[PATCH v2 2/2] mm: memcontrol: make cgroup_memory_noswap a static key

Kairui Song
2022-09-19T18:06:34Z

From: Kairui Song <kasong-1Nz4purKYjRBDgjK7y7TUQ@public.gmane.org>

cgroup_memory_noswap is used in many hot path, so make it a static key
to lower the kernel overhead.

Using 8G of ZRAM as SWAP, benchmark using `perf stat -d -d -d --repeat 100`
with the following code snip in a non-root cgroup:

#include <stdio.h>
#include <string.h>
#include <linux/mman.h>
#include <sys/mman.h>
#define MB 1024UL * 1024UL
int main(int argc, char **argv){
void *p = mmap(NULL, 8000 * MB, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
memset(p, 0xff, 8000 * MB);
madvise(p, 8000 * MB, MADV_PAGEOUT);
memset(p, 0xff, 8000 * MB);
return 0;
}

Before:
7,021.43 msec task-clock # 0.967 CPUs utilized ( +- 0.03% )
4,010 context-switches # 573.853 /sec ( +- 0.01% )
0 cpu-migrations # 0.000 /sec
2,052,057 page-faults # 293.661 K/sec ( +- 0.00% )
12,616,546,027 cycles # 1.805 GHz ( +- 0.06% ) (39.92%)
156,823,666 stalled-cycles-frontend # 1.25% frontend cycles idle ( +- 0.10% ) (40.25%)
310,130,812 stalled-cycles-backend # 2.47% backend cycles idle ( +- 4.39% ) (40.73%)
18,692,516,591 instructions # 1.49 insn per cycle
# 0.01 stalled cycles per insn ( +- 0.04% ) (40.75%)
4,907,447,976 branches # 702.283 M/sec ( +- 0.05% ) (40.30%)
13,002,578 branch-misses # 0.26% of all branches ( +- 0.08% ) (40.48%)
7,069,786,296 L1-dcache-loads # 1.012 G/sec ( +- 0.03% ) (40.32%)
649,385,847 L1-dcache-load-misses # 9.13% of all L1-dcache accesses ( +- 0.07% ) (40.10%)
1,485,448,688 L1-icache-loads # 212.576 M/sec ( +- 0.15% ) (39.49%)
31,628,457 L1-icache-load-misses # 2.13% of all L1-icache accesses ( +- 0.40% ) (39.57%)
6,667,311 dTLB-loads # 954.129 K/sec ( +- 0.21% ) (39.50%)
5,668,555 dTLB-load-misses # 86.40% of all dTLB cache accesses ( +- 0.12% ) (39.03%)
765 iTLB-loads # 109.476 /sec ( +- 21.81% ) (39.44%)
4,370,351 iTLB-load-misses # 214320.09% of all iTLB cache accesses ( +- 1.44% ) (39.86%)
149,207,254 L1-dcache-prefetches # 21.352 M/sec ( +- 0.13% ) (40.27%)

7.25869 +- 0.00203 seconds time elapsed ( +- 0.03% )

After:
6,576.16 msec task-clock # 0.953 CPUs utilized ( +- 0.10% )
4,020 context-switches # 605.595 /sec ( +- 0.01% )
0 cpu-migrations # 0.000 /sec
2,052,056 page-faults # 309.133 K/sec ( +- 0.00% )
11,967,619,180 cycles # 1.803 GHz ( +- 0.36% ) (38.76%)
161,259,240 stalled-cycles-frontend # 1.38% frontend cycles idle ( +- 0.27% ) (36.58%)
253,605,302 stalled-cycles-backend # 2.16% backend cycles idle ( +- 4.45% ) (34.78%)
19,328,171,892 instructions # 1.65 insn per cycle
# 0.01 stalled cycles per insn ( +- 0.10% ) (31.46%)
5,213,967,902 branches # 785.461 M/sec ( +- 0.18% ) (30.68%)
12,385,170 branch-misses # 0.24% of all branches ( +- 0.26% ) (34.13%)
7,271,687,822 L1-dcache-loads # 1.095 G/sec ( +- 0.12% ) (35.29%)
649,873,045 L1-dcache-load-misses # 8.93% of all L1-dcache accesses ( +- 0.11% ) (41.41%)
1,950,037,608 L1-icache-loads # 293.764 M/sec ( +- 0.33% ) (43.11%)
31,365,566 L1-icache-load-misses # 1.62% of all L1-icache accesses ( +- 0.39% ) (45.89%)
6,767,809 dTLB-loads # 1.020 M/sec ( +- 0.47% ) (48.42%)
6,339,590 dTLB-load-misses # 95.43% of all dTLB cache accesses ( +- 0.50% ) (46.60%)
736 iTLB-loads # 110.875 /sec ( +- 1.79% ) (48.60%)
4,314,836 iTLB-load-misses # 518653.73% of all iTLB cache accesses ( +- 0.63% ) (42.91%)
144,950,156 L1-dcache-prefetches # 21.836 M/sec ( +- 0.37% ) (41.39%)

6.89935 +- 0.00703 seconds time elapsed ( +- 0.10% )

The performance is clearly better. There is no significant hotspot
improvement according to perf report, as there are quite a few
callers of memcg_swap_enabled and do_memsw_account (which calls
memcg_swap_enabled). Many pieces of minor optimizations resulted
in lower overhead for the branch predictor, and bettter performance.

Acked-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
Signed-off-by: Kairui Song <kasong-1Nz4purKYjRBDgjK7y7TUQ@public.gmane.org>
---
mm/memcontrol.c | 27 +++++++++++++++++++--------
1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b69979c9ced5..5bb89c745233 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -90,9 +90,18 @@ static bool cgroup_memory_nokmem __ro_after_init;

/* Whether the swap controller is active */
#ifdef CONFIG_MEMCG_SWAP
-static bool cgroup_memory_noswap __ro_after_init;
+static bool cgroup_memory_noswap __initdata;
+
+static DEFINE_STATIC_KEY_FALSE(memcg_swap_enabled_key);
+static inline bool memcg_swap_enabled(void)
+{
+ return static_branch_likely(&memcg_swap_enabled_key);
+}
#else
-#define cgroup_memory_noswap 1
+static inline bool memcg_swap_enabled(void)
+{
+ return false;
+}
#endif

#ifdef CONFIG_CGROUP_WRITEBACK
@@ -102,7 +111,7 @@ static DECLARE_WAIT_QUEUE_HEAD(memcg_cgwb_frn_waitq);
/* Whether legacy memory+swap accounting is active */
static bool do_memsw_account(void)
{
- return !cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_noswap;
+ return !cgroup_subsys_on_dfl(memory_cgrp_subsys) && memcg_swap_enabled();
}

#define THRESHOLDS_EVENTS_TARGET 128
@@ -7267,7 +7276,7 @@ void mem_cgroup_swapout(struct folio *folio, swp_entry_t entry)
if (!mem_cgroup_is_root(memcg))
page_counter_uncharge(&memcg->memory, nr_entries);

- if (!cgroup_memory_noswap && memcg != swap_memcg) {
+ if (memcg_swap_enabled() && memcg != swap_memcg) {
if (!mem_cgroup_is_root(swap_memcg))
page_counter_charge(&swap_memcg->memsw, nr_entries);
page_counter_uncharge(&memcg->memsw, nr_entries);
@@ -7319,7 +7328,7 @@ int __mem_cgroup_try_charge_swap(struct folio *folio, swp_entry_t entry)

memcg = mem_cgroup_id_get_online(memcg);

- if (!cgroup_memory_noswap && !mem_cgroup_is_root(memcg) &&
+ if (memcg_swap_enabled() && !mem_cgroup_is_root(memcg) &&
!page_counter_try_charge(&memcg->swap, nr_pages, &counter)) {
memcg_memory_event(memcg, MEMCG_SWAP_MAX);
memcg_memory_event(memcg, MEMCG_SWAP_FAIL);
@@ -7351,7 +7360,7 @@ void __mem_cgroup_uncharge_swap(swp_entry_t entry, unsigned int nr_pages)
rcu_read_lock();
memcg = mem_cgroup_from_id(id);
if (memcg) {
- if (!cgroup_memory_noswap && !mem_cgroup_is_root(memcg)) {
+ if (memcg_swap_enabled() && !mem_cgroup_is_root(memcg)) {
if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
page_counter_uncharge(&memcg->swap, nr_pages);
else
@@ -7367,7 +7376,7 @@ long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
{
long nr_swap_pages = get_nr_swap_pages();

- if (cgroup_memory_noswap || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
+ if (!memcg_swap_enabled() || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
return nr_swap_pages;
for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg))
nr_swap_pages = min_t(long, nr_swap_pages,
@@ -7384,7 +7393,7 @@ bool mem_cgroup_swap_full(struct page *page)

if (vm_swap_full())
return true;
- if (cgroup_memory_noswap || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
+ if (!memcg_swap_enabled() || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
return false;

memcg = page_memcg(page);
@@ -7692,6 +7701,8 @@ static int __init mem_cgroup_swap_init(void)
if (cgroup_memory_noswap)
return 0;

+ static_branch_enable(&memcg_swap_enabled_key);
+
WARN_ON(cgroup_add_dfl_cftypes(&memory_cgrp_subsys, swap_files));
WARN_ON(cgroup_add_legacy_cftypes(&memory_cgrp_subsys, memsw_files));
#if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
--
2.35.2