[PATCH v3 00/11] Mitigate a vmap lock contention v3

Uladzislau Rezki (Sony)
2024-01-02T18:46:38Z

This is v3. It is based on the 6.7.0-rc8.

1. Motivation

- Offload global vmap locks making it scaled to number of CPUS;
- If possible and there is an agreement, we can remove the "Per cpu kva allocator"
to make the vmap code to be more simple;
- There were complains from XFS folk that a vmalloc might be contented
on the their workloads.

2. Design(high level overview)

We introduce an effective vmap node logic. A node behaves as independent
entity to serve an allocation request directly(if possible) from its pool.
That way it bypasses a global vmap space that is protected by its own lock.

An access to pools are serialized by CPUs. Number of nodes are equal to
number of CPUs in a system. Please note the high threshold is bound to
128 nodes.

Pools are size segregated and populated based on system demand. The maximum
alloc request that can be stored into a segregated storage is 256 pages. The
lazily drain path decays a pool by 25% as a first step and as second populates
it by fresh freed VAs for reuse instead of returning them into a global space.

When a VA is obtained(alloc path), it is stored in separate nodes. A va->va_start
address is converted into a correct node where it should be placed and resided.
Doing so we balance VAs across the nodes as a result an access becomes scalable.
The addr_to_node() function does a proper address conversion to a correct node.

A vmap space is divided on segments with fixed size, it is 16 pages. That way
any address can be associated with a segment number. Number of segments are
equal to num_possible_cpus() but not grater then 128. The numeration starts
from 0. See below how it is converted:

static inline unsigned int
addr_to_node_id(unsigned long addr)
{
return (addr / zone_size) % nr_nodes;
}

On a free path, a VA can be easily found by converting its "va_start" address
to a certain node it resides. It is moved from "busy" data to "lazy" data structure.
Later on, as noted earlier, the lazy kworker decays each node pool and populates it
by fresh incoming VAs. Please note, a VA is returned to a node that did an alloc
request.

3. Test on AMD Ryzen Threadripper 3970X 32-Core Processor

sudo ./test_vmalloc.sh run_test_mask=7 nr_threads=64

<default perf>
94.41% 0.89% [kernel] [k] _raw_spin_lock
93.35% 93.07% [kernel] [k] native_queued_spin_lock_slowpath
76.13% 0.28% [kernel] [k] __vmalloc_node_range
72.96% 0.81% [kernel] [k] alloc_vmap_area
56.94% 0.00% [kernel] [k] __get_vm_area_node
41.95% 0.00% [kernel] [k] vmalloc
37.15% 0.01% [test_vmalloc] [k] full_fit_alloc_test
35.17% 0.00% [kernel] [k] ret_from_fork_asm
35.17% 0.00% [kernel] [k] ret_from_fork
35.17% 0.00% [kernel] [k] kthread
35.08% 0.00% [test_vmalloc] [k] test_func
34.45% 0.00% [test_vmalloc] [k] fix_size_alloc_test
28.09% 0.01% [test_vmalloc] [k] long_busy_list_alloc_test
23.53% 0.25% [kernel] [k] vfree.part.0
21.72% 0.00% [kernel] [k] remove_vm_area
20.08% 0.21% [kernel] [k] find_unlink_vmap_area
2.34% 0.61% [kernel] [k] free_vmap_area_noflush
<default perf>
vs
<patch-series perf>
82.32% 0.22% [test_vmalloc] [k] long_busy_list_alloc_test
63.36% 0.02% [kernel] [k] vmalloc
63.34% 2.64% [kernel] [k] __vmalloc_node_range
30.42% 4.46% [kernel] [k] vfree.part.0
28.98% 2.51% [kernel] [k] __alloc_pages_bulk
27.28% 0.19% [kernel] [k] __get_vm_area_node
26.13% 1.50% [kernel] [k] alloc_vmap_area
21.72% 21.67% [kernel] [k] clear_page_rep
19.51% 2.43% [kernel] [k] _raw_spin_lock
16.61% 16.51% [kernel] [k] native_queued_spin_lock_slowpath
13.40% 2.07% [kernel] [k] free_unref_page
10.62% 0.01% [kernel] [k] remove_vm_area
9.02% 8.73% [kernel] [k] insert_vmap_area
8.94% 0.00% [kernel] [k] ret_from_fork_asm
8.94% 0.00% [kernel] [k] ret_from_fork
8.94% 0.00% [kernel] [k] kthread
8.29% 0.00% [test_vmalloc] [k] test_func
7.81% 0.05% [test_vmalloc] [k] full_fit_alloc_test
5.30% 4.73% [kernel] [k] purge_vmap_node
4.47% 2.65% [kernel] [k] free_vmap_area_noflush
<patch-series perf>

confirms that a native_queued_spin_lock_slowpath goes down to
16.51% percent from 93.07%.

The throughput is ~12x higher:

urezki@pc638:~$ time sudo ./test_vmalloc.sh run_test_mask=7 nr_threads=64
Run the test with following parameters: run_test_mask=7 nr_threads=64
Done.
Check the kernel ring buffer to see the summary.

real 10m51.271s
user 0m0.013s
sys 0m0.187s
urezki@pc638:~$

urezki@pc638:~$ time sudo ./test_vmalloc.sh run_test_mask=7 nr_threads=64
Run the test with following parameters: run_test_mask=7 nr_threads=64
Done.
Check the kernel ring buffer to see the summary.

real 0m51.301s
user 0m0.015s
sys 0m0.040s
urezki@pc638:~$

4. Changelog

v1: https://lore.kernel.org/linux-mm/ZIAqojPKjChJTssg@pc636/T/
v2: https://lore.kernel.org/lkml/20230829081142.3619-1-urezki@gmail.com/

Delta v2 -> v3:
- fix comments from v2 feedback;
- switch from pre-fetch chunk logic to a less complex size based pools.

Baoquan He (1):
mm/vmalloc: remove vmap_area_list

Uladzislau Rezki (Sony) (10):
mm: vmalloc: Add va_alloc() helper
mm: vmalloc: Rename adjust_va_to_fit_type() function
mm: vmalloc: Move vmap_init_free_space() down in vmalloc.c
mm: vmalloc: Remove global vmap_area_root rb-tree
mm: vmalloc: Remove global purge_vmap_area_root rb-tree
mm: vmalloc: Offload free_vmap_area_lock lock
mm: vmalloc: Support multiple nodes in vread_iter
mm: vmalloc: Support multiple nodes in vmallocinfo
mm: vmalloc: Set nr_nodes based on CPUs in a system
mm: vmalloc: Add a shrinker to drain vmap pools

.../admin-guide/kdump/vmcoreinfo.rst | 8 +-
arch/arm64/kernel/crash_core.c | 1 -
arch/riscv/kernel/crash_core.c | 1 -
include/linux/vmalloc.h | 1 -
kernel/crash_core.c | 4 +-
kernel/kallsyms_selftest.c | 1 -
mm/nommu.c | 2 -
mm/vmalloc.c | 1049 ++++++++++++-----
8 files changed, 786 insertions(+), 281 deletions(-)

--
2.39.2