To help with debugging the sluggishness caused by TLB miss/reload, we introduce monotonic lifetime hugepage split event counts since system state: SYSTEM_RUNNING to be displayed as part of /proc/vmstat in x86 servers The lifetime split event information will be displayed at the bottom of /proc/vmstat .... swap_ra 0 swap_ra_hit 0 direct_map_level2_splits 94 direct_map_level3_splits 4 nr_unstable 0 .... One of the many lasting (as we don't coalesce back) sources for huge page splits is tracing as the granular page attribute/permission changes would force the kernel to split code segments mapped to huge pages to smaller ones thereby increasing the probability of TLB miss/reload even after tracing has been stopped. Documentation regarding linear mapping split events added to admin-guide as requested in V3 of the patch. Signed-off-by: Saravanan D <saravanand@xxxxxx> --- .../admin-guide/mm/direct_mapping_splits.rst | 59 +++++++++++++++++++ Documentation/admin-guide/mm/index.rst | 1 + arch/x86/mm/pat/set_memory.c | 8 +++ include/linux/vm_event_item.h | 4 ++ mm/vmstat.c | 4 ++ 5 files changed, 76 insertions(+) create mode 100644 Documentation/admin-guide/mm/direct_mapping_splits.rst diff --git a/Documentation/admin-guide/mm/direct_mapping_splits.rst b/Documentation/admin-guide/mm/direct_mapping_splits.rst new file mode 100644 index 000000000000..298751391deb --- /dev/null +++ b/Documentation/admin-guide/mm/direct_mapping_splits.rst @@ -0,0 +1,59 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================== +Direct Mapping Splits +===================== + +Kernel maps all of physical memory in linear/direct mapped pages with +translation of virtual kernel address to physical address is achieved +through a simple subtraction of offset. CPUs maintain a cache of these +translations on fast caches called TLBs. CPU architectures like x86 allow +direct mapping large portions of memory into hugepages (2M, 1G, etc) in +various page table levels. + +Maintaining huge direct mapped pages greatly reduces TLB miss pressure. +The splintering of huge direct pages into smaller ones does result in +a measurable performance hit caused by frequent TLB miss and reloads. + +One of the many lasting (as we don't coalesce back) sources for huge page +splits is tracing as the granular page attribute/permission changes would +force the kernel to split code segments mapped to hugepages to smaller +ones thus increasing the probability of TLB miss/reloads even after +tracing has been stopped. + +On x86 systems, we can track the splitting of huge direct mapped pages +through lifetime event counters in ``/proc/vmstat`` + + direct_map_level2_splits xxx + direct_map_level3_splits yyy + +where: + +direct_map_level2_splits + are 2M/4M hugepage split events +direct_map_level3_splits + are 1G hugepage split events + +The distribution of direct mapped system memory in various page sizes +post splits can be viewed through ``/proc/meminfo`` whose output +will include the following lines depending upon supporting CPU +architecture + + DirectMap4k: xxxxx kB + DirectMap2M: yyyyy kB + DirectMap1G: zzzzz kB + +where: + +DirectMap4k + is the total amount of direct mapped memory (in kB) + accessed through 4k pages +DirectMap2M + is the total amount of direct mapped memory (in kB) + accessed through 2M pages +DirectMap1G + is the total amount of direct mapped memory (in kB) + accessed through 1G pages + + +-- Saravanan D, Jan 27, 2021 diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst index 4b14d8b50e9e..9439780f3f07 100644 --- a/Documentation/admin-guide/mm/index.rst +++ b/Documentation/admin-guide/mm/index.rst @@ -38,3 +38,4 @@ the Linux memory management. soft-dirty transhuge userfaultfd + direct_mapping_splits diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c index 16f878c26667..a7b3c5f1d316 100644 --- a/arch/x86/mm/pat/set_memory.c +++ b/arch/x86/mm/pat/set_memory.c @@ -16,6 +16,8 @@ #include <linux/pci.h> #include <linux/vmalloc.h> #include <linux/libnvdimm.h> +#include <linux/vmstat.h> +#include <linux/kernel.h> #include <asm/e820/api.h> #include <asm/processor.h> @@ -91,6 +93,12 @@ static void split_page_count(int level) return; direct_pages_count[level]--; + if (system_state == SYSTEM_RUNNING) { + if (level == PG_LEVEL_2M) + count_vm_event(DIRECT_MAP_LEVEL2_SPLIT); + else if (level == PG_LEVEL_1G) + count_vm_event(DIRECT_MAP_LEVEL3_SPLIT); + } direct_pages_count[level - 1] += PTRS_PER_PTE; } diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h index 18e75974d4e3..7c06c2bdc33b 100644 --- a/include/linux/vm_event_item.h +++ b/include/linux/vm_event_item.h @@ -120,6 +120,10 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT, #ifdef CONFIG_SWAP SWAP_RA, SWAP_RA_HIT, +#endif +#ifdef CONFIG_X86 + DIRECT_MAP_LEVEL2_SPLIT, + DIRECT_MAP_LEVEL3_SPLIT, #endif NR_VM_EVENT_ITEMS }; diff --git a/mm/vmstat.c b/mm/vmstat.c index f8942160fc95..a43ac4ac98a2 100644 --- a/mm/vmstat.c +++ b/mm/vmstat.c @@ -1350,6 +1350,10 @@ const char * const vmstat_text[] = { "swap_ra", "swap_ra_hit", #endif +#ifdef CONFIG_X86 + "direct_map_level2_splits", + "direct_map_level3_splits", +#endif #endif /* CONFIG_VM_EVENT_COUNTERS || CONFIG_MEMCG */ }; #endif /* CONFIG_PROC_FS || CONFIG_SYSFS || CONFIG_NUMA || CONFIG_MEMCG */ -- 2.24.1