WARNING: CPU: 3 PID: 261 at kernel/bpf/memalloc.c:342

[Alexei Starovoitov]

On Fri, Aug 25, 2023 at 8:28 AM Yonghong Song <yonghong.song at linux.dev> wrote:
>
>
>
> On 8/25/23 3:32 AM, Björn Töpel wrote:
> > I'm chasing a workqueue hang on RISC-V/qemu (TCG), using the bpf
> > selftests on bpf-next 9e3b47abeb8f.
> >
> > I'm able to reproduce the hang by multiple runs of:
> >   | ./test_progs -a link_api -a linked_list
> > I'm currently investigating that.
> >
> > But! Sometimes (every blue moon) I get a warn_on_once hit:
> >   | ------------[ cut here ]------------
> >   | WARNING: CPU: 3 PID: 261 at kernel/bpf/memalloc.c:342 bpf_mem_refill+0x1fc/0x206
> >   | Modules linked in: bpf_testmod(OE)
> >   | CPU: 3 PID: 261 Comm: test_progs-cpuv Tainted: G           OE    N 6.5.0-rc5-01743-gdcb152bb8328 #2
> >   | Hardware name: riscv-virtio,qemu (DT)
> >   | epc : bpf_mem_refill+0x1fc/0x206
> >   |  ra : irq_work_single+0x68/0x70
> >   | epc : ffffffff801b1bc4 ra : ffffffff8015fe84 sp : ff2000000001be20
> >   |  gp : ffffffff82d26138 tp : ff6000008477a800 t0 : 0000000000046600
> >   |  t1 : ffffffff812b6ddc t2 : 0000000000000000 s0 : ff2000000001be70
> >   |  s1 : ff5ffffffffe8998 a0 : ff5ffffffffe8998 a1 : ff600003fef4b000
> >   |  a2 : 000000000000003f a3 : ffffffff80008250 a4 : 0000000000000060
> >   |  a5 : 0000000000000080 a6 : 0000000000000000 a7 : 0000000000735049
> >   |  s2 : ff5ffffffffe8998 s3 : 0000000000000022 s4 : 0000000000001000
> >   |  s5 : 0000000000000007 s6 : ff5ffffffffe8570 s7 : ffffffff82d6bd30
> >   |  s8 : 000000000000003f s9 : ffffffff82d2c5e8 s10: 000000000000ffff
> >   |  s11: ffffffff82d2c5d8 t3 : ffffffff81ea8f28 t4 : 0000000000000000
> >   |  t5 : ff6000008fd28278 t6 : 0000000000040000
> >   | status: 0000000200000100 badaddr: 0000000000000000 cause: 0000000000000003
> >   | [<ffffffff801b1bc4>] bpf_mem_refill+0x1fc/0x206
> >   | [<ffffffff8015fe84>] irq_work_single+0x68/0x70
> >   | [<ffffffff8015feb4>] irq_work_run_list+0x28/0x36
> >   | [<ffffffff8015fefa>] irq_work_run+0x38/0x66
> >   | [<ffffffff8000828a>] handle_IPI+0x3a/0xb4
> >   | [<ffffffff800a5c3a>] handle_percpu_devid_irq+0xa4/0x1f8
> >   | [<ffffffff8009fafa>] generic_handle_domain_irq+0x28/0x36
> >   | [<ffffffff800ae570>] ipi_mux_process+0xac/0xfa
> >   | [<ffffffff8000a8ea>] sbi_ipi_handle+0x2e/0x88
> >   | [<ffffffff8009fafa>] generic_handle_domain_irq+0x28/0x36
> >   | [<ffffffff807ee70e>] riscv_intc_irq+0x36/0x4e
> >   | [<ffffffff812b5d3a>] handle_riscv_irq+0x54/0x86
> >   | [<ffffffff812b6904>] do_irq+0x66/0x98
> >   | ---[ end trace 0000000000000000 ]---
> >
> > Code:
> >   | static void free_bulk(struct bpf_mem_cache *c)
> >   | {
> >   |   struct bpf_mem_cache *tgt = c->tgt;
> >   |   struct llist_node *llnode, *t;
> >   |   unsigned long flags;
> >   |   int cnt;
> >   |
> >   |   WARN_ON_ONCE(tgt->unit_size != c->unit_size);
> >   | ...
> >
> > I'm not well versed in the memory allocator; Before I dive into it --
> > has anyone else hit it? Ideas on why the warn_on_once is hit?
>
> Maybe take a look at the patch
>    822fb26bdb55  bpf: Add a hint to allocated objects.
>
> In the above patch, we have
>
> +       /*
> +        * Remember bpf_mem_cache that allocated this object.
> +        * The hint is not accurate.
> +        */
> +       c->tgt = *(struct bpf_mem_cache **)llnode;
>
> I suspect that the warning may be related to the above.
> I tried the above ./test_progs command line (running multiple
> at the same time) and didn't trigger the issue.

Interesting. Thanks for the report.
I wasn't able to repo the warn either, but I can confirm that linked_list
test is super slow on the kernel with debugs on and may appear to "hang",
since it doesn't respond to key press for many seconds.

time ./test_progs -a linked_list
Summary: 1/132 PASSED, 0 SKIPPED, 0 FAILED

real    0m35.897s
user    0m1.860s
sys    0m32.105s

That's not normal.

Kumar, Dave,

do you have any suggestions?