[PATCH bpf v2 0/3] bpf: Enable IRQ after irq_work_raise() completes

[Hou Tao <houtao@huaweicloud.com>]

From: Hou Tao <houtao1@huawei.com>

Hi,

The patchset aims to fix the problem that bpf_mem_alloc() may return
NULL unexpectedly when multiple bpf_mem_alloc() are invoked concurrently
under process context and there is still free memory available. The
problem was found when doing stress test for qp-trie but the same
problem also exists for bpf_obj_new() as demonstrated in patch #3.

As pointed out by Alexei, the patchset can only fix ENOMEM problem for
normal process context and can not fix the problem for irq-disabled
context or RT-enabled kernel.

Patch #1 fixes the race between unit_alloc() and unit_alloc(). Patch #2
fixes the race between unit_alloc() and unit_free(). And patch #3 adds
a selftest for the problem. The major change compared with v1 is using
local_irq_{save,restore)() pair to disable and enable preemption
instead of preempt_{disable,enable}_notrace pair. The main reason is to
prevent potential overhead from __preempt_schedule_notrace(). I also
run htab_mem benchmark and hash_map_perf on a 8-CPUs KVM VM to compare
the performance between local_irq_{save,restore} and
preempt_{disable,enable}_notrace(), but the results are similar as shown
below:

(1) use preempt_{disable,enable}_notrace()

[root@hello bpf]# ./map_perf_test 4 8
0:hash_map_perf kmalloc 652179 events per sec
1:hash_map_perf kmalloc 651880 events per sec
2:hash_map_perf kmalloc 651382 events per sec
3:hash_map_perf kmalloc 650791 events per sec
5:hash_map_perf kmalloc 650140 events per sec
6:hash_map_perf kmalloc 652773 events per sec
7:hash_map_perf kmalloc 652751 events per sec
4:hash_map_perf kmalloc 648199 events per sec

[root@hello bpf]# ./benchs/run_bench_htab_mem.sh
normal bpf ma
=============
overwrite            per-prod-op: 110.82 ± 0.02k/s, avg mem: 2.00 ± 0.00MiB, peak mem: 2.73MiB
batch_add_batch_del  per-prod-op: 89.79 ± 0.75k/s, avg mem: 1.68 ± 0.38MiB, peak mem: 2.73MiB
add_del_on_diff_cpu  per-prod-op: 17.83 ± 0.07k/s, avg mem: 25.68 ± 2.92MiB, peak mem: 35.10MiB

(2) use local_irq_{save,restore}

[root@hello bpf]# ./map_perf_test 4 8
0:hash_map_perf kmalloc 656299 events per sec
1:hash_map_perf kmalloc 656397 events per sec
2:hash_map_perf kmalloc 656046 events per sec
3:hash_map_perf kmalloc 655723 events per sec
5:hash_map_perf kmalloc 655221 events per sec
4:hash_map_perf kmalloc 654617 events per sec
6:hash_map_perf kmalloc 650269 events per sec
7:hash_map_perf kmalloc 653665 events per sec

[root@hello bpf]# ./benchs/run_bench_htab_mem.sh
normal bpf ma
=============
overwrite            per-prod-op: 116.10 ± 0.02k/s, avg mem: 2.00 ± 0.00MiB, peak mem: 2.74MiB
batch_add_batch_del  per-prod-op: 88.76 ± 0.61k/s, avg mem: 1.94 ± 0.33MiB, peak mem: 2.74MiB
add_del_on_diff_cpu  per-prod-op: 18.12 ± 0.08k/s, avg mem: 25.10 ± 2.70MiB, peak mem: 34.78MiB

As ususal comments are always welcome.

Change Log:
v2:
  * Use local_irq_save to disable preemption instead of using
    preempt_{disable,enable}_notrace pair to prevent potential overhead

v1: https://lore.kernel.org/bpf/20230822133807.3198625-1-houtao@huaweicloud.com/

Hou Tao (3):
  bpf: Enable IRQ after irq_work_raise() completes in unit_alloc()
  bpf: Enable IRQ after irq_work_raise() completes in unit_free{_rcu}()
  selftests/bpf: Test preemption between bpf_obj_new() and
    bpf_obj_drop()

 kernel/bpf/memalloc.c                         |  16 ++-
 .../bpf/prog_tests/preempted_bpf_ma_op.c      |  89 +++++++++++++++
 .../selftests/bpf/progs/preempted_bpf_ma_op.c | 106 ++++++++++++++++++
 3 files changed, 208 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/preempted_bpf_ma_op.c
 create mode 100644 tools/testing/selftests/bpf/progs/preempted_bpf_ma_op.c

-- 
2.29.2