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Date: Thu, 15 Jul 2021 12:55:56 +0100
From: Mark Gray <mark.d.gray@...hat.com>
To: Pravin Shelar <pravin.ovn@...il.com>
Cc: ovs dev <dev@...nvswitch.org>,
Linux Kernel Network Developers <netdev@...r.kernel.org>,
Flavio Leitner <fbl@...close.org>, dan.carpenter@...cle.com
Subject: Re: [PATCH net-next] openvswitch: Introduce per-cpu upcall dispatch
On 15/07/2021 05:45, Pravin Shelar wrote:
> On Wed, Jun 30, 2021 at 2:53 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>
>> ---
>>
>> 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.
>
>> 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
>
>> +{
>> + 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
>
>> +{
>> + 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.
>
>> +
>> + 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;
>> + }
>> +}
>> +
>
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