mm/memcg: Skip high limit check in root memcg

  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.

Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
---
 mm/memcontrol.c | 4 ++++
 1 file changed, 4 insertions(+)
  

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
  

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;
}
  

On Tue 21-02-23 18:29:39, Haifeng Xu wrote:
> 
> 
> 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.

This is in line with my expectation. So the question is whether the
additional check is really worth it.
  

On 2023/2/21 20:20, Michal Hocko wrote:
> On Tue 21-02-23 18:29:39, Haifeng Xu wrote:
>>
>>
>> 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.
> 
> This is in line with my expectation. So the question is whether the
> additional check is really worth it. 

This patch doesn't bring any obvious benifit or harm, but the high limit check in root memcg seems a little weird.
Maybe we can add this check？It all depends on your viewpoint.

Thanks.
  

On Tue 21-02-23 22:21:45, Haifeng Xu wrote:
[...]
> >> The test result show that with or without the patch, the time taken is almost the same.
> > 
> > This is in line with my expectation. So the question is whether the
> > additional check is really worth it. 
> 
> This patch doesn't bring any obvious benifit or harm, but the high
> limit check in root memcg seems a little weird.  Maybe we can add this
> check

Well, I do not see the code to look weird TBH. There is nothing wrong in
doing the check for the root memcg. It is a bit pointless but it is not
incorrect.

> It all depends on your viewpoint.

From my POV, I prefer changes that either fix something (correctness
issue or a performance issue/improvement) or improve readbility. The
check doesn't fix anything and I am not convinced about an improved
readabilit either.

Thanks for the patch anyway!