Re: [PATCH v7] mm: shrink skip folio mapped by an exiting process

[Barry Song <21cnbao@gmail.com>]

On Wed, Jul 10, 2024 at 3:59 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 10.07.24 05:32, Barry Song wrote:
> > On Wed, Jul 10, 2024 at 9:23 AM Andrew Morton <akpm@linux-foundation.org> wrote:
> >>
> >> On Tue,  9 Jul 2024 20:31:15 +0800 Zhiguo Jiang <justinjiang@vivo.com> wrote:
> >>
> >>> The releasing process of the non-shared anonymous folio mapped solely by
> >>> an exiting process may go through two flows: 1) the anonymous folio is
> >>> firstly is swaped-out into swapspace and transformed into a swp_entry
> >>> in shrink_folio_list; 2) then the swp_entry is released in the process
> >>> exiting flow. This will result in the high cpu load of releasing a
> >>> non-shared anonymous folio mapped solely by an exiting process.
> >>>
> >>> When the low system memory and the exiting process exist at the same
> >>> time, it will be likely to happen, because the non-shared anonymous
> >>> folio mapped solely by an exiting process may be reclaimed by
> >>> shrink_folio_list.
> >>>
> >>> This patch is that shrink skips the non-shared anonymous folio solely
> >>> mapped by an exting process and this folio is only released directly in
> >>> the process exiting flow, which will save swap-out time and alleviate
> >>> the load of the process exiting.
> >>
> >> It would be helpful to provide some before-and-after runtime
> >> measurements, please.  It's a performance optimization so please let's
> >> see what effect it has.
> >
> > Hi Andrew,
> >
> > This was something I was curious about too, so I created a small test program
> > that allocates and continuously writes to 256MB of memory. Using QEMU, I set
> > up a small machine with only 300MB of RAM to trigger kswapd.
> >
> > qemu-system-aarch64 -M virt,gic-version=3,mte=off -nographic \
> >   -smp cpus=4 -cpu max \
> >   -m 300M -kernel arch/arm64/boot/Image
> >
> > The test program will be randomly terminated by its subprocess to trigger
> > the use case of this patch.
> >
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <unistd.h>
> > #include <string.h>
> > #include <sys/types.h>
> > #include <sys/wait.h>
> > #include <time.h>
> > #include <signal.h>
> >
> > #define MEMORY_SIZE (256 * 1024 * 1024)
> >
> > unsigned char *memory;
> >
> > void allocate_and_write_memory()
> > {
> >      memory = (unsigned char *)malloc(MEMORY_SIZE);
> >      if (memory == NULL) {
> >          perror("malloc");
> >          exit(EXIT_FAILURE);
> >      }
> >
> >      while (1)
> >          memset(memory, 0x11, MEMORY_SIZE);
> > }
> >
> > int main()
> > {
> >      pid_t pid;
> >      srand(time(NULL));
> >
> >      pid = fork();
> >
> >      if (pid < 0) {
> >          perror("fork");
> >          exit(EXIT_FAILURE);
> >      }
> >
> >      if (pid == 0) {
> >          int delay = (rand() % 10000) + 10000;
> >          usleep(delay * 1000);
> >
> >       /* kill parent when it is busy on swapping */
> >          kill(getppid(), SIGKILL);
> >          _exit(0);
> >      } else {
> >          allocate_and_write_memory();
> >
> >          wait(NULL);
> >
> >          free(memory);
> >      }
> >
> >      return 0;
> > }
> >
> > I tracked the number of folios that could be redundantly
> > swapped out by adding a simple counter as shown below:
> >
> > @@ -879,6 +880,9 @@ static bool folio_referenced_one(struct folio *folio,
> >                      check_stable_address_space(vma->vm_mm)) &&
> >                      folio_test_swapbacked(folio) &&
> >                      !folio_likely_mapped_shared(folio)) {
> > +                       static long i, size;
> > +                       size += folio_size(folio);
> > +                       pr_err("index: %d skipped folio:%lx total size:%d\n", i++, (unsigned long)folio, size);
> >                          pra->referenced = -1;
> >                          page_vma_mapped_walk_done(&pvmw);
> >                          return false;
> >
> >
> > This is what I have observed:
> >
> > / # /home/barry/develop/linux/skip_swap_out_test
> > [   82.925645] index: 0 skipped folio:fffffdffc0425400 total size:65536
> > [   82.925960] index: 1 skipped folio:fffffdffc0425800 total size:131072
> > [   82.927524] index: 2 skipped folio:fffffdffc0425c00 total size:196608
> > [   82.928649] index: 3 skipped folio:fffffdffc0426000 total size:262144
> > [   82.929383] index: 4 skipped folio:fffffdffc0426400 total size:327680
> > [   82.929995] index: 5 skipped folio:fffffdffc0426800 total size:393216
> > ...
> > [   88.469130] index: 6112 skipped folio:fffffdffc0390080 total size:97230848
> > [   88.469966] index: 6113 skipped folio:fffffdffc038d000 total size:97296384
> > [   89.023414] index: 6114 skipped folio:fffffdffc0366cc0 total size:97300480
> >
> > I observed that this patch effectively skipped 6114 folios (either 4KB or 64KB
> > mTHP), potentially reducing the swap-out by up to 92MB (97,300,480 bytes) during
> > the process exit.
> >
> > Despite the numerous mistakes Zhiguo made in sending this patch, it is still
> > quite valuable. Please consider pulling his v9 into the mm tree for testing.
>
> BTW, we dropped the folio_test_anon() check, but what about shmem? They
> also do __folio_set_swapbacked()?

my point is that the purpose is skipping redundant swap-out, if shmem is single
mapped, they could be also skipped.

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry