Re: [PATCH] mm: remove redundant check in handle_mm_fault

[Haifeng Xu <haifeng.xu@xxxxxxxxxx>]

On 2023/3/7 10:48, Matthew Wilcox wrote:
> On Tue, Mar 07, 2023 at 10:36:55AM +0800, Haifeng Xu wrote:
>> On 2023/3/6 21:49, David Hildenbrand wrote:
>>> On 06.03.23 03:49, Haifeng Xu wrote:
>>>> mem_cgroup_oom_synchronize() has checked whether current memcg_in_oom is
>>>> set or not, so remove the check in handle_mm_fault().
>>>
>>> "mem_cgroup_oom_synchronize() will returned immediately if memcg_in_oom is not set, so remove the check from handle_mm_fault()".
>>>
>>> However, that requires now always an indirect function call -- do we care about dropping that optimization?
>>>
>>>
>>
>> If memcg_in_oom is set, we will check it twice, one is from handle_mm_fault(), the other is from mem_cgroup_oom_synchronize(). That seems a bit redundant.
>>
>> if memcg_in_oom is not set, mem_cgroup_oom_synchronize() returns directly. Though it's an indirect function call, but the time spent can be negligible 
>> compare to the whole mm user falut preocess. And that won't cause stack overflow error.
> 
> I suggest you measure it.

test steps:
1) Run command: ./mmap_anon_test(global alloc, so the memcg_in_oom is not set)
2) Calculate the quotient of cost time and page-fault counts, run 10 rounds and average the results.

The test result shows that whether using indirect function call or not, the time spent in user fault
is almost the same, about 2.3ms.

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/time.h>
#include <inttypes.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <ctype.h>
#include <string.h>

#define SIZE (1 * 1024 * 1024 * 1024)
#define PAGE_SIZE 4096

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;
	char * ptr;
	size_t size = SIZE, step = PAGE_SIZE;
	int64_t start, cost;

	buf = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
	if (buf ==  MAP_FAILED) {
		printf("mmap failed\n");
	}

	start = current_time_ms();

	for(size_t i = 0; i < size; i += step ) {
		ptr = (char *)(buf + i);
		*ptr = 'a';
	}

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

	munmap(buf, size);

        return 0;
}