Re: [PATCH 0/1] prctl: allow overriding system THP policy to always

[Zi Yan <ziy@nvidia.com>]

+Yafang, who is also looking at changing THP config at cgroup/container level.

On 7 May 2025, at 10:00, Usama Arif wrote:

> Allowing override of global THP policy per process allows workloads
> that have shown to benefit from hugepages to do so, without regressing
> workloads that wouldn't benefit. This will allow such types of
> workloads to be run/stacked on the same machine.
>
> It also helps in rolling out hugepages in hyperscaler configurations
> for workloads that benefit from them, where a single THP policy is
> likely to be used across the entire fleet, and prctl will help override it.
>
> An advantage of doing it via prctl vs creating a cgroup specific
> option (like /sys/fs/cgroup/test/memory.transparent_hugepage.enabled) is
> that this will work even when there are no cgroups present, and my
> understanding is there is a strong preference of cgroups controls being
> hierarchical which usually means them having a numerical value.

Hi Usama,

Do you mind giving an example on how to change THP policy for a set of
processes running in a container (under a cgroup)?

Yafang mentioned that the prctl approach would require restarting all running
services[1] and other inflexiblities, so he proposed to use BPF to change THP
policy[2]. I wonder if Yafang's issues also apply to your case and if you
have a solution to them.

Thanks.

[1] https://lore.kernel.org/linux-mm/CALOAHbCXMi2GaZdHJaNLXxGsJf-hkDTrztsQiceaBcJ8d8p3cA@mail.gmail.com/
[2] https://lore.kernel.org/linux-mm/20250429024139.34365-1-laoar.shao@gmail.com/
>
>
> The output and code of test program is below:
>
> [root@vm4 vmuser]# echo madvise > /sys/kernel/mm/transparent_hugepage/enabled
> [root@vm4 vmuser]# echo inherit > /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
> [root@vm4 vmuser]# ./a.out
> Default THP setting:
> THP is not set to 'always'.
> PR_SET_THP_ALWAYS = 1
> THP is set to 'always'.
> PR_SET_THP_ALWAYS = 0
> THP is not set to 'always'.
>
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <unistd.h>
> #include <sys/mman.h>
> #include <sys/prctl.h>
>
> #define PR_SET_THP_ALWAYS 78
> #define SIZE 12 * (2 * 1024 * 1024) // 24 MB
>
> void check_smaps(void) {
>     FILE *file = fopen("/proc/self/smaps", "r");
>     if (!file) {
>         perror("fopen");
>         return;
>     }
>
>     char line[256];
>     int is_hugepage = 0;
>     while (fgets(line, sizeof(line), file)) {
>         // if (strstr(line, "AnonHugePages:"))
>         //     printf("%s\n", line);
>         if (strstr(line, "AnonHugePages:") && strstr(line, "24576 kB"))
> {
>             // printf("%s\n", line);
>             is_hugepage = 1;
>             break;
>         }
>     }
>     fclose(file);
>     if (is_hugepage) {
>         printf("THP is set to 'always'.\n");
>     } else {
>         printf("THP is not set to 'always'.\n");
>     }
> }
>
> void test_mmap_thp(void) {
>     char *buffer = (char *)mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
>                                 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>     if (buffer == MAP_FAILED) {
>         perror("mmap");
>         return;
>     }
>     // Touch the memory to ensure it's allocated
>     memset(buffer, 0, SIZE);
>     check_smaps();
>     munmap(buffer, SIZE);
> }
>
> int main() {
>     printf("Default THP setting: \n");
>     test_mmap_thp();
>     printf("PR_SET_THP_ALWAYS = 1 \n");
>     prctl(PR_SET_THP_ALWAYS, 1, NULL, NULL, NULL);
>     test_mmap_thp();
>     printf("PR_SET_THP_ALWAYS = 0 \n");
>     prctl(PR_SET_THP_ALWAYS, 0, NULL, NULL, NULL);
>     test_mmap_thp();
>
>     return 0;
> }
>
>
> Usama Arif (1):
>   prctl: allow overriding system THP policy to always per process
>
>  include/linux/huge_mm.h                          |  3 ++-
>  include/linux/mm_types.h                         |  7 ++-----
>  include/uapi/linux/prctl.h                       |  3 +++
>  kernel/sys.c                                     | 16 ++++++++++++++++
>  tools/include/uapi/linux/prctl.h                 |  3 +++
>  .../perf/trace/beauty/include/uapi/linux/prctl.h |  3 +++
>  6 files changed, 29 insertions(+), 6 deletions(-)
>
> -- 
> 2.47.1


--
Best Regards,
Yan, Zi