Re: [PATCH v7] mm: shrink skip folio mapped by an exiting process
From: Barry Song
Date: Wed Jul 10 2024 - 00:44:45 EST
On Wed, Jul 10, 2024 at 4:04 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 10.07.24 06:02, Barry Song wrote:
> > On Wed, Jul 10, 2024 at 3:59 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 10.07.24 05:32, Barry Song wrote:
> >>> On Wed, Jul 10, 2024 at 9:23 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >>>>
> >>>> On Tue, 9 Jul 2024 20:31:15 +0800 Zhiguo Jiang <justinjiang@xxxxxxxx> 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.
>
> But they won't get necessarily *freed* when unmapping them. They might
> just continue living in tmpfs? where some other process might just map
> them later?
>
You're correct. I overlooked this aspect, focusing on swap and thinking of shmem
solely in terms of swap.
> IMHO, there is a big difference here between anon and shmem. (well,
> anon_shmem would actually be different :) )
Even though anon_shmem behaves similarly to anonymous memory when
releasing memory, it doesn't seem worth the added complexity?
So unfortunately it seems Zhiguo still needs v10 to take folio_test_anon()
back? Sorry for my bad, Zhiguo.
>
> --
> Cheers,
>
> David / dhildenb
>
Thanks
Barry