Re: [PATCH] mm/memcg: Skip high limit check in root memcg

[Haifeng Xu <haifeng.xu@xxxxxxxxxx>]

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@xxxxxxxxxx>
>> ---
>>  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;
}