Message-ID: <20210125132502.3165-1-alobakin@pm.me>
Date:   Mon, 25 Jan 2021 13:25:45 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     Alexander Lobakin <alobakin@...me>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, bjorn@...nel.org,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        virtualization@...ts.linux-foundation.org, bpf@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 3/3] xsk: build skb by page

From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Date: Mon, 25 Jan 2021 11:10:43 +0800

> On Fri, 22 Jan 2021 16:24:17 +0000, Alexander Lobakin <alobakin@...me> wrote:
> > From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > Date: Fri, 22 Jan 2021 23:36:29 +0800
> >
> > > On Fri, 22 Jan 2021 12:08:00 +0000, Alexander Lobakin <alobakin@...me> wrote:
> > > > From: Alexander Lobakin <alobakin@...me>
> > > > Date: Fri, 22 Jan 2021 11:55:35 +0000
> > > >
> > > > > From: Alexander Lobakin <alobakin@...me>
> > > > > Date: Fri, 22 Jan 2021 11:47:45 +0000
> > > > >
> > > > > > From: Eric Dumazet <eric.dumazet@...il.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@...ux.alibaba.com>
> > > > > > > > Reviewed-by: Dust Li <dust.li@...ux.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);
> > > >
> > > > Also,
> > > > maybe we should allocate it with NET_SKB_PAD so NIC drivers could
> > > > use some reserved space?
> > > >
> > > > 		skb = sock_alloc_send_skb(&xs->sk, NET_SKB_PAD, 1, &err);
> > > > 		...
> > > > 		skb_reserve(skb, NET_SKB_PAD);
> > > >
> > > > Eric, what do you think?
> > >
> > > I think you are right. Some space should be added to continuous equipment. This
> > > space should also be added in the copy mode below. Is LL_RESERVED_SPACE more
> > > appropriate?
> >
> > No. If you look at __netdev_alloc_skb() and __napi_alloc_skb(), they
> > reserve NET_SKB_PAD at the beginning of linear area. Documentation of
> > __build_skb() also says that driver should reserve NET_SKB_PAD before
> > the actual frame, so it is a standartized hardware-independent
> > headroom.
> 
> I understand that these scenarios are in the case of receiving packets, and the
> increased space is used by the protocol stack, especially RPS. I don't know if
> this also applies to the sending scenario?
> 
> > Leaving that space in skb->head will allow developers to implement
> > IFF_TX_SKB_NO_LINEAR in a wider variety of drivers, especially when
> > a driver has to prepend some sort of data before the actual frame.
> > Since it's usually of a size of one cacheline, shouldn't be a big
> > deal.
> >
> 
> I agree with this. Some network cards require some space. For example,
> virtio-net needs to add a virtio_net_hdr_mrg_rxbuf before skb->data, so my
> original understanding is used here. When we send the skb to the
> driver, the driver may need a memory space. So I refer to the
> implementation of __ip_append_data, I feel that adding
> LL_RESERVED_SPACE is a suitable solution.
> 
> I feel that I may still not understand the use scene you mentioned. Can you
> elaborate on what you understand this space will be used for?

LL_RESERVED_SPACE() consists of L2 header size (Ethernet for the most
cases) and dev->needed_headroom. That is not a value to count on, as:
 - L2 header is already here in XSK buffer;
 - not all drivers set dev->needed_headroom;
 - it's aligned by 16, not L1_CACHE_SIZE.

As this path is XSK generic path, i.e. when driver-side XSK is not
present or not requested, it can be applied to every driver. Many
of them call skb_cow_head() + skb_push() on their xmit path:
 - nearly all virtual drivers (to insert their specific headers);
 - nearly all switch drivers (to insert switch CPU port tags);
 - some enterprise NIC drivers (ChelsIO for LSO, Netronome
   for TLS etc.).

skb_cow_head() + skb_push() relies on a required NET_SKB_PAD headroom.
In case where there is no enough space (and you allocate an skb with
no headroom at all), skb will be COWed, which is a huge overhead and
will cause slowdowns.
So, adding NET_SKB_PAD would save from almost all, if not all, such
reallocations.

> Thanks.
> 
> >
> > [ I also had an idea of allocating an skb with a headroom of
> > NET_SKB_PAD + 256 bytes, so nearly all drivers could just call
> > pskb_pull_tail() to support such type of skbuffs without much
> > effort, but I think that it's better to teach drivers to support
> > xmitting of really headless ones. If virtio_net can do it, why
> > shouldn't the others ]
> >
> > > > > > > > +	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.
> > > > >
> > > > > > > > +
> > > > > > > > +	refcount_add(len, &xs->sk.sk_wmem_alloc);
> > > > > > > > +
> > > > > > > > +	return skb;
> > > > > > > > +}
> > > > > > > > +
> > > > > >
> > > > > > Al
> > > > >
> > > > > Thanks,
> > > > > Al
> > > >
> > > > Al

Thanks,
Al

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