[PATCH v2 9/9] mm: vmalloc: Set nr_nodes/node_size based on CPU-cores

Uladzislau Rezki (Sony)
2023-08-29T08:12:41Z

The density ratio is set to 2, i.e. two users per one node.
For example if there are 6 cores in a system the "nr_nodes"
is 3.

The "node_size" also depends on number of physical cores.
A high-threshold limit is hard-coded and set to SZ_4M.

For 32-bit, single/dual core systems an access to a global
vmap heap is not balanced. Such small systems do not suffer
from lock contentions due to limitation of CPU-cores.

Test on AMD Ryzen Threadripper 3970X 32-Core Processor:
sudo ./test_vmalloc.sh run_test_mask=127 nr_threads=64

<default perf>
94.17% 0.90% [kernel] [k] _raw_spin_lock
93.27% 93.05% [kernel] [k] native_queued_spin_lock_slowpath
74.69% 0.25% [kernel] [k] __vmalloc_node_range
72.64% 0.01% [kernel] [k] __get_vm_area_node
72.04% 0.89% [kernel] [k] alloc_vmap_area
42.17% 0.00% [kernel] [k] vmalloc
32.53% 0.00% [kernel] [k] __vmalloc_node
24.91% 0.25% [kernel] [k] vfree
24.32% 0.01% [kernel] [k] remove_vm_area
22.63% 0.21% [kernel] [k] find_unlink_vmap_area
15.51% 0.00% [unknown] [k] 0xffffffffc09a74ac
14.35% 0.00% [kernel] [k] ret_from_fork_asm
14.35% 0.00% [kernel] [k] ret_from_fork
14.35% 0.00% [kernel] [k] kthread
<default perf>
vs
<patch-series perf>
74.32% 2.42% [kernel] [k] __vmalloc_node_range
69.58% 0.01% [kernel] [k] vmalloc
54.21% 1.17% [kernel] [k] __alloc_pages_bulk
48.13% 47.91% [kernel] [k] clear_page_orig
43.60% 0.01% [unknown] [k] 0xffffffffc082f16f
32.06% 0.00% [kernel] [k] ret_from_fork_asm
32.06% 0.00% [kernel] [k] ret_from_fork
32.06% 0.00% [kernel] [k] kthread
31.30% 0.00% [unknown] [k] 0xffffffffc082f889
22.98% 4.16% [kernel] [k] vfree
14.36% 0.28% [kernel] [k] __get_vm_area_node
13.43% 3.35% [kernel] [k] alloc_vmap_area
10.86% 0.04% [kernel] [k] remove_vm_area
8.89% 2.75% [kernel] [k] _raw_spin_lock
7.19% 0.00% [unknown] [k] 0xffffffffc082fba3
6.65% 1.37% [kernel] [k] free_unref_page
6.13% 6.11% [kernel] [k] native_queued_spin_lock_slowpath
<patch-series perf>

confirms that a native_queued_spin_lock_slowpath bottle-neck
can be considered as negligible for the patch-series version.

The throughput is ~15x higher:

urezki@pc638:~$ time sudo ./test_vmalloc.sh run_test_mask=127 nr_threads=64
Run the test with following parameters: run_test_mask=127 nr_threads=64
Done.
Check the kernel ring buffer to see the summary.

real 24m3.305s
user 0m0.361s
sys 0m0.013s
urezki@pc638:~$

urezki@pc638:~$ time sudo ./test_vmalloc.sh run_test_mask=127 nr_threads=64
Run the test with following parameters: run_test_mask=127 nr_threads=64
Done.
Check the kernel ring buffer to see the summary.

real 1m28.382s
user 0m0.014s
sys 0m0.026s
urezki@pc638:~$

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
mm/vmalloc.c | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 9cce012aecdb..08990f630c21 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -796,6 +796,9 @@ struct vmap_node {
atomic_t fill_in_progress;
};

+#define MAX_NODES U8_MAX
+#define MAX_NODE_SIZE SZ_4M
+
static struct vmap_node *nodes, snode;
static __read_mostly unsigned int nr_nodes = 1;
static __read_mostly unsigned int node_size = 1;
@@ -4803,11 +4806,24 @@ static void vmap_init_free_space(void)
}
}

+static unsigned int calculate_nr_nodes(void)
+{
+ unsigned int nr_cpus;
+
+ nr_cpus = num_present_cpus();
+ if (nr_cpus <= 1)
+ nr_cpus = num_possible_cpus();
+
+ /* Density factor. Two users per a node. */
+ return clamp_t(unsigned int, nr_cpus >> 1, 1, MAX_NODES);
+}
+
static void vmap_init_nodes(void)
{
struct vmap_node *vn;
int i;

+ nr_nodes = calculate_nr_nodes();
nodes = &snode;

if (nr_nodes > 1) {
@@ -4830,6 +4846,16 @@ static void vmap_init_nodes(void)
INIT_LIST_HEAD(&vn->free.head);
spin_lock_init(&vn->free.lock);
}
+
+ /*
+ * Scale a node size to number of CPUs. Each power of two
+ * value doubles a node size. A high-threshold limit is set
+ * to 4M.
+ */
+#if BITS_PER_LONG == 64
+ if (nr_nodes > 1)
+ node_size = min(SZ_64K << fls(num_possible_cpus()), SZ_4M);
+#endif
}

void __init vmalloc_init(void)
--
2.30.2