[PATCH 8/8] migrate_pages: batch flushing TLB

Huang Ying
2022-12-27T00:31:11Z

The TLB flushing will cost quite some CPU cycles during the folio
migration in some situations. For example, when migrate a folio of a
process with multiple active threads that run on multiple CPUs. After
batching the _unmap and _move in migrate_pages(), the TLB flushing can
be batched easily with the existing TLB flush batching mechanism.
This patch implements that.

We use the following test case to test the patch.

On a 2-socket Intel server,

- Run pmbench memory accessing benchmark

- Run `migratepages` to migrate pages of pmbench between node 0 and
node 1 back and forth.

With the patch, the TLB flushing IPI reduces 99.1% during the test and
the number of pages migrated successfully per second increases 291.7%.

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Bharata B Rao <bharata@amd.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: haoxin <xhao@linux.alibaba.com>
---
mm/migrate.c | 4 +++-
mm/rmap.c | 20 +++++++++++++++++---
2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 70a40b8fee1f..d7413164e748 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1215,7 +1215,7 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
/* Establish migration ptes */
VM_BUG_ON_FOLIO(folio_test_anon(src) &&
!folio_test_ksm(src) && !anon_vma, src);
- try_to_migrate(src, 0);
+ try_to_migrate(src, TTU_BATCH_FLUSH);
page_was_mapped = 1;
}

@@ -1732,6 +1732,8 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
stats->nr_thp_failed += thp_retry;
stats->nr_failed_pages += nr_retry_pages;
move:
+ try_to_unmap_flush();
+
retry = 1;
for (pass = 0; pass < 10 && (retry || large_retry); pass++) {
retry = 0;
diff --git a/mm/rmap.c b/mm/rmap.c
index b616870a09be..2e125f3e462e 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1976,7 +1976,21 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
} else {
flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
/* Nuke the page table entry. */
- pteval = ptep_clear_flush(vma, address, pvmw.pte);
+ if (should_defer_flush(mm, flags)) {
+ /*
+ * We clear the PTE but do not flush so potentially
+ * a remote CPU could still be writing to the folio.
+ * If the entry was previously clean then the
+ * architecture must guarantee that a clear->dirty
+ * transition on a cached TLB entry is written through
+ * and traps if the PTE is unmapped.
+ */
+ pteval = ptep_get_and_clear(mm, address, pvmw.pte);
+
+ set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
+ } else {
+ pteval = ptep_clear_flush(vma, address, pvmw.pte);
+ }
}

/* Set the dirty flag on the folio now the pte is gone. */
@@ -2148,10 +2162,10 @@ void try_to_migrate(struct folio *folio, enum ttu_flags flags)

/*
* Migration always ignores mlock and only supports TTU_RMAP_LOCKED and
- * TTU_SPLIT_HUGE_PMD and TTU_SYNC flags.
+ * TTU_SPLIT_HUGE_PMD, TTU_SYNC, and TTU_BATCH_FLUSH flags.
*/
if (WARN_ON_ONCE(flags & ~(TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD |
- TTU_SYNC)))
+ TTU_SYNC | TTU_BATCH_FLUSH)))
return;

if (folio_is_zone_device(folio) &&
--
2.35.1