Re: [PATCHv2 8/8] khugepaged: Introduce 'max_ptes_shared' tunable

From: Zi Yan
Date: Mon Apr 06 2020 - 09:17:45 EST

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On 3 Apr 2020, at 7:29, Kirill A. Shutemov wrote:

> External email: Use caution opening links or attachments
>
>
> ``max_ptes_shared`` speicies how many pages can be shared across multiple

s/speicies/specifies

> processes. Exeeding the number woul block the collapse::

s/Exeeding/Exceeding

s/woul/would

>
> /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_shared
>
> A higher value may increase memory footprint for some workloads.
>
> By default, at least half of pages has to be not shared.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
> Documentation/admin-guide/mm/transhuge.rst | 7 ++
> include/trace/events/huge_memory.h | 3 +-
> mm/khugepaged.c | 52 ++++++++++++--
> tools/testing/selftests/vm/khugepaged.c | 83 ++++++++++++++++++++++
> 4 files changed, 140 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index bd5714547cee..d16e4f2bb70f 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -220,6 +220,13 @@ memory. A lower value can prevent THPs from being
> collapsed, resulting fewer pages being collapsed into
> THPs, and lower memory access performance.
>
> +``max_ptes_shared`` speicies how many pages can be shared across multiple
> +processes. Exeeding the number woul block the collapse::

s/speicies/specifies

s/Exeeding/Exceeding

s/woul/would
> +
> + /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_shared
> +
> +A higher value may increase memory footprint for some workloads.
> +
> Boot parameter
> ==============
>
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index d82a0f4e824d..53532f5925c3 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -12,6 +12,8 @@
> EM( SCAN_SUCCEED, "succeeded") \
> EM( SCAN_PMD_NULL, "pmd_null") \
> EM( SCAN_EXCEED_NONE_PTE, "exceed_none_pte") \
> + EM( SCAN_EXCEED_SWAP_PTE, "exceed_swap_pte") \
> + EM( SCAN_EXCEED_SHARED_PTE, "exceed_shared_pte") \
> EM( SCAN_PTE_NON_PRESENT, "pte_non_present") \
> EM( SCAN_PAGE_RO, "no_writable_page") \
> EM( SCAN_LACK_REFERENCED_PAGE, "lack_referenced_page") \
> @@ -30,7 +32,6 @@
> EM( SCAN_DEL_PAGE_LRU, "could_not_delete_page_from_lru")\
> EM( SCAN_ALLOC_HUGE_PAGE_FAIL, "alloc_huge_page_failed") \
> EM( SCAN_CGROUP_CHARGE_FAIL, "ccgroup_charge_failed") \
> - EM( SCAN_EXCEED_SWAP_PTE, "exceed_swap_pte") \
> EM( SCAN_TRUNCATED, "truncated") \
> EMe(SCAN_PAGE_HAS_PRIVATE, "page_has_private") \
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 49e56e4e30d1..bfb6155f1d69 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -28,6 +28,8 @@ enum scan_result {
> SCAN_SUCCEED,
> SCAN_PMD_NULL,
> SCAN_EXCEED_NONE_PTE,
> + SCAN_EXCEED_SWAP_PTE,
> + SCAN_EXCEED_SHARED_PTE,
> SCAN_PTE_NON_PRESENT,
> SCAN_PAGE_RO,
> SCAN_LACK_REFERENCED_PAGE,
> @@ -46,7 +48,6 @@ enum scan_result {
> SCAN_DEL_PAGE_LRU,
> SCAN_ALLOC_HUGE_PAGE_FAIL,
> SCAN_CGROUP_CHARGE_FAIL,
> - SCAN_EXCEED_SWAP_PTE,
> SCAN_TRUNCATED,
> SCAN_PAGE_HAS_PRIVATE,
> };
> @@ -71,6 +72,7 @@ static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);
> */
> static unsigned int khugepaged_max_ptes_none __read_mostly;
> static unsigned int khugepaged_max_ptes_swap __read_mostly;
> +static unsigned int khugepaged_max_ptes_shared __read_mostly;
>
> #define MM_SLOTS_HASH_BITS 10
> static __read_mostly DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
> @@ -290,15 +292,43 @@ static struct kobj_attribute khugepaged_max_ptes_swap_attr =
> __ATTR(max_ptes_swap, 0644, khugepaged_max_ptes_swap_show,
> khugepaged_max_ptes_swap_store);
>
> +static ssize_t khugepaged_max_ptes_shared_show(struct kobject *kobj,
> + struct kobj_attribute *attr,
> + char *buf)
> +{
> + return sprintf(buf, "%u\n", khugepaged_max_ptes_shared);
> +}
> +
> +static ssize_t khugepaged_max_ptes_shared_store(struct kobject *kobj,
> + struct kobj_attribute *attr,
> + const char *buf, size_t count)
> +{
> + int err;
> + unsigned long max_ptes_shared;
> +
> + err = kstrtoul(buf, 10, &max_ptes_shared);
> + if (err || max_ptes_shared > HPAGE_PMD_NR-1)
> + return -EINVAL;
> +
> + khugepaged_max_ptes_shared = max_ptes_shared;
> +
> + return count;
> +}
> +
> +static struct kobj_attribute khugepaged_max_ptes_shared_attr =
> + __ATTR(max_ptes_shared, 0644, khugepaged_max_ptes_shared_show,
> + khugepaged_max_ptes_shared_store);
> +
> static struct attribute *khugepaged_attr[] = {
> &khugepaged_defrag_attr.attr,
> &khugepaged_max_ptes_none_attr.attr,
> + &khugepaged_max_ptes_swap_attr.attr,
> + &khugepaged_max_ptes_shared_attr.attr,
> &pages_to_scan_attr.attr,
> &pages_collapsed_attr.attr,
> &full_scans_attr.attr,
> &scan_sleep_millisecs_attr.attr,
> &alloc_sleep_millisecs_attr.attr,
> - &khugepaged_max_ptes_swap_attr.attr,
> NULL,
> };
>
> @@ -360,6 +390,7 @@ int __init khugepaged_init(void)
> khugepaged_pages_to_scan = HPAGE_PMD_NR * 8;
> khugepaged_max_ptes_none = HPAGE_PMD_NR - 1;
> khugepaged_max_ptes_swap = HPAGE_PMD_NR / 8;
> + khugepaged_max_ptes_shared = HPAGE_PMD_NR / 2;
>
> return 0;
> }
> @@ -549,7 +580,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> {
> struct page *page = NULL;
> pte_t *_pte;
> - int none_or_zero = 0, result = 0, referenced = 0;
> + int none_or_zero = 0, shared = 0, result = 0, referenced = 0;
> bool writable = false;
>
> for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
> @@ -577,6 +608,12 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>
> VM_BUG_ON_PAGE(!PageAnon(page), page);
>
> + if (page_mapcount(page) > 1 &&
> + ++shared > khugepaged_max_ptes_shared) {
> + result = SCAN_EXCEED_SHARED_PTE;
> + goto out;
> + }
> +
> if (PageCompound(page)) {
> struct page *p;
> page = compound_head(page);
> @@ -1168,7 +1205,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> {
> pmd_t *pmd;
> pte_t *pte, *_pte;
> - int ret = 0, none_or_zero = 0, result = 0, referenced = 0;
> + int ret = 0, result = 0, referenced = 0;
> + int none_or_zero = 0, shared = 0;
> struct page *page = NULL;
> unsigned long _address;
> spinlock_t *ptl;
> @@ -1218,6 +1256,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> goto out_unmap;
> }
>
> + if (page_mapcount(page) > 1 &&
> + ++shared > khugepaged_max_ptes_shared) {
> + result = SCAN_EXCEED_SHARED_PTE;
> + goto out_unmap;
> + }
> +
> page = compound_head(page);
>
> /*
> diff --git a/tools/testing/selftests/vm/khugepaged.c b/tools/testing/selftests/vm/khugepaged.c
> index 3eeff4a0fbc9..9ae119234a39 100644
> --- a/tools/testing/selftests/vm/khugepaged.c
> +++ b/tools/testing/selftests/vm/khugepaged.c
> @@ -77,6 +77,7 @@ struct khugepaged_settings {
> unsigned int scan_sleep_millisecs;
> unsigned int max_ptes_none;
> unsigned int max_ptes_swap;
> + unsigned int max_ptes_shared;
> unsigned long pages_to_scan;
> };
>
> @@ -276,6 +277,7 @@ static void write_settings(struct settings *settings)
> khugepaged->scan_sleep_millisecs);
> write_num("khugepaged/max_ptes_none", khugepaged->max_ptes_none);
> write_num("khugepaged/max_ptes_swap", khugepaged->max_ptes_swap);
> + write_num("khugepaged/max_ptes_shared", khugepaged->max_ptes_shared);
> write_num("khugepaged/pages_to_scan", khugepaged->pages_to_scan);
> }
>
> @@ -312,6 +314,7 @@ static void save_settings(void)
> read_num("khugepaged/scan_sleep_millisecs"),
> .max_ptes_none = read_num("khugepaged/max_ptes_none"),
> .max_ptes_swap = read_num("khugepaged/max_ptes_swap"),
> + .max_ptes_shared = read_num("khugepaged/max_ptes_shared"),
> .pages_to_scan = read_num("khugepaged/pages_to_scan"),
> };
> success("OK");
> @@ -843,12 +846,90 @@ static void collapse_fork_compound(void)
> fail("Fail");
> fill_memory(p, 0, page_size);
>
> + write_num("khugepaged/max_ptes_shared", hpage_pmd_nr - 1);
> if (wait_for_scan("Collapse PTE table full of compound pages in child", p))
> fail("Timeout");
> else if (check_huge(p))
> success("OK");
> else
> fail("Fail");
> + write_num("khugepaged/max_ptes_shared",
> + default_settings.khugepaged.max_ptes_shared);
> +
> + validate_memory(p, 0, hpage_pmd_size);
> + munmap(p, hpage_pmd_size);
> + exit(exit_status);
> + }
> +
> + wait(&wstatus);
> + exit_status += WEXITSTATUS(wstatus);
> +
> + printf("Check if parent still has huge page...");
> + if (check_huge(p))
> + success("OK");
> + else
> + fail("Fail");
> + validate_memory(p, 0, hpage_pmd_size);
> + munmap(p, hpage_pmd_size);
> +}
> +
> +static void collapse_max_ptes_shared()
> +{
> + int max_ptes_shared = read_num("khugepaged/max_ptes_shared");
> + int wstatus;
> + void *p;
> +
> + p = alloc_mapping();
> +
> + printf("Allocate huge page...");
> + madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
> + fill_memory(p, 0, hpage_pmd_size);
> + if (check_huge(p))
> + success("OK");
> + else
> + fail("Fail");
> +
> + printf("Share huge page over fork()...");
> + if (!fork()) {
> + /* Do not touch settings on child exit */
> + skip_settings_restore = true;
> + exit_status = 0;
> +
> + if (check_huge(p))
> + success("OK");
> + else
> + fail("Fail");
> +
> + printf("Trigger CoW in %d of %d...",
> + hpage_pmd_nr - max_ptes_shared - 1, hpage_pmd_nr);
> + fill_memory(p, 0, (hpage_pmd_nr - max_ptes_shared - 1) * page_size);
> + if (!check_huge(p))
> + success("OK");
> + else
> + fail("Fail");
> +
> + if (wait_for_scan("Do not collapse with max_ptes_shared exeeded", p))
> + fail("Timeout");
> + else if (!check_huge(p))
> + success("OK");
> + else
> + fail("Fail");
> +
> + printf("Trigger CoW in %d of %d...",
> + hpage_pmd_nr - max_ptes_shared, hpage_pmd_nr);
> + fill_memory(p, 0, (hpage_pmd_nr - max_ptes_shared) * page_size);
> + if (!check_huge(p))
> + success("OK");
> + else
> + fail("Fail");
> +
> +
> + if (wait_for_scan("Collapse with max_ptes_shared PTEs shared", p))
> + fail("Timeout");
> + else if (check_huge(p))
> + success("OK");
> + else
> + fail("Fail");
>
> validate_memory(p, 0, hpage_pmd_size);
> munmap(p, hpage_pmd_size);
> @@ -877,6 +958,7 @@ int main(void)
>
> default_settings.khugepaged.max_ptes_none = hpage_pmd_nr - 1;
> default_settings.khugepaged.max_ptes_swap = hpage_pmd_nr / 8;
> + default_settings.khugepaged.max_ptes_shared = hpage_pmd_nr / 2;
> default_settings.khugepaged.pages_to_scan = hpage_pmd_nr * 8;
>
> save_settings();
> @@ -894,6 +976,7 @@ int main(void)
> collapse_compound_extreme();
> collapse_fork();
> collapse_fork_compound();
> + collapse_max_ptes_shared();
>
> restore_settings(0);
> }
> --
> 2.26.0

â
Best Regards,
Yan Zi

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