[PATCH V6] x86/mm: Tracking linear mapping split events

Saravanan D
2021-02-18T23:58:54Z

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
....

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>
---
This patch has been acked and can be routed through either x86 or -mm
Please let me know if there's anything needed. Thanks.
---
arch/x86/mm/pat/set_memory.c | 8 ++++++++
include/linux/vm_event_item.h | 4 ++++
mm/vmstat.c | 4 ++++
3 files changed, 16 insertions(+)

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