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Date:   Mon, 16 Aug 2021 10:48:17 -0500
From:   Tyler Hicks <tyhicks@linux.microsoft.com>
To:     Zhansaya Bagdauletkyzy <zhansayabagdaulet@gmail.com>
Cc:     akpm@linux-foundation.org, shuah@kernel.org, linux-mm@kvack.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
        pasha.tatashin@soleen.com
Subject: Re: [PATCH v2 2/2] selftests: vm: add COW time test for KSM pages
Message-ID: <20210816154817.GM5469@sequoia>
References: <cover.1628199399.git.zhansayabagdaulet@gmail.com>
 <eb4d348a90329b20d9659b86a501c9625efe4fab.1628199399.git.zhansayabagdaulet@gmail.com>
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On 2021-08-06 16:10:29, Zhansaya Bagdauletkyzy wrote:
> Since merged pages are copied every time they need to be modified,
> the write access time is different between shared and non-shared pages.
> Add ksm_cow_time() function which evaluates latency of these COW
> breaks. First, 4000 pages are allocated and the time, required to modify
> 1 byte in every other page, is measured. After this, the pages are
> merged into 2000 pairs and in each pair, 1 page is modified (i.e. they
> are decoupled) to detect COW breaks. The time needed to break COW of
> merged pages is then compared with performance of non-shared pages.
> 
> The test is run as follows: ./ksm_tests -C
> The output:
>         Total size:    15 MiB
> 
>         Not merged pages:
>         Total time:     0.001903112 s
>         Average speed:  3678.186 MiB/s
> 
>         Merged pages:
>         Total time:     0.006215680 s
>         Average speed:  1126.184 MiB/s
> 
> Signed-off-by: Zhansaya Bagdauletkyzy <zhansayabagdaulet@gmail.com>
> ---
> v1 -> v2:
>  As suggested by Pavel,
>  - add baseline figures with non-shared pages
>  - instead of having all pages merged together, create pairs of
>    duplicated pages
> 
> Pavel's review comments:
> https://lore.kernel.org/lkml/CA+CK2bDYZBBaU3pC369o01tCgydaJ6y91GZ0_MWONMMCajZOUw@mail.gmail.com/
> 
>  tools/testing/selftests/vm/ksm_tests.c | 84 +++++++++++++++++++++++++-
>  1 file changed, 81 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/vm/ksm_tests.c b/tools/testing/selftests/vm/ksm_tests.c
> index 432dfe615e50..382ee6ccd13a 100644
> --- a/tools/testing/selftests/vm/ksm_tests.c
> +++ b/tools/testing/selftests/vm/ksm_tests.c
> @@ -33,7 +33,8 @@ enum ksm_test_name {
>  	CHECK_KSM_UNMERGE,
>  	CHECK_KSM_ZERO_PAGE_MERGE,
>  	CHECK_KSM_NUMA_MERGE,
> -	KSM_MERGE_TIME
> +	KSM_MERGE_TIME,
> +	KSM_COW_TIME
>  };
>  
>  static int ksm_write_sysfs(const char *file_path, unsigned long val)
> @@ -98,7 +99,8 @@ static void print_help(void)
>  	       " -U (page unmerging)\n"
>  	       " -P evaluate merging time and speed.\n"
>  	       "    For this test, the size of duplicated memory area (in MiB)\n"
> -	       "    must be provided using -s option\n\n");
> +	       "    must be provided using -s option\n"
> +	       " -C evaluate the time required to break COW of merged pages.\n\n");
>  
>  	printf(" -a: specify the access protections of pages.\n"
>  	       "     <prot> must be of the form [rwx].\n"
> @@ -455,6 +457,75 @@ static int ksm_merge_time(int mapping, int prot, int timeout, size_t map_size)
>  	return KSFT_FAIL;
>  }
>  
> +static int ksm_cow_time(int mapping, int prot, int timeout, size_t page_size)
> +{
> +	void *map_ptr;
> +	struct timespec start_time, end_time;
> +	unsigned long cow_time_ns;
> +	int page_count = 4000;

size_t is more correct.

> +
> +	map_ptr = allocate_memory(NULL, prot, mapping, '*', page_size * page_count);
> +	if (!map_ptr)
> +		return KSFT_FAIL;
> +
> +	if (clock_gettime(CLOCK_MONOTONIC_RAW, &start_time)) {
> +		perror("clock_gettime");
> +		return KSFT_FAIL;
> +	}
> +	for (size_t i = 0; i < page_count; i = i + 2)
                           ^
I think it is best to use the correct test here even though you're
stepping by 2 and won't have any problems in practice:

 i < (page_count - 1)

This should be changed in all for loops in this function.

> +		memset(map_ptr + page_size * i, '-', 1);
> +	if (clock_gettime(CLOCK_MONOTONIC_RAW, &end_time)) {
> +		perror("clock_gettime");
> +		return KSFT_FAIL;
> +	}
> +
> +	cow_time_ns = (end_time.tv_sec - start_time.tv_sec) * NSEC_PER_SEC +
> +		       (end_time.tv_nsec - start_time.tv_nsec);
> +
> +	printf("Total size:    %lu MiB\n\n", (page_size * page_count) / MB);
> +	printf("Not merged pages:\n");
> +	printf("Total time:     %ld.%09ld s\n", cow_time_ns / NSEC_PER_SEC,
> +	       cow_time_ns % NSEC_PER_SEC);
> +	printf("Average speed:  %.3f MiB/s\n\n", ((page_size * (page_count / 2)) / MB) /
> +					       ((double)cow_time_ns / NSEC_PER_SEC));
> +
> +	/* Create 2000 pairs of duplicated pages */
> +	for (size_t i = 0; i < page_count; i = i + 2) {
> +		memset(map_ptr + page_size * i, '+', i + 1);
> +		memset(map_ptr + page_size * (i + 1), '+', i + 1);
> +	}

Since i is being used as the third argument to memset(), this loop only
works if page_count is less than page_size. I could see someone quickly
bumping page_count higher and then unknowingly getting bad test results.

Lets gain a little headroom in page_count. We are creating page_count/2
pairs of duplicate pages so we can use these memset() calls:

 memset(map_ptr + page_size * i, '+', i / 2 + 1);
 memset(map_ptr + page_size * (i + 1), '+', i / 2 + 1);

Then add a short comment to the page_count declaration mentioning that
page_count must be < (page_size / 2).


Everything else looks good to me.

Tyler

> +	if (ksm_merge_pages(map_ptr, page_size * page_count, start_time, timeout))
> +		goto err_out;
> +
> +	if (clock_gettime(CLOCK_MONOTONIC_RAW, &start_time)) {
> +		perror("clock_gettime");
> +		goto err_out;
> +	}
> +	for (size_t i = 0; i < page_count; i = i + 2)
> +		memset(map_ptr + page_size * i, '-', 1);
> +	if (clock_gettime(CLOCK_MONOTONIC_RAW, &end_time)) {
> +		perror("clock_gettime");
> +		goto err_out;
> +	}
> +
> +	cow_time_ns = (end_time.tv_sec - start_time.tv_sec) * NSEC_PER_SEC +
> +		       (end_time.tv_nsec - start_time.tv_nsec);
> +
> +	printf("Merged pages:\n");
> +	printf("Total time:     %ld.%09ld s\n", cow_time_ns / NSEC_PER_SEC,
> +	       cow_time_ns % NSEC_PER_SEC);
> +	printf("Average speed:  %.3f MiB/s\n", ((page_size * (page_count / 2)) / MB) /
> +					       ((double)cow_time_ns / NSEC_PER_SEC));
> +
> +	munmap(map_ptr, page_size * page_count);
> +	return KSFT_PASS;
> +
> +err_out:
> +	printf("Not OK\n");
> +	munmap(map_ptr, page_size * page_count);
> +	return KSFT_FAIL;
> +}
> +
>  int main(int argc, char *argv[])
>  {
>  	int ret, opt;
> @@ -468,7 +539,7 @@ int main(int argc, char *argv[])
>  	bool merge_across_nodes = KSM_MERGE_ACROSS_NODES_DEFAULT;
>  	long size_MB = 0;
>  
> -	while ((opt = getopt(argc, argv, "ha:p:l:z:m:s:MUZNP")) != -1) {
> +	while ((opt = getopt(argc, argv, "ha:p:l:z:m:s:MUZNPC")) != -1) {
>  		switch (opt) {
>  		case 'a':
>  			prot = str_to_prot(optarg);
> @@ -522,6 +593,9 @@ int main(int argc, char *argv[])
>  		case 'P':
>  			test_name = KSM_MERGE_TIME;
>  			break;
> +		case 'C':
> +			test_name = KSM_COW_TIME;
> +			break;
>  		default:
>  			return KSFT_FAIL;
>  		}
> @@ -571,6 +645,10 @@ int main(int argc, char *argv[])
>  		ret = ksm_merge_time(MAP_PRIVATE | MAP_ANONYMOUS, prot, ksm_scan_limit_sec,
>  				     size_MB);
>  		break;
> +	case KSM_COW_TIME:
> +		ret = ksm_cow_time(MAP_PRIVATE | MAP_ANONYMOUS, prot, ksm_scan_limit_sec,
> +				   page_size);
> +		break;
>  	}
>  
>  	if (ksm_restore(&ksm_sysfs_old)) {
> -- 
> 2.25.1
>