Message-ID: <6413ae41-1c26-9f5d-3d3e-5eaa82a764dd@redhat.com>
Date:   Wed, 30 Jun 2021 10:44:36 +0100
From:   Mark Gray <mark.d.gray@...hat.com>
To:     Pravin Shelar <pravin.ovn@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        ovs dev <dev@...nvswitch.org>
Subject: Re: [RFC net-next] openvswitch: Introduce per-cpu upcall dispatch

On 19/05/2021 22:47, Pravin Shelar wrote:

Thanks for the review and sorry for the delay. I will be more responsive
to any further changes from this point on.

> On Fri, Apr 30, 2021 at 8:33 AM Mark Gray <mark.d.gray@...hat.com> 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
>>
>> Bugzilla: https://bugzilla.redhat.com/1844576
>> Signed-off-by: Mark Gray <mark.d.gray@...hat.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);
>> +
>>                 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)
>> +               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);
>> +       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) {
>> +               if (cpu_id < dp_portids->n_ids) {
>> +                       return dp_portids->ids[cpu_id];
> Have you considered more than one port per CPU?

No. Do you think there would be an advantage to this?

Maybe I am missing something but I suppose only one handler thread can
ever be resident on a CPU at any one time so we couldn't handle more
than one netlink socket.

> 
>> +               } 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];
>> +               } 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;
>> +       }
>> +
> Since this takes precedence over OVS_DP_F_VPORT_PIDS, can we reject
> datapath with both options.

It's a good idea and I explored it but I came across a problem that made
this difficult (impossible?) to implement. The scenario follows:

* User updates to the latest kernel module.
* User updates to the latest vswitchd.
* User creates a bridge which will be using, by default, the per-cpu
dispatch mode.
* User downgrades to an older vswitchd. At this point, the older
vswitchd will open (rather than create) the datapath and feature
negotiate with kernel space. It will get the kernel's currently used
feature bitmask. This bitmask will have the per-cpu dispatch mode
enabled but the older vswitchd will ignore it because it will not
"recognise" it. vswitchd will then enable the "vport_portids" feature
flag (and not disable the per-cpu dispatch mode flag). vswitchd will
send the feature list to kernel space with both flags enabled. This is a
legitimate use case so I think we need to allow this.

FYI: While investigating this, I realised that I wasn't explicitly
disabling the vport_pids flag in userspace during feature negotiation so
I changed 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[];
>> +};
>> +
>>  /**
>>   * 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;
>> -
> We need to keep this check for compatibility reasons.
> 

Yes, I fixed that. Thanks

>>         return 0;
>>  }
> 
>>
>> --
>> 2.27.0
>>
> 

Powered by blists - more mailing lists