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SubjectRe: [PATCH] mm/memcg: Skip high limit check in root memcg
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On 2023/2/14 23:56, Michal Hocko wrote:
> On Fri 10-02-23 09:45:50, Haifeng Xu wrote:
>> The high limit checks the memory usage from given memcg to root memcg.
>> However, there is no limit in root memcg. So this check makes no sense
>> and we can ignore it.
>
> Is this check actually addining any benefit? Have you measured aby
> performance gains by this change?
>
>> Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
>> ---
>> mm/memcontrol.c | 4 ++++
>> 1 file changed, 4 insertions(+)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 73afff8062f9..a31a56598f29 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -2780,6 +2780,10 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>> do {
>> bool mem_high, swap_high;
>>
>> + /* There is no need for root memcg to check high limit */
>> + if (mem_cgroup_is_root(memcg))
>> + break;
>> +
>> mem_high = page_counter_read(&memcg->memory) >
>> READ_ONCE(memcg->memory.high);
>> swap_high = page_counter_read(&memcg->swap) >
>> --
>> 2.25.1
>

test steps:
1. mkdir -p /sys/fs/cgroup/memory/test
2. echo $$ > /sys/fs/cgroup/memory/test/cgroup.procs
3. ./mmap_test

The test result show that with or without the patch, the time taken is almost the same.#include <sys/mman.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <ctype.h>
#include <string.h>
#include <inttypes.h>

#define SIZE (5 * 1024 * 1024 * 1024)

int64_t current_time_ms() {
struct timeval time;
gettimeofday(&time, NULL);
int64_t s1 = (int64_t)(time.tv_sec) * 1000;
int64_t s2 = (time.tv_usec / 1000);
return s1 + s2;
}

int main(int argc, char* argv[])
{
void * buf;
size_t size = SIZE;
int64_t start, cost;

buf = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, 0, 0);
if (buf < 0 ) {
printf("mmap failed\n");
exit(-1);
}

start = current_time_ms();

mlock(buf, size);

cost = current_time_ms() - start;
printf("cost: %" PRId64 " ms\n", cost);

munmap(buf, size);

return 0;
}
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