From: Vernon Yang Currently, each scan always increases "progress" by HPAGE_PMD_NR, even if only scanning a single PTE/PMD entry. - When only scanning a sigle PTE entry, let me provide a detailed example: static int hpage_collapse_scan_pmd() { for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR; _pte++, addr += PAGE_SIZE) { pte_t pteval = ptep_get(_pte); ... if (pte_uffd_wp(pteval)) { <-- first scan hit result = SCAN_PTE_UFFD_WP; goto out_unmap; } } } During the first scan, if pte_uffd_wp(pteval) is true, the loop exits directly. In practice, only one PTE is scanned before termination. Here, "progress += 1" reflects the actual number of PTEs scanned, but previously "progress += HPAGE_PMD_NR" always. - When the memory has been collapsed to PMD, let me provide a detailed example: The following data is traced by bpftrace on a desktop system. After the system has been left idle for 10 minutes upon booting, a lot of SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE are observed during a full scan by khugepaged. From trace_mm_khugepaged_scan_pmd and trace_mm_khugepaged_scan_file, the following statuses were observed, with frequency mentioned next to them: SCAN_SUCCEED : 1 SCAN_EXCEED_SHARED_PTE: 2 SCAN_PMD_MAPPED : 142 SCAN_NO_PTE_TABLE : 178 total progress size : 674 MB Total time : 419 seconds, include khugepaged_scan_sleep_millisecs The khugepaged_scan list save all task that support collapse into hugepage, as long as the task is not destroyed, khugepaged will not remove it from the khugepaged_scan list. This exist a phenomenon where task has already collapsed all memory regions into hugepage, but khugepaged continues to scan it, which wastes CPU time and invalid, and due to khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for scanning a large number of invalid task, so scanning really valid task is later. After applying this patch, when the memory is either SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE, just skip it, as follow: SCAN_EXCEED_SHARED_PTE: 2 SCAN_PMD_MAPPED : 147 SCAN_NO_PTE_TABLE : 173 total progress size : 45 MB Total time : 20 seconds Signed-off-by: Vernon Yang --- mm/khugepaged.c | 38 ++++++++++++++++++++++++++++---------- 1 file changed, 28 insertions(+), 10 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 4049234e1c8b..8b68ae3bc2c5 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -68,7 +68,10 @@ enum scan_result { static struct task_struct *khugepaged_thread __read_mostly; static DEFINE_MUTEX(khugepaged_mutex); -/* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */ +/* + * default scan 8*HPAGE_PMD_NR ptes, pmd_mapped, no_pte_table or vmas + * every 10 second. + */ static unsigned int khugepaged_pages_to_scan __read_mostly; static unsigned int khugepaged_pages_collapsed; static unsigned int khugepaged_full_scans; @@ -1240,7 +1243,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a } static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, - struct vm_area_struct *vma, unsigned long start_addr, bool *mmap_locked, + struct vm_area_struct *vma, unsigned long start_addr, + bool *mmap_locked, unsigned int *cur_progress, struct collapse_control *cc) { pmd_t *pmd; @@ -1256,19 +1260,27 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK); result = find_pmd_or_thp_or_none(mm, start_addr, &pmd); - if (result != SCAN_SUCCEED) + if (result != SCAN_SUCCEED) { + if (cur_progress) + *cur_progress = 1; goto out; + } memset(cc->node_load, 0, sizeof(cc->node_load)); nodes_clear(cc->alloc_nmask); pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl); if (!pte) { + if (cur_progress) + *cur_progress = 1; result = SCAN_NO_PTE_TABLE; goto out; } for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR; _pte++, addr += PAGE_SIZE) { + if (cur_progress) + *cur_progress += 1; + pte_t pteval = ptep_get(_pte); if (pte_none_or_zero(pteval)) { ++none_or_zero; @@ -2288,8 +2300,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, return result; } -static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr, - struct file *file, pgoff_t start, struct collapse_control *cc) +static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, + unsigned long addr, struct file *file, pgoff_t start, + unsigned int *cur_progress, struct collapse_control *cc) { struct folio *folio = NULL; struct address_space *mapping = file->f_mapping; @@ -2378,6 +2391,8 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned cond_resched_rcu(); } } + if (cur_progress) + *cur_progress = HPAGE_PMD_NR; rcu_read_unlock(); if (result == SCAN_SUCCEED) { @@ -2457,6 +2472,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result while (khugepaged_scan.address < hend) { bool mmap_locked = true; + unsigned int cur_progress = 0; cond_resched(); if (unlikely(hpage_collapse_test_exit_or_disable(mm))) @@ -2473,7 +2489,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result mmap_read_unlock(mm); mmap_locked = false; *result = hpage_collapse_scan_file(mm, - khugepaged_scan.address, file, pgoff, cc); + khugepaged_scan.address, file, pgoff, + &cur_progress, cc); fput(file); if (*result == SCAN_PTE_MAPPED_HUGEPAGE) { mmap_read_lock(mm); @@ -2487,7 +2504,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result } } else { *result = hpage_collapse_scan_pmd(mm, vma, - khugepaged_scan.address, &mmap_locked, cc); + khugepaged_scan.address, &mmap_locked, + &cur_progress, cc); } if (*result == SCAN_SUCCEED) @@ -2495,7 +2513,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result /* move to next address */ khugepaged_scan.address += HPAGE_PMD_SIZE; - progress += HPAGE_PMD_NR; + progress += cur_progress; if (!mmap_locked) /* * We released mmap_lock so break loop. Note @@ -2818,7 +2836,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, mmap_locked = false; *lock_dropped = true; result = hpage_collapse_scan_file(mm, addr, file, pgoff, - cc); + NULL, cc); if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb && mapping_can_writeback(file->f_mapping)) { @@ -2833,7 +2851,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, fput(file); } else { result = hpage_collapse_scan_pmd(mm, vma, addr, - &mmap_locked, cc); + &mmap_locked, NULL, cc); } if (!mmap_locked) *lock_dropped = true; -- 2.51.0