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

From: Michal Hocko
Date: Tue Aug 30 2022 - 03:01:29 EST

On Tue 30-08-22 13:59:49, Kairui Song wrote:
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> 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% )

Do you happen to have a perf profile before and after to see which of
the paths really benefits from this?

> The performance is clearly better.
>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>

Anyway, this looks good to me. I like memcg_swap_enabled() better than
!cgroup_memory_noswap. The double negative was quite confusing.

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> ---
> mm/memcontrol.c | 27 +++++++++++++++++++--------
> 1 file changed, 19 insertions(+), 8 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 20e26ccd7dddc..8ea5589345a14 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -90,9 +90,18 @@ static bool cgroup_memory_nokmem __initdata;
>
> /* 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
> @@ -7264,7 +7273,7 @@ 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);
> @@ -7316,7 +7325,7 @@ 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);
> @@ -7348,7 +7357,7 @@ 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
> @@ -7364,7 +7373,7 @@ 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,
> @@ -7381,7 +7390,7 @@ 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);
> @@ -7689,6 +7698,8 @@ 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

--
Michal Hocko
SUSE Labs