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@xxxxxxxxxxxxxxx/
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(-)
