Re: [PATCH bpf-next v6 0/8] bpf: Reduce memory usage for bpf_global_percpu_ma

[Yonghong Song <yonghong.song@linux.dev>]

On 12/22/23 1:35 AM, Hou Tao wrote:
> Hi,
>
> On 12/22/2023 11:17 AM, Yonghong Song wrote:
>> Currently when a bpf program intends to allocate memory for percpu kptr,
>> the verifier will call bpf_mem_alloc_init() to prefill all supported
>> unit sizes and this caused memory consumption very big for large number
>> of cpus. For example, for 128-cpu system, the total memory consumption
>> with initial prefill is ~175MB. Things will become worse for systems
>> with even more cpus.
>>
>> Patch 1 avoids unnecessary extra percpu memory allocation.
>> Patch 2 adds objcg to bpf_mem_alloc at init stage so objcg can be
>> associated with root cgroup and objcg can be passed to later
>> bpf_mem_alloc_percpu_unit_init().
>> Patch 3 addresses memory consumption issue by avoiding to prefill
>> with all unit sizes, i.e. only prefilling with user specified size.
>> Patch 4 further reduces memory consumption by limiting the
>> number of prefill entries for percpu memory allocation.
>> Patch 5 has much smaller low/high watermarks for percpu allocation
>> to reduce memory consumption.
>> Patch 6 rejects percpu memory allocation with bpf_global_percpu_ma
>> when allocation size is greater than 512 bytes.
>> Patch 7 fixed test_bpf_ma test due to Patch 5.
>> Patch 8 added one test to show the verification failure log message.
> FYI. After applying the patch set, the memory consumption in bpf memory
> benchmark [1] on 8-CPU VM decreases a lot:
>
> Before the patch set:
>
> $ for i in 1 4 8; do ./bench -w3 -d10 bpf_ma -p${i} -a --percpu; done |
> grep Summary
> Summary: per-prod alloc   14.16 ± 0.59M/s free   36.18 ± 0.39M/s, total
> memory usage  183.71 ± 10.38MiB
> Summary: per-prod alloc   12.35 ± 1.10M/s free   35.79 ± 0.51M/s, total
> memory usage  744.52 ± 11.64MiB
> Summary: per-prod alloc   11.15 ± 0.20M/s free   35.72 ± 0.27M/s, total
> memory usage 2545.98 ± 537.57MiB
>
> After the patch set:
>
> $ for i in 1 4 8; do ./bench -w3 -d10 bpf_ma -p${i} -a --percpu; done |
> grep Summary
> Summary: per-prod alloc    0.86 ± 0.00M/s free   37.29 ± 0.11M/s, total
> memory usage    0.00 ± 0.00MiB
> Summary: per-prod alloc    0.85 ± 0.00M/s free   36.70 ± 0.24M/s, total
> memory usage    0.00 ± 0.00MiB
> Summary: per-prod alloc    0.84 ± 0.00M/s free   37.21 ± 0.17M/s, total
> memory usage    0.00 ± 0.00MiB
>
> However the allocation performance also degrades a lot. It seems it is
> due to patch 5 (bpf: Use smaller low/high marks for percpu allocation),
> because c->batch is 1 now, so each allocation needs one run of irq_work.

Thanks for benchmarking! With low watermark to be 1 and c->batch to 1
as well, there will be more overhead due to irq_work. In practice,
I expect we should not see a lot of such percpu map element updates or
percpu kptr allocaitons.

>
> [1]:
> https://lore.kernel.org/bpf/20231221141501.3588586-1-houtao@huaweicloud.com/
>> Changelogs:
>>    v5 -> v6:
>>      . Change bpf_mem_alloc_percpu_init() to add objcg as one of parameters.
>>        For bpf_global_percpu_ma, the objcg is NULL, corresponding root memcg.
>>    v4 -> v5:
>>      . Do not do bpf_global_percpu_ma initialization at init stage, instead
>>        doing initialization when the verifier knows it is going to be used
>>        by bpf prog.
>>      . Using much smaller low/high watermarks for percpu allocation.
>>    v3 -> v4:
>>      . Add objcg to bpf_mem_alloc during init stage.
>>      . Initialize objcg at init stage but use it in bpf_mem_alloc_percpu_unit_init().
>>      . Remove check_obj_size() in bpf_mem_alloc_percpu_unit_init().
>>    v2 -> v3:
>>      . Clear the bpf_mem_cache if prefill fails.
>>      . Change test_bpf_ma percpu allocation tests to use bucket_size
>>        as allocation size instead of bucket_size - 8.
>>      . Remove __GFP_ZERO flag from __alloc_percpu_gfp() call.
>>    v1 -> v2:
>>      . Avoid unnecessary extra percpu memory allocation.
>>      . Add a separate function to do bpf_global_percpu_ma initialization
>>      . promote.
>>      . Promote function static 'sizes' array to file static.
>>      . Add comments to explain to refill only one item for percpu alloc.
>>
>> Yonghong Song (8):
>>    bpf: Avoid unnecessary extra percpu memory allocation
>>    bpf: Add objcg to bpf_mem_alloc
>>    bpf: Allow per unit prefill for non-fix-size percpu memory allocator
>>    bpf: Refill only one percpu element in memalloc
>>    bpf: Use smaller low/high marks for percpu allocation
>>    bpf: Limit up to 512 bytes for bpf_global_percpu_ma allocation
>>    selftests/bpf: Cope with 512 bytes limit with bpf_global_percpu_ma
>>    selftests/bpf: Add a selftest with > 512-byte percpu allocation size
>>
>>   include/linux/bpf_mem_alloc.h                 |  8 ++
>>   kernel/bpf/memalloc.c                         | 93 ++++++++++++++++---
>>   kernel/bpf/verifier.c                         | 45 ++++++---
>>   .../selftests/bpf/prog_tests/test_bpf_ma.c    | 20 ++--
>>   .../selftests/bpf/progs/percpu_alloc_fail.c   | 18 ++++
>>   .../testing/selftests/bpf/progs/test_bpf_ma.c | 66 ++++++-------
>>   6 files changed, 184 insertions(+), 66 deletions(-)
>>