+ migrate_pages-batch-flushing-tlb.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: migrate_pages: batch flushing TLB
has been added to the -mm mm-unstable branch.  Its filename is
     migrate_pages-batch-flushing-tlb.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/migrate_pages-batch-flushing-tlb.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Huang Ying <ying.huang@intel.com>
Subject: migrate_pages: batch flushing TLB
Date: Tue, 10 Jan 2023 15:53:26 +0800

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%.

NOTE: TLB flushing is batched only for normal folios, not for THP folios. 
Because the overhead of TLB flushing for THP folios is much lower than
that for normal folios (about 1/512 on x86 platform).

Link: https://lkml.kernel.org/r/20230110075327.590514-9-ying.huang@intel.com
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/migrate.c |    4 +++-
 mm/rmap.c    |   20 +++++++++++++++++---
 2 files changed, 20 insertions(+), 4 deletions(-)

--- a/mm/migrate.c~migrate_pages-batch-flushing-tlb
+++ a/mm/migrate.c
@@ -1247,7 +1247,7 @@ static int migrate_folio_unmap(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;
 	}
 
@@ -1790,6 +1790,8 @@ retry:
 	stats->nr_thp_failed += thp_retry;
 	stats->nr_failed_pages += nr_retry_pages;
 move:
+	try_to_unmap_flush();
+
 	retry = 1;
 	for (pass = 0;
 	     pass < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);
--- a/mm/rmap.c~migrate_pages-batch-flushing-tlb
+++ a/mm/rmap.c
@@ -1960,7 +1960,21 @@ static bool try_to_migrate_one(struct fo
 		} 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. */
@@ -2132,10 +2146,10 @@ void try_to_migrate(struct folio *folio,
 
 	/*
 	 * 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) &&
_

Patches currently in -mm which might be from ying.huang@intel.com are

migrate_pages-organize-stats-with-struct-migrate_pages_stats.patch
migrate_pages-separate-hugetlb-folios-migration.patch
migrate_pages-restrict-number-of-pages-to-migrate-in-batch.patch
migrate_pages-split-unmap_and_move-to-_unmap-and-_move.patch
migrate_pages-batch-_unmap-and-_move.patch
migrate_pages-move-migrate_folio_unmap.patch
migrate_pages-share-more-code-between-_unmap-and-_move.patch
migrate_pages-batch-flushing-tlb.patch
migrate_pages-move-thp-hugetlb-migration-support-check-to-simplify-code.patch