[RFC v2 3/3] mm: make swapin readahead to improve thp collapse rate

Ebru Akagunduz
2015-06-20T11:29:13Z

This patch makes swapin readahead to improve thp collapse rate.
When khugepaged scanned pages, there can be a few of the pages
in swap area.

With the patch THP can collapse 4kB pages into a THP when
there are up to max_ptes_swap swap ptes in a 2MB range.

The patch was tested with a test program that allocates
800MB of memory, writes to it, and then sleeps. I force
the system to swap out all. Afterwards, the test program
touches the area by writing, it skips a page in each
20 pages of the area.

Without the patch, system did not swap in readahead.
THP rate was %47 of the program of the memory, it
did not change over time.

With this patch, after 10 minutes of waiting khugepaged had
collapsed %99 of the program's memory.

Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Acked-by: Rik van Riel <riel@redhat.com>
---
Changes in v2:
- Use FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_RETRY_NOWAIT flag
instead of 0x0 when called do_swap_page
from __collapse_huge_page_swapin

Test results:

After swapped out
-------------------------------------------------------------------
| Anonymous | AnonHugePages | Swap | Fraction |
-------------------------------------------------------------------
With patch | 265772 kB | 264192 kB | 534232 kB | %99 |
-------------------------------------------------------------------
Without patch | 238160 kB | 235520 kB | 561844 kB | %98 |
-------------------------------------------------------------------

After swapped in
-------------------------------------------------------------------
| Anonymous | AnonHugePages | Swap | Fraction |
-------------------------------------------------------------------
With patch | 532756 kB | 528384 kB | 267248 kB | %99 |
-------------------------------------------------------------------
Without patch | 499956 kB | 235520 kB | 300048 kB | %47 |
-------------------------------------------------------------------

include/linux/mm.h | 4 ++++
include/trace/events/huge_memory.h | 24 ++++++++++++++++++++++++
mm/huge_memory.c | 37 +++++++++++++++++++++++++++++++++++++
mm/memory.c | 2 +-
4 files changed, 66 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7f47178..f66ff8a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -29,6 +29,10 @@ struct user_struct;
struct writeback_control;
struct bdi_writeback;

+extern int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
+ unsigned long address, pte_t *page_table, pmd_t *pmd,
+ unsigned int flags, pte_t orig_pte);
+
#ifndef CONFIG_NEED_MULTIPLE_NODES /* Don't use mapnrs, do it properly */
extern unsigned long max_mapnr;

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index 53c9f2e..0117ab9 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -95,5 +95,29 @@ TRACE_EVENT(mm_collapse_huge_page_isolate,
__entry->writable)
);

+TRACE_EVENT(mm_collapse_huge_page_swapin,
+
+ TP_PROTO(struct mm_struct *mm, unsigned long vm_start, int swap_pte),
+
+ TP_ARGS(mm, vm_start, swap_pte),
+
+ TP_STRUCT__entry(
+ __field(struct mm_struct *, mm)
+ __field(unsigned long, vm_start)
+ __field(int, swap_pte)
+ ),
+
+ TP_fast_assign(
+ __entry->mm = mm;
+ __entry->vm_start = vm_start;
+ __entry->swap_pte = swap_pte;
+ ),
+
+ TP_printk("mm=%p, vm_start=%04lx, swap_pte=%d",
+ __entry->mm,
+ __entry->vm_start,
+ __entry->swap_pte)
+);
+
#endif /* __HUGE_MEMORY_H */
#include <trace/define_trace.h>
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 22bc0bf..064fd72 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2496,6 +2496,41 @@ static bool hugepage_vma_check(struct vm_area_struct *vma)
return true;
}

+/*
+ * Bring missing pages in from swap, to complete THP collapse.
+ * Only done if khugepaged_scan_pmd believes it is worthwhile.
+ *
+ * Called and returns without pte mapped or spinlocks held,
+ * but with mmap_sem held to protect against vma changes.
+ */
+
+static void __collapse_huge_page_swapin(struct mm_struct *mm,
+ struct vm_area_struct *vma,
+ unsigned long address, pmd_t *pmd,
+ pte_t *pte)
+{
+ unsigned long _address;
+ pte_t pteval = *pte;
+ int swap_pte = 0;
+
+ pte = pte_offset_map(pmd, address);
+ for (_address = address; _address < address + HPAGE_PMD_NR*PAGE_SIZE;
+ pte++, _address += PAGE_SIZE) {
+ pteval = *pte;
+ if (is_swap_pte(pteval)) {
+ swap_pte++;
+ do_swap_page(mm, vma, _address, pte, pmd,
+ FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_RETRY_NOWAIT,
+ pteval);
+ /* pte is unmapped now, we need to map it */
+ pte = pte_offset_map(pmd, _address);
+ }
+ }
+ pte--;
+ pte_unmap(pte);
+ trace_mm_collapse_huge_page_swapin(mm, vma->vm_start, swap_pte);
+}
+
static void collapse_huge_page(struct mm_struct *mm,
unsigned long address,
struct page **hpage,
@@ -2551,6 +2586,8 @@ static void collapse_huge_page(struct mm_struct *mm,
if (!pmd)
goto out;

+ __collapse_huge_page_swapin(mm, vma, address, pmd, pte);
+
anon_vma_lock_write(vma->anon_vma);

pte = pte_offset_map(pmd, address);
diff --git a/mm/memory.c b/mm/memory.c
index e1c45d0..d801dc5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2443,7 +2443,7 @@ EXPORT_SYMBOL(unmap_mapping_range);
* We return with the mmap_sem locked or unlocked in the same cases
* as does filemap_fault().
*/
-static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
+int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
unsigned long address, pte_t *page_table, pmd_t *pmd,
unsigned int flags, pte_t orig_pte)
{
--
1.9.1