On 15/07/2021 05:45, Pravin Shelar wrote:

On Wed, Jun 30, 2021 at 2:53 AM Mark Gray [off-list ref] wrote:

quoted

The Open vSwitch kernel module uses the upcall mechanism to send
packets from kernel space to user space when it misses in the kernel
space flow table. The upcall sends packets via a Netlink socket.
Currently, a Netlink socket is created for every vport. In this way,
there is a 1:1 mapping between a vport and a Netlink socket.
When a packet is received by a vport, if it needs to be sent to
user space, it is sent via the corresponding Netlink socket.

This mechanism, with various iterations of the corresponding user
space code, has seen some limitations and issues:

* On systems with a large number of vports, there is a correspondingly
large number of Netlink sockets which can limit scaling.
(https://bugzilla.redhat.com/show_bug.cgi?id=1526306)
* Packet reordering on upcalls.
(https://bugzilla.redhat.com/show_bug.cgi?id=1844576)
* A thundering herd issue.
(https://bugzilla.redhat.com/show_bug.cgi?id=1834444)

This patch introduces an alternative, feature-negotiated, upcall
mode using a per-cpu dispatch rather than a per-vport dispatch.

In this mode, the Netlink socket to be used for the upcall is
selected based on the CPU of the thread that is executing the upcall.
In this way, it resolves the issues above as:

a) The number of Netlink sockets scales with the number of CPUs
rather than the number of vports.
b) Ordering per-flow is maintained as packets are distributed to
CPUs based on mechanisms such as RSS and flows are distributed
to a single user space thread.
c) Packets from a flow can only wake up one user space thread.

The corresponding user space code can be found at:
https://mail.openvswitch.org/pipermail/ovs-dev/2021-April/382618.html

Bugzilla: https://bugzilla.redhat.com/1844576
Signed-off-by: Mark Gray <redacted>
---

Notes:
    v1 - Reworked based on Flavio's comments:
         * Fixed handling of userspace action case
         * Renamed 'struct dp_portids'
         * Fixed handling of return from kmalloc()
         * Removed check for dispatch type from ovs_dp_get_upcall_portid()
       - Reworked based on Dan's comments:
         * Fixed handling of return from kmalloc()
       - Reworked based on Pravin's comments:
         * Fixed handling of userspace action case
       - Added kfree() in destroy_dp_rcu() to cleanup netlink port ids

Patch looks good to me. I have the following minor comments.

quoted

 include/uapi/linux/openvswitch.h |  8 ++++
 net/openvswitch/actions.c        |  6 ++-
 net/openvswitch/datapath.c       | 70 +++++++++++++++++++++++++++++++-
 net/openvswitch/datapath.h       | 20 +++++++++
 4 files changed, 101 insertions(+), 3 deletions(-)

diff --git a/include/uapi/linux/openvswitch.h b/include/uapi/linux/openvswitch.h
index 8d16744edc31..6571b57b2268 100644
--- a/include/uapi/linux/openvswitch.h
+++ b/include/uapi/linux/openvswitch.h

@@ -70,6 +70,8 @@ enum ovs_datapath_cmd {
  * set on the datapath port (for OVS_ACTION_ATTR_MISS).  Only valid on
  * %OVS_DP_CMD_NEW requests. A value of zero indicates that upcalls should
  * not be sent.
+ * OVS_DP_ATTR_PER_CPU_PIDS: Per-cpu array of PIDs for upcalls when
+ * OVS_DP_F_DISPATCH_UPCALL_PER_CPU feature is set.
  * @OVS_DP_ATTR_STATS: Statistics about packets that have passed through the
  * datapath.  Always present in notifications.
  * @OVS_DP_ATTR_MEGAFLOW_STATS: Statistics about mega flow masks usage for the

@@ -87,6 +89,9 @@ enum ovs_datapath_attr {
        OVS_DP_ATTR_USER_FEATURES,      /* OVS_DP_F_*  */
        OVS_DP_ATTR_PAD,
        OVS_DP_ATTR_MASKS_CACHE_SIZE,
+       OVS_DP_ATTR_PER_CPU_PIDS,   /* Netlink PIDS to receive upcalls in per-cpu
+                                    * dispatch mode
+                                    */
        __OVS_DP_ATTR_MAX
 };

@@ -127,6 +132,9 @@ struct ovs_vport_stats {
 /* Allow tc offload recirc sharing */
 #define OVS_DP_F_TC_RECIRC_SHARING     (1 << 2)

+/* Allow per-cpu dispatch of upcalls */
+#define OVS_DP_F_DISPATCH_UPCALL_PER_CPU       (1 << 3)
+
 /* Fixed logical ports. */
 #define OVSP_LOCAL      ((__u32)0)

diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
index ef15d9eb4774..f79679746c62 100644
--- a/net/openvswitch/actions.c
+++ b/net/openvswitch/actions.c

@@ -924,7 +924,11 @@ static int output_userspace(struct datapath *dp, struct sk_buff *skb,
                        break;

                case OVS_USERSPACE_ATTR_PID:
-                       upcall.portid = nla_get_u32(a);
+                       if (dp->user_features & OVS_DP_F_DISPATCH_UPCALL_PER_CPU)
+                               upcall.portid =
+                                  ovs_dp_get_upcall_portid(dp, smp_processor_id());
+                       else
+                               upcall.portid = nla_get_u32(a);
                        break;

                case OVS_USERSPACE_ATTR_EGRESS_TUN_PORT: {

diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
index bc164b35e67d..8d54fa323543 100644
--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c

@@ -166,6 +166,7 @@ static void destroy_dp_rcu(struct rcu_head *rcu)
        free_percpu(dp->stats_percpu);
        kfree(dp->ports);
        ovs_meters_exit(dp);
+       kfree(dp->upcall_portids);
        kfree(dp);
 }

@@ -239,7 +240,12 @@ void ovs_dp_process_packet(struct sk_buff *skb, struct sw_flow_key *key)

                memset(&upcall, 0, sizeof(upcall));
                upcall.cmd = OVS_PACKET_CMD_MISS;
-               upcall.portid = ovs_vport_find_upcall_portid(p, skb);
+
+               if (dp->user_features & OVS_DP_F_DISPATCH_UPCALL_PER_CPU)
+                       upcall.portid = ovs_dp_get_upcall_portid(dp, smp_processor_id());
+               else
+                       upcall.portid = ovs_vport_find_upcall_portid(p, skb);
+
                upcall.mru = OVS_CB(skb)->mru;
                error = ovs_dp_upcall(dp, skb, key, &upcall, 0);
                if (unlikely(error))

@@ -1594,16 +1600,67 @@ static void ovs_dp_reset_user_features(struct sk_buff *skb,

 DEFINE_STATIC_KEY_FALSE(tc_recirc_sharing_support);

+int ovs_dp_set_upcall_portids(struct datapath *dp,
+                             const struct nlattr *ids)

this can be static function.

Yes

quoted

+{
+       struct dp_nlsk_pids *old, *dp_nlsk_pids;
+
+       if (!nla_len(ids) || nla_len(ids) % sizeof(u32))
+               return -EINVAL;
+
+       old = ovsl_dereference(dp->upcall_portids);
+
+       dp_nlsk_pids = kmalloc(sizeof(*dp_nlsk_pids) + nla_len(ids),
+                              GFP_KERNEL);
+       if (!dp_nlsk_pids)
+               return -ENOMEM;
+
+       dp_nlsk_pids->n_pids = nla_len(ids) / sizeof(u32);
+       nla_memcpy(dp_nlsk_pids->pids, ids, nla_len(ids));
+
+       rcu_assign_pointer(dp->upcall_portids, dp_nlsk_pids);
+
+       kfree_rcu(old, rcu);
+
+       return 0;
+}
+
+u32 ovs_dp_get_upcall_portid(const struct datapath *dp, uint32_t cpu_id)

same here, it can be static.

This cannot as it gets called in actions.c

quoted

+{
+       struct dp_nlsk_pids *dp_nlsk_pids;
+
+       dp_nlsk_pids = rcu_dereference_ovsl(dp->upcall_portids);

I dont think this function is only called under ovs-lock, so we can
change it to rcu_dereference().

I had a quick look through the code and I think you are right so I will
change it.

quoted

+
+       if (dp_nlsk_pids) {
+               if (cpu_id < dp_nlsk_pids->n_pids) {
+                       return dp_nlsk_pids->pids[cpu_id];
+               } else if (dp_nlsk_pids->n_pids > 0 && cpu_id >= dp_nlsk_pids->n_pids) {
+                       /* If the number of netlink PIDs is mismatched with the number of
+                        * CPUs as seen by the kernel, log this and send the upcall to an
+                        * arbitrary socket (0) in order to not drop packets
+                        */
+                       pr_info_ratelimited("cpu_id mismatch with handler threads");
+                       return dp_nlsk_pids->pids[cpu_id % dp_nlsk_pids->n_pids];
+               } else {
+                       return 0;
+               }
+       } else {
+               return 0;
+       }
+}
+