[ovs-dev] [RFC net-next] openvswitch: Introduce per-cpu upcall dispatch
Flavio Leitner
fbl at sysclose.org
Fri May 28 19:49:46 UTC 2021
Hi Mark,
I think this patch is going in the right direction but there
are some points that I think we should address. See below.
On Fri, Apr 30, 2021 at 11:33:25AM -0400, Mark Gray wrote:
> 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
Thanks for writing a nice commit description.
>
> Bugzilla: https://bugzilla.redhat.com/1844576
> Signed-off-by: Mark Gray <mark.d.gray at redhat.com>
> ---
> include/uapi/linux/openvswitch.h | 8 ++++
> net/openvswitch/datapath.c | 70 +++++++++++++++++++++++++++++++-
> net/openvswitch/datapath.h | 18 ++++++++
> net/openvswitch/flow_netlink.c | 4 --
> 4 files changed, 94 insertions(+), 6 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/datapath.c b/net/openvswitch/datapath.c
> index 9d6ef6cb9b26..98d54f41fdaa 100644
> --- a/net/openvswitch/datapath.c
> +++ b/net/openvswitch/datapath.c
> @@ -121,6 +121,8 @@ int lockdep_ovsl_is_held(void)
> #endif
>
> static struct vport *new_vport(const struct vport_parms *);
> +static u32 ovs_dp_get_upcall_portid(const struct datapath *, uint32_t);
> +static int ovs_dp_set_upcall_portids(struct datapath *, const struct nlattr *);
> static int queue_gso_packets(struct datapath *dp, struct sk_buff *,
> const struct sw_flow_key *,
> const struct dp_upcall_info *,
> @@ -238,7 +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);
> +
We don't expect that to change during packet processing, so I
wondered if that could be improved. However, user_feature is
in the same cache line as stats_percpu which is also used.
> upcall.mru = OVS_CB(skb)->mru;
> error = ovs_dp_upcall(dp, skb, key, &upcall, 0);
> if (unlikely(error))
> @@ -1590,16 +1597,67 @@ static void ovs_dp_reset_user_features(struct sk_buff *skb,
>
> DEFINE_STATIC_KEY_FALSE(tc_recirc_sharing_support);
>
> +static int ovs_dp_set_upcall_portids(struct datapath *dp,
> + const struct nlattr *ids)
> +{
> + struct dp_portids *old, *dp_portids;
> +
> + if (!nla_len(ids) || nla_len(ids) % sizeof(u32))
> + return -EINVAL;
> +
> + old = ovsl_dereference(dp->upcall_portids);
> +
> + dp_portids = kmalloc(sizeof(*dp_portids) + nla_len(ids),
> + GFP_KERNEL);
> + if (!dp)
I suspect you meant dp_portids.
> + return -ENOMEM;
> +
> + dp_portids->n_ids = nla_len(ids) / sizeof(u32);
> + nla_memcpy(dp_portids->ids, ids, nla_len(ids));
> +
> + rcu_assign_pointer(dp->upcall_portids, dp_portids);
> +
> + if (old)
> + kfree_rcu(old, rcu);
IIRC, the kfree_rcu() checks for NULL pointers.
> + return 0;
> +}
> +
> +static u32 ovs_dp_get_upcall_portid(const struct datapath *dp, uint32_t cpu_id)
> +{
> + struct dp_portids *dp_portids;
> +
> + dp_portids = rcu_dereference_ovsl(dp->upcall_portids);
> +
> + if (dp->user_features & OVS_DP_F_DISPATCH_UPCALL_PER_CPU && dp_portids) {
The OVS_DP_F_DISPATCH_UPCALL_PER_CPU needs to be verified
by the caller to decide whether to use this function or
ovs_vport_find_upcall_portid(), so perhaps function could
check only if dp_portids is set.
> + if (cpu_id < dp_portids->n_ids) {
> + return dp_portids->ids[cpu_id];
> + } else if (dp_portids->n_ids > 0 && cpu_id >= dp_portids->n_ids) {
> + /* 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_portids->ids[0];
Instead of returning a fixed pid for all other unmapped CPUs, perhaps
it could distribute among mapped ones to help with the load:
return dp_portids->ids[cpu_id % dp_portids->n_ids]
> + } else {
> + return 0;
> + }
> + } else {
> + return 0;
> + }
> +}
> +
> static int ovs_dp_change(struct datapath *dp, struct nlattr *a[])
> {
> u32 user_features = 0;
> + int err;
>
> if (a[OVS_DP_ATTR_USER_FEATURES]) {
> user_features = nla_get_u32(a[OVS_DP_ATTR_USER_FEATURES]);
>
> if (user_features & ~(OVS_DP_F_VPORT_PIDS |
> OVS_DP_F_UNALIGNED |
> - OVS_DP_F_TC_RECIRC_SHARING))
> + OVS_DP_F_TC_RECIRC_SHARING |
> + OVS_DP_F_DISPATCH_UPCALL_PER_CPU))
> return -EOPNOTSUPP;
>
> #if !IS_ENABLED(CONFIG_NET_TC_SKB_EXT)
> @@ -1620,6 +1678,14 @@ static int ovs_dp_change(struct datapath *dp, struct nlattr *a[])
>
> dp->user_features = user_features;
>
> + if (dp->user_features & OVS_DP_F_DISPATCH_UPCALL_PER_CPU &&
> + a[OVS_DP_ATTR_PER_CPU_PIDS]) {
> + /* Upcall Netlink Port IDs have been updated */
> + err = ovs_dp_set_upcall_portids(dp, a[OVS_DP_ATTR_PER_CPU_PIDS]);
> + if (err)
> + return err;
> + }
It is possible to switch between per_cpu_pids and per_ports_pids
and vice versa. It seems this patch doesn't count for that.
> +
> if (dp->user_features & OVS_DP_F_TC_RECIRC_SHARING)
> static_branch_enable(&tc_recirc_sharing_support);
> else
> diff --git a/net/openvswitch/datapath.h b/net/openvswitch/datapath.h
> index 38f7d3e66ca6..6003eba81958 100644
> --- a/net/openvswitch/datapath.h
> +++ b/net/openvswitch/datapath.h
> @@ -50,6 +50,21 @@ struct dp_stats_percpu {
> struct u64_stats_sync syncp;
> };
>
> +/**
> + * struct dp_portids - array of netlink portids of for a datapath.
> + * This is used when OVS_DP_F_DISPATCH_UPCALL_PER_CPU
> + * is enabled and must be protected by rcu.
> + * @rcu: RCU callback head for deferred destruction.
> + * @n_ids: Size of @ids array.
> + * @ids: Array storing the Netlink socket PIDs indexed by CPU ID for packets
> + * that miss the flow table.
> + */
> +struct dp_portids {
> + struct rcu_head rcu;
> + u32 n_ids;
> + u32 ids[];
> +};
It doesn't have any relation to ports, so the name is somewhat
misleading and confusing with the existing one. What do you
think about the suggestion below:
struct dp_nlsk_pids {
struct rcu_head rcu;
u32 n_pids;
u32 pids[];
};
> +
> /**
> * struct datapath - datapath for flow-based packet switching
> * @rcu: RCU callback head for deferred destruction.
> @@ -61,6 +76,7 @@ struct dp_stats_percpu {
> * @net: Reference to net namespace.
> * @max_headroom: the maximum headroom of all vports in this datapath; it will
> * be used by all the internal vports in this dp.
> + * @upcall_portids: RCU protected 'struct dp_portids'.
> *
> * Context: See the comment on locking at the top of datapath.c for additional
> * locking information.
> @@ -87,6 +103,8 @@ struct datapath {
>
> /* Switch meters. */
> struct dp_meter_table meter_tbl;
> +
> + struct dp_portids __rcu *upcall_portids;
> };
>
> /**
> diff --git a/net/openvswitch/flow_netlink.c b/net/openvswitch/flow_netlink.c
> index fd1f809e9bc1..97242bc1d960 100644
> --- a/net/openvswitch/flow_netlink.c
> +++ b/net/openvswitch/flow_netlink.c
> @@ -2928,10 +2928,6 @@ static int validate_userspace(const struct nlattr *attr)
> if (error)
> return error;
>
> - if (!a[OVS_USERSPACE_ATTR_PID] ||
> - !nla_get_u32(a[OVS_USERSPACE_ATTR_PID]))
> - return -EINVAL;
> -
It can't be removed because it is still required by ovs_dp_upcall().
There is a problem with the action output_userspace which still
uses the netlink OVS_USERSPACE_ATTR_PID to set upcall.portid.
Later ovs_dp_upcall() returns -ENOTCONN if that is missing.
The userspace patchset posted together with this patch does:
static uint32_t
dpif_netlink_port_get_pid(const struct dpif *dpif_, odp_port_t port_no)
{
const struct dpif_netlink *dpif = dpif_netlink_cast(dpif_);
uint32_t ret;
/* In per-cpu dispatch mode, vports do not have an associated PID */
if (DISPATCH_MODE_PER_CPU(dpif)) {
return 0;
}
That is used by compose_slow_path() to create userspace actions. So,
it seems all userspace actions would be using id 0 and that would
discarded by the kernel with -ENOTCONN.
--
fbl
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