Re: [PATCH v2 net-next] net: introduce gro_frag_list_enable sysctl

Jerry Chu
2013-10-30T00:03:21Z

On Tue, Oct 29, 2013 at 8:12 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> From: Eric Dumazet <edumazet@google.com>
>
> Christoph Paasch and Jerry Chu reported crashes in skb_segment() caused
> by commit 8a29111c7ca6 ("net: gro: allow to build full sized skb")
>
> (Jerry is working on adding native GRO support for tunnels)
>
> skb_segment() only deals with a frag_list chain containing MSS sized
> fragments.
>
> This patch adds support any kind of frag, and adds a new sysctl,
> as clearly the GRO layer should avoid building frag_list skbs
> on a router, as the segmentation is adding cpu overhead.
>
> Note that we could try to reuse page fragments instead of doing
> copy to linear skbs, but this requires a fair amount of work,
> and possible truesize nightmares, as we do not track individual
> (per page fragment) truesizes.
>
> /proc/sys/net/core/gro_frag_list_enable possible values are :
>
> 0 : GRO layer is not allowed to use frag_list to extend skb capacity
> 1 : GRO layer is allowed to use frag_list, but skb_segment()
> automatically sets the sysctl to 0.
> 2 : GRO is allowed to use frag_list, and skb_segment() wont
> clear the sysctl.
>
> Default value is 1 : automatic discovery
>
> Reported-by: Christoph Paasch <christoph.paasch@uclouvain.be>
> Reported-by: Jerry Chu <hkchu@google.com>
> Cc: Michael Dalton <mwdalton@google.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> ---
> v2: added missing sysctl definition in skbuff.c
>
> Documentation/sysctl/net.txt | 19 +++++++++++++++++++
> include/linux/netdevice.h | 1 +
> net/core/skbuff.c | 31 ++++++++++++++++++++++---------
> net/core/sysctl_net_core.c | 10 ++++++++++
> 4 files changed, 52 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/sysctl/net.txt b/Documentation/sysctl/net.txt
> index 9a0319a82470..8778568ae64e 100644
> --- a/Documentation/sysctl/net.txt
> +++ b/Documentation/sysctl/net.txt
> @@ -87,6 +87,25 @@ sysctl.net.busy_read globally.
> Will increase power usage.
> Default: 0 (off)
>
> +gro_frag_list_enable
> +--------------------
> +
> +GRO layer can build full size GRO packets (~64K of payload) if it is allowed
> +to extend skb using the frag_list pointer. However, this strategy is a win
> +on hosts, where TCP flows are terminated. For a router, using frag_list
> +skbs is not a win because we have to segment skbs before transmit,
> +as most NIC drivers do not support frag_list.
> +As soon as one frag_list skb has to be segmented, this sysctl is automatically
> +changed from 1 to 0.
> +If the value is set to 2, kernel wont change it.
> +
> +Choices : 0 (off),
> + 1 (on, with automatic change to 0)
> + 2 (on, permanent)
> +
> +Default: 1 (on, with automatic downgrade on a router)
> +
> +
> rmem_default
> ------------
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 27f62f746621..b82ff52f301e 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -2807,6 +2807,7 @@ extern int netdev_max_backlog;
> extern int netdev_tstamp_prequeue;
> extern int weight_p;
> extern int bpf_jit_enable;
> +extern int sysctl_gro_frag_list_enable;
>
> bool netdev_has_upper_dev(struct net_device *dev, struct net_device *upper_dev);
> bool netdev_has_any_upper_dev(struct net_device *dev);
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 0ab32faa520f..e089cd2782e5 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -74,6 +74,8 @@
> struct kmem_cache *skbuff_head_cache __read_mostly;
> static struct kmem_cache *skbuff_fclone_cache __read_mostly;
>
> +int sysctl_gro_frag_list_enable __read_mostly = 1;
> +
> static void sock_pipe_buf_release(struct pipe_inode_info *pipe,
> struct pipe_buffer *buf)
> {
> @@ -2761,7 +2763,7 @@ struct sk_buff *skb_segment(struct sk_buff *skb, netdev_features_t features)
> unsigned int len;
> __be16 proto;
> bool csum;
> - int sg = !!(features & NETIF_F_SG);
> + bool sg = !!(features & NETIF_F_SG);
> int nfrags = skb_shinfo(skb)->nr_frags;
> int err = -ENOMEM;
> int i = 0;
> @@ -2793,7 +2795,13 @@ struct sk_buff *skb_segment(struct sk_buff *skb, netdev_features_t features)
> hsize = len;
>
> if (!hsize && i >= nfrags) {
> - BUG_ON(fskb->len != len);
> + if (fskb->len != len) {
> + if (sysctl_gro_frag_list_enable == 1)
> + sysctl_gro_frag_list_enable = 0;
> + hsize = len;
> + sg = false;
> + goto do_linear;
> + }
>
> pos += len;
> nskb = skb_clone(fskb, GFP_ATOMIC);
> @@ -2812,6 +2820,7 @@ struct sk_buff *skb_segment(struct sk_buff *skb, netdev_features_t features)
> skb_release_head_state(nskb);
> __skb_push(nskb, doffset);
> } else {
> +do_linear:
> nskb = __alloc_skb(hsize + doffset + headroom,
> GFP_ATOMIC, skb_alloc_rx_flag(skb),
> NUMA_NO_NODE);
> @@ -2838,9 +2847,6 @@ struct sk_buff *skb_segment(struct sk_buff *skb, netdev_features_t features)
> nskb->data - tnl_hlen,
> doffset + tnl_hlen);
>
> - if (fskb != skb_shinfo(skb)->frag_list)
> - goto perform_csum_check;
> -
> if (!sg) {
> nskb->ip_summed = CHECKSUM_NONE;
> nskb->csum = skb_copy_and_csum_bits(skb, offset,
> @@ -2849,6 +2855,9 @@ struct sk_buff *skb_segment(struct sk_buff *skb, netdev_features_t features)
> continue;
> }
>
> + if (fskb != skb_shinfo(skb)->frag_list)
> + goto perform_csum_check;
> +
> frag = skb_shinfo(nskb)->frags;
>
> skb_copy_from_linear_data_offset(skb, offset,
> @@ -2944,9 +2953,11 @@ int skb_gro_receive(struct sk_buff **head, struct sk_buff *skb)
> int i = skbinfo->nr_frags;
> int nr_frags = pinfo->nr_frags + i;
>
> - if (nr_frags > MAX_SKB_FRAGS)
> + if (unlikely(nr_frags > MAX_SKB_FRAGS)) {
> + if (!sysctl_gro_frag_list_enable)
> + return -E2BIG;
> goto merge;
> -
> + }
> offset -= headlen;
> pinfo->nr_frags = nr_frags;
> skbinfo->nr_frags = 0;
> @@ -2977,9 +2988,11 @@ int skb_gro_receive(struct sk_buff **head, struct sk_buff *skb)
> unsigned int first_size = headlen - offset;
> unsigned int first_offset;
>
> - if (nr_frags + 1 + skbinfo->nr_frags > MAX_SKB_FRAGS)
> + if (unlikely(nr_frags + 1 + skbinfo->nr_frags > MAX_SKB_FRAGS)) {
> + if (!sysctl_gro_frag_list_enable)
> + return -E2BIG;
> goto merge;
> -
> + }
> first_offset = skb->data -
> (unsigned char *)page_address(page) +
> offset;
> diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
> index cca444190907..2d6aaf6d5838 100644
> --- a/net/core/sysctl_net_core.c
> +++ b/net/core/sysctl_net_core.c
> @@ -24,6 +24,7 @@
>
> static int zero = 0;
> static int one = 1;
> +static int two = 2;
> static int ushort_max = USHRT_MAX;
>
> #ifdef CONFIG_RPS
> @@ -360,6 +361,15 @@ static struct ctl_table net_core_table[] = {
> .mode = 0644,
> .proc_handler = proc_dointvec
> },
> + {
> + .procname = "gro_frag_list_enable",
> + .data = &sysctl_gro_frag_list_enable,
> + .maxlen = sizeof(int),
> + .mode = 0644,
> + .proc_handler = proc_dointvec_minmax,
> + .extra1 = &zero,
> + .extra2 = &two,
> + },
> { }
> };
>
>
>

I've tested all three values of sysctl_gro_frag_list_enable with my
yet-to-submitted GRE-GRO patch running on a router. The BUG panic is
gone and the patch seems to work as documented. Also setting the value
to 0 or 1 does seem to save a bit of CPU time on the forwarding path
and also help throughput a little (a combined of 5-8%).

Tested by: Jerry Chu <hkchu@google.com>