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x86/mm: track linear mapping split events

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To help with debugging the sluggishness caused by TLB miss/reload, we
introduce monotonic hugepage [direct mapped] 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 sources of direct hugepage splits is kernel
tracing (kprobes, tracepoints).

Note that the kernel's code segment [512 MB] points to the same physical
addresses that have been already mapped in the kernel's direct mapping
range.

Source : Documentation/x86/x86_64/mm.rst

When we enable kernel tracing, the kernel has to modify
attributes/permissions of the text segment hugepages that are direct
mapped causing them to split.

Kernel's direct mapped hugepages do not coalesce back after split and
remain in place for the remainder of the lifetime.

An instance of direct page splits when we turn on dynamic kernel tracing
....
cat /proc/vmstat | grep -i direct_map_level
direct_map_level2_splits 784
direct_map_level3_splits 12
bpftrace -e 'tracepoint:raw_syscalls:sys_enter { @ [pid, comm] =
count(); }'
cat /proc/vmstat | grep -i
direct_map_level
direct_map_level2_splits 789
direct_map_level3_splits 12
....

Link: https://lkml.kernel.org/r/20210218235744.1040634-1-saravanand@fb.com
Signed-off-by: Saravanan D <saravanand@fb.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Saravanan D 2021-05-04 18:38:03 -07:00 committed by Linus Torvalds
parent c675790972
commit 575299ea18
3 changed files with 16 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
arch/x86/mm/pat/set_memory.c
View File

@ -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;
}

4
include/linux/vm_event_item.h
View File

@ -124,6 +124,10 @@ 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
};

4
mm/vmstat.c
View File

@ -1369,6 +1369,10 @@ 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 */