
> > > > > + skb = sock_alloc_send_skb(&xs->sk, 0, 1, &err);
> > > > > + if (unlikely(!skb))
> > > > > +         return ERR_PTR(err);
> > > > > +
> > > > > + addr = desc->addr;
> > > > > + len = desc->len;
> > > > > +
> > > > > + buffer = xsk_buff_raw_get_data(xs->pool, addr);
> > > > > + offset = offset_in_page(buffer);
> > > > > + addr = buffer - xs->pool->addrs;
> > > > > +
> > > > > + for (copied = 0, i = 0; copied < len; i++) {
> > > > > +         page = xs->pool->umem->pgs[addr >> PAGE_SHIFT];
> > > > > +
> > > > > +         get_page(page);
> > > > > +
> > > > > +         copy = min_t(u32, PAGE_SIZE - offset, len - copied);
> > > > > +
> > > > > +         skb_fill_page_desc(skb, i, page, offset, copy);
> > > > > +
> > > > > +         copied += copy;
> > > > > +         addr += copy;
> > > > > +         offset = 0;
> > > > > + }
> > > > > +
> > > > > + skb->len += len;
> > > > > + skb->data_len += len;
> > > >
> > > > > + skb->truesize += len;
> > > >
> > > > This is not the truesize, unfortunately.
> > > >
> > > > We need to account for the number of pages, not number of bytes.
> > >
> > > The easiest solution is:
> > >
> > >       skb->truesize += PAGE_SIZE * i;
> > >
> > > i would be equal to skb_shinfo(skb)->nr_frags after exiting the loop.
> >
> > Oops, pls ignore this. I forgot that XSK buffers are not
> > "one per page".
> > We need to count the number of pages manually and then do
> >
> >         skb->truesize += PAGE_SIZE * npages;
> >
> > Right.
> 
> There are two possible packet buffer (chunks) sizes in a umem, 2K and
> 4K on a system with a PAGE_SIZE of 4K. If I remember correctly, and
> please correct me if wrong, truesize is used for memory accounting.
> But in this code, no kernel memory has been allocated (apart from the
> skb). The page is just a part of the umem that has been already
> allocated beforehand and by user-space in this case. So what should
> truesize be in this case? Do we add 0, chunk_size * i, or the
> complicated case of counting exactly how many 4K pages that are used
> when the chunk_size is 2K, as two chunks could occupy the same page,
> or just the upper bound of PAGE_SIZE * i that is likely a good
> approximation in most cases? Just note that there might be other uses
> of truesize that I am unaware of that could impact this choice.

Truesize is "what amount of memory does this skb occupy with all its
fragments, linear space and struct sk_buff itself". The closest it
will be to the actual value, the better.
In this case, I think adding of chunk_size * i would be enough.

(PAGE_SIZE * i can be overwhelming when chunk_size is 2K, especially
for setups with PAGE_SIZE > SZ_4K)

> > > > > +
> > > > > + refcount_add(len, &xs->sk.sk_wmem_alloc);
> > > > > +
> > > > > + return skb;
> > > > > +}
> > > > > +
> > >
> > > Al
> >
> > Thanks,
> > Al

Al