Re: [PATCH bpf-next 00/16] Add percpu variable support for bpf arena

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Tue, Oct 20, 2026 at 06:22 AM Xu Kuohai <xukuohai@huaweicloud.com> wrote:
> To use percpu data in a bpf arena today, we need to define an array
> sized to the maximum possible cpu number, like the cpu_ctxs array
> defined in tools/sched_ext/scx_qmap.h. This is not a convenient
> pattern. It lacks performance and scalability, and wastes memory.

It depends on how you use it.
Patches 10 and 11 emit the same thing as
&arr[bpf_get_smp_processor_id()]: load the cpu id, multiply, add.
The x86 verifier and the arm64 JIT inline that helper already.

__u64 hits takes a page per possible cpu with this series and 8 bytes
per cpu in an array.

cpu_ctxs[] is indexed by cid and qmap_dump_cid() reads other cpus'
slots from bpf, so scx_qmap cannot use this.

Keep it in the bpf prog.

Any arena offset is safe to access, so the kernel doesn't need to know
about percpu chunks. bpf_arena_alloc.h does it already with
page_frag_cur_page[NR_CPUS]: a table of pointers and a page that the
cpu allocates on the first use. NUMA_NO_NODE gives the local node and
user space reads the table through its mmap.
Then no need for BPF_PSEUDO_ARENA_PERCPU, the map_create attrs, verifier
and JIT changes, new libbpf APIs and libbpf changing thread affinity
during load. What is lost is plain 'hits++', but kernel C doesn't have
that either. Initial values are left to the prog below.

Something like this:

+/*
+ * One copy of 'type' per cpu. A cpu allocates its copy on the first use,
+ * so it comes from the local NUMA node and cpus that never run the prog
+ * cost nothing. User space reads any cpu's copy through name[cpu].
+ */
+#define DEFINE_ARENA_PER_CPU(type, name)				\
+type __arena * __arena name[NR_CPUS];					\
+									\
+static inline type __arena *this_cpu_##name(void)			\
+{									\
+	__u32 cpu = bpf_get_smp_processor_id();				\
+	type __arena *p = name[cpu];					\
+									\
+	if (!p) {							\
+		p = bpf_arena_alloc_pages(&arena, NULL,			\
+					  round_up(sizeof(type), PAGE_SIZE) / PAGE_SIZE, \
+					  NUMA_NO_NODE, 0);		\
+		name[cpu] = p;						\
+	}								\
+	cast_kern(p);							\
+	return p;							\
+}

Orthogonal to that people are working on slab-quality allocator
for arena. It's also going to be done completely on bpf side.

pw-bot: cr