From: Maryam Tahhan <mtahhan@xxxxxxxxxx>
Add documentation for BPF_MAP_TYPE_CPUMAP including
kernel version introduced, usage and examples.
Signed-off-by: Maryam Tahhan <mtahhan@xxxxxxxxxx>
Signed-off-by: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
Co-developed-by: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
---
Documentation/bpf/map_cpumap.rst | 166 +++++++++++++++++++++++++++++++
1 file changed, 166 insertions(+)
create mode 100644 Documentation/bpf/map_cpumap.rst
diff --git a/Documentation/bpf/map_cpumap.rst b/Documentation/bpf/map_cpumap.rst
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+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2022 Red Hat, Inc.
+
+===================
+BPF_MAP_TYPE_CPUMAP
+===================
+
+.. note::
+ - ``BPF_MAP_TYPE_CPUMAP`` was introduced in kernel version 4.15
+
+``BPF_MAP_TYPE_CPUMAP`` is primarily used as a backend map for the XDP BPF helpers
+``bpf_redirect_map()`` and ``XDP_REDIRECT`` action. This map type redirects raw
+XDP frames to another CPU.
+
+A CPUMAP is a scalability and isolation mechanism, that allows separating the driver
+network XDP layer, from the rest of the network stack, and assigning dedicated
+CPUs for this stage. An example use case for this map type is software based Receive
+Side Scaling (RSS) at the XDP layer.
+
+The CPUMAP represents the CPUs in the system indexed as the map-key, and the
+map-value is the config setting (per CPUMAP entry). Each CPUMAP entry has a dedicated
+kernel thread bound to the given CPU to represent the remote CPU execution unit.
+
+The CPUMAP entry represents a multi-producer single-consumer (MPSC) queue
+(implemented via ``ptr_ring`` in the kernel). The single consumer is the CPUMAP
+``kthread`` that can access the ``ptr_ring`` queue without taking any lock. It also
+tries to bulk dequeue eight xdp_frame objects, as they represent one cache line.
+The multi-producers can be RX IRQ line CPUs queuing up packets simultaneously for
+the remote CPU. To avoid queue lock contention for each producer CPU, there is a
+small eight-object queue to generate bulk enqueueing into the cross-CPU queue.
+This careful queue usage means that each cache line transfers eight frames across
+the CPUs.
+
+.. note::
+
+ XDP packets getting XDP redirected to another CPU, will maximum be stored/queued
+ for one ``driver ->poll()`` call. Queueing the frame and the flush operation
+ are guaranteed to happen on same CPU. Thus, ``cpu_map_flush`` operation can deduce
+ via ``this_cpu_ptr()`` which queue in bpf_cpu_map_entry contains packets.
+
+Usage
+=====
+
+.. c:function::
+ long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
+
+ CPU entries can be added or updated using the ``bpf_map_update_elem()``
+ helper. This helper replaces existing elements atomically. The ``value`` parameter
+ can be ``struct bpf_cpumap_val``.
+
+ .. note::
+ The maps can only be updated from user space and not from a BPF program.
+
+ .. code-block:: c
+
+ struct bpf_cpumap_val {
+ __u32 qsize; /* queue size to remote target CPU */
+ union {
+ int fd; /* prog fd on map write */
+ __u32 id; /* prog id on map read */
+ } bpf_prog;
+ };
+
+ Starting from Linux kernel version 5.9 the CPUMAP can run a second XDP program
+ on the remote CPU. This helps with scalability as the receive CPU should spend
+ as few cycles as possible processing packets. The remote CPU (to which the packet is
+ directed) can afford to spend more cycles processing the frame. For example, packets
+ are received on a CPU to which the IRQ of the NIC RX queue is steered. This CPU
+ is the one that initially sees the packets. This is where the XDP redirect program
+ is executed. Because the objective is to scale the CPU usage across multiple CPUs,
+ the eBPF program should use as few cycles as possible on this initial CPU; just
+ enough to determine which remote CPU to send the packet to, and then move the
+ packet to a remote CPU for continued processing. The remote CPUMAP ``kthread``
+ receives raw XDP frame (``xdp_frame``) objects. If the frames are to be passed
+ to the networking stack, the SKB objects are allocated by the remote CPU, and
+ the SKBs are passed to the networking stack.
+
+.. c:function::
+ void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
+
+ CPU entries can be retrieved using the ``bpf_map_lookup_elem()``
+ helper.
+
+.. c:function::
+ long bpf_map_delete_elem(struct bpf_map *map, const void *key)
+
+ CPU entries can be deleted using the ``bpf_map_delete_elem()``
+ helper. This helper will return 0 on success, or negative error in case of
+ failure.
+
+.. c:function::
+ long bpf_redirect_map(struct bpf_map *map, u32 key, u64 flags)
+
+ Redirect the packet to the endpoint referenced by ``map`` at index ``key``.
+ For ``BPF_MAP_TYPE_CPUMAP`` this map contains references to CPUs.
+
+ The lower two bits of *flags* are used as the return code if the map lookup
+ fails. This is so that the return value can be one of the XDP program return
+ codes up to ``XDP_TX``, as chosen by the caller.
+
+Examples
+========
+Kernel
+------
+
+The following code snippet shows how to declare a BPF_MAP_TYPE_CPUMAP called cpu_map.
+
+.. code-block:: c
+
+ struct {
+ __uint(type, BPF_MAP_TYPE_CPUMAP);
+ __type(key, u32);
+ __type(value, struct bpf_cpumap_val);
+ } cpu_map SEC(".maps");
+
+The following code snippet shows how to redirect packets to a remote CPU.
+
+.. code-block:: c
+
+ struct {
+ __uint(type, BPF_MAP_TYPE_ARRAY);
+ __type(key, u32);
+ __type(value, u32);
+ } cpus_available SEC(".maps"); /* Map populated by user space program as selectable redirect CPUs*/
+
+ SEC("xdp")
+ int xdp_redir_cpu(struct xdp_md *ctx)
+ {
+ u32 key = bpf_get_smp_processor_id();
+ u32 *cpu_selected;
+ u32 cpu_dest = 0;
+
+ cpu_selected = bpf_map_lookup_elem(&cpus_available, &key);
+ if (!cpu_selected)
+ return XDP_ABORTED;
+ cpu_dest = *cpu_selected;
+
+ if (cpu_dest >= bpf_num_possible_cpus()) {
+ return XDP_ABORTED;
+ }
+ return bpf_redirect_map(&cpu_map, cpu_dest, 0);
+ }
+
+User Space
+----------
+
+The following code snippet shows how to update a CPUMAP called cpumap.
+
+.. code-block:: c
+
+ static int create_cpu_entry(__u32 cpu, struct bpf_cpumap_val *value)
+ {
+ int ret;
+
+ ret = bpf_map_update_elem(bpf_map__fd(cpu_map), &cpu, value, 0);
+ if (ret < 0)
+ fprintf(stderr, "Create CPU entry failed: %s\n", strerror(errno));
+
+ return ret;
+ }
+
+References
+===========
+
+- https://elixir.bootlin.com/linux/v6.0.1/source/kernel/bpf/cpumap.c
+- https://developers.redhat.com/blog/2021/05/13/receive-side-scaling-rss-with-ebpf-and-cpumap#redirecting_into_a_cpumap
--
2.35.3
