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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 10.07.24 06:02, Barry Song wrote: > On Wed, Jul 10, 2024 at 3:59 PM David Hildenbrand wrote: >> >> On 10.07.24 05:32, Barry Song wrote: >>> On Wed, Jul 10, 2024 at 9:23 AM Andrew Morton wrote: >>>> >>>> On Tue, 9 Jul 2024 20:31:15 +0800 Zhiguo Jiang 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 >>> #include >>> #include >>> #include >>> #include >>> #include >>> #include >>> #include >>> >>> #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? IMHO, there is a big difference here between anon and shmem. (well, anon_shmem would actually be different :) ) -- Cheers, David / dhildenb