Subject: Re: [PATCH net-next v2 12/18] openvswitch: Move ovs_frag_data_storage into the struct ovs_pcpu_storage

From: Aaron Conole <[email-masked]>
Date: Tue, 15 Apr 2025 12:26:13 -0400

Sebastian Andrzej Siewior <[email-masked]> writes:

> ovs_frag_data_storage is a per-CPU variable and relies on disabled BH for its
> locking. Without per-CPU locking in local_bh_disable() on PREEMPT_RT
> this data structure requires explicit locking.
>
> Move ovs_frag_data_storage into the struct ovs_pcpu_storage which already
> provides locking for the structure.
>
> Cc: Aaron Conole <[email-masked]>
> Cc: Eelco Chaudron <[email-masked]>
> Cc: Ilya Maximets <[email-masked]>
> Cc: [email-masked]
> Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
> ---

I'm going to reply here, but I need to bisect a bit more (though I
suspect the results below are due to 11/18).  When I tested with this
patch there were lots of "unexplained" latency spikes during processing
(note, I'm not doing PREEMPT_RT in my testing, but I guess it would
smooth the spikes out at the cost of max performance).

With the series:
[SUM]   0.00-300.00 sec  3.28 TBytes  96.1 Gbits/sec  9417             sender
[SUM]   0.00-300.00 sec  3.28 TBytes  96.1 Gbits/sec                  receiver

Without the series:
[SUM]   0.00-300.00 sec  3.26 TBytes  95.5 Gbits/sec  149             sender
[SUM]   0.00-300.00 sec  3.26 TBytes  95.5 Gbits/sec                  receiver

And while the 'final' numbers might look acceptable, one thing I'll note
is I saw multiple stalls as:

[  5]  57.00-58.00  sec   128 KBytes   903 Kbits/sec    0   4.02 MBytes

But without the patch, I didn't see such stalls.  My testing:

1. Install openvswitch userspace and ipcalc
2. start userspace.
3. Setup two netns and connect them (I have a more complicated script to
   set up the flows, and I can send that to you)
4. Use iperf3 to test (-P5 -t 300)

As I wrote I suspect the locking in 11 is leading to these stalls, as
the data I'm sending shouldn't be hitting the frag path.

Do these results seem expected to you?

>  net/openvswitch/actions.c  | 20 ++------------------
>  net/openvswitch/datapath.h | 16 ++++++++++++++++
>  2 files changed, 18 insertions(+), 18 deletions(-)
>
> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
> index f4996c11aefac..4d20eadd77ceb 100644
> --- a/net/openvswitch/actions.c
> +++ b/net/openvswitch/actions.c
> @@ -39,22 +39,6 @@
>  #include "flow_netlink.h"
>  #include "openvswitch_trace.h"
>  
> -#define MAX_L2_LEN	(VLAN_ETH_HLEN + 3 * MPLS_HLEN)
> -struct ovs_frag_data {
> -	unsigned long dst;
> -	struct vport *vport;
> -	struct ovs_skb_cb cb;
> -	__be16 inner_protocol;
> -	u16 network_offset;	/* valid only for MPLS */
> -	u16 vlan_tci;
> -	__be16 vlan_proto;
> -	unsigned int l2_len;
> -	u8 mac_proto;
> -	u8 l2_data[MAX_L2_LEN];
> -};
> -
> -static DEFINE_PER_CPU(struct ovs_frag_data, ovs_frag_data_storage);
> -
>  DEFINE_PER_CPU(struct ovs_pcpu_storage, ovs_pcpu_storage) = {
>  	.bh_lock = INIT_LOCAL_LOCK(bh_lock),
>  };
> @@ -771,7 +755,7 @@ static int set_sctp(struct sk_buff *skb, struct sw_flow_key *flow_key,
>  static int ovs_vport_output(struct net *net, struct sock *sk,
>  			    struct sk_buff *skb)
>  {
> -	struct ovs_frag_data *data = this_cpu_ptr(&ovs_frag_data_storage);
> +	struct ovs_frag_data *data = this_cpu_ptr(&ovs_pcpu_storage.frag_data);
>  	struct vport *vport = data->vport;
>  
>  	if (skb_cow_head(skb, data->l2_len) < 0) {
> @@ -823,7 +807,7 @@ static void prepare_frag(struct vport *vport, struct sk_buff *skb,
>  	unsigned int hlen = skb_network_offset(skb);
>  	struct ovs_frag_data *data;
>  
> -	data = this_cpu_ptr(&ovs_frag_data_storage);
> +	data = this_cpu_ptr(&ovs_pcpu_storage.frag_data);
>  	data->dst = skb->_skb_refdst;
>  	data->vport = vport;
>  	data->cb = *OVS_CB(skb);
> diff --git a/net/openvswitch/datapath.h b/net/openvswitch/datapath.h
> index 4a665c3cfa906..1b5348b0f5594 100644
> --- a/net/openvswitch/datapath.h
> +++ b/net/openvswitch/datapath.h
> @@ -13,6 +13,7 @@
>  #include <linux/skbuff.h>
>  #include <linux/u64_stats_sync.h>
>  #include <net/ip_tunnels.h>
> +#include <net/mpls.h>
>  
>  #include "conntrack.h"
>  #include "flow.h"
> @@ -173,6 +174,20 @@ struct ovs_net {
>  	bool xt_label;
>  };
>  
> +#define MAX_L2_LEN	(VLAN_ETH_HLEN + 3 * MPLS_HLEN)
> +struct ovs_frag_data {
> +	unsigned long dst;
> +	struct vport *vport;
> +	struct ovs_skb_cb cb;
> +	__be16 inner_protocol;
> +	u16 network_offset;	/* valid only for MPLS */
> +	u16 vlan_tci;
> +	__be16 vlan_proto;
> +	unsigned int l2_len;
> +	u8 mac_proto;
> +	u8 l2_data[MAX_L2_LEN];
> +};
> +
>  struct deferred_action {
>  	struct sk_buff *skb;
>  	const struct nlattr *actions;
> @@ -200,6 +215,7 @@ struct action_flow_keys {
>  struct ovs_pcpu_storage {
>  	struct action_fifo action_fifos;
>  	struct action_flow_keys flow_keys;
> +	struct ovs_frag_data frag_data;
>  	int exec_level;
>  	struct task_struct *owner;
>  	local_lock_t bh_lock;