SCX eBPF macro bpf_obj_new

v6.12

The bpf_obj_new macro wraps bpf_obj_new_impl to provide a more ergonomic interface.

Definition

#define bpf_obj_new(type) ((type *)bpf_obj_new_impl(bpf_core_type_id_local(type), NULL))

Usage

The bpf_obj_new_impl kfunc is used to allocate objects of a custom type in kernel memory. These objects outlast a BPF programs lifetime and can be used to store state across invocations. In order for the kernel to understand the type, we have to pass it the BTF type ID. Which we can get using the bpf_core_type_id_local macro. So this macro is a shorthand for that, and casts the resulting void * to the correct type.

Example

/* SPDX-License-Identifier: GPL-2.0 */
/* Copyright (c) 2022 Tejun Heo <tj@kernel.org> */

int BPF_STRUCT_OPS_SLEEPABLE(fcg_cgroup_init, struct cgroup *cgrp,
                 struct scx_cgroup_init_args *args)
{
    struct fcg_cgrp_ctx *cgc;
    struct cgv_node *cgv_node;
    struct cgv_node_stash empty_stash = {}, *stash;
    u64 cgid = cgrp->kn->id;
    int ret;

    /*
     * Technically incorrect as cgroup ID is full 64bit while dsq ID is
     * 63bit. Should not be a problem in practice and easy to spot in the
     * unlikely case that it breaks.
     */
    ret = scx_bpf_create_dsq(cgid, -1);
    if (ret)
        return ret;

    cgc = bpf_cgrp_storage_get(&cgrp_ctx, cgrp, 0,
                   BPF_LOCAL_STORAGE_GET_F_CREATE);
    if (!cgc) {
        ret = -ENOMEM;
        goto err_destroy_dsq;
    }

    cgc->weight = args->weight;
    cgc->hweight = FCG_HWEIGHT_ONE;

    ret = bpf_map_update_elem(&cgv_node_stash, &cgid, &empty_stash,
                  BPF_NOEXIST);
    if (ret) {
        if (ret != -ENOMEM)
            scx_bpf_error("unexpected stash creation error (%d)",
                      ret);
        goto err_destroy_dsq;
    }

    stash = bpf_map_lookup_elem(&cgv_node_stash, &cgid);
    if (!stash) {
        scx_bpf_error("unexpected cgv_node stash lookup failure");
        ret = -ENOENT;
        goto err_destroy_dsq;
    }

    cgv_node = bpf_obj_new(struct cgv_node);
    if (!cgv_node) {
        ret = -ENOMEM;
        goto err_del_cgv_node;
    }

    cgv_node->cgid = cgid;
    cgv_node->cvtime = cvtime_now;

    cgv_node = bpf_kptr_xchg(&stash->node, cgv_node);
    if (cgv_node) {
        scx_bpf_error("unexpected !NULL cgv_node stash");
        ret = -EBUSY;
        goto err_drop;
    }

    return 0;

err_drop:
    bpf_obj_drop(cgv_node);
err_del_cgv_node:
    bpf_map_delete_elem(&cgv_node_stash, &cgid);
err_destroy_dsq:
    scx_bpf_destroy_dsq(cgid);
    return ret;
}