Commit 7e12beb8 authored by Huang Ying's avatar Huang Ying Committed by Andrew Morton
Browse files

migrate_pages: batch flushing TLB 

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%.

Haoxin helped to test the patchset on an ARM64 server with 128 cores, 2
NUMA nodes.  Test results show that the page migration performance
increases up to 78%.

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/20230213123444.155149-9-ying.huang@intel.com


Signed-off-by: default avatar"Huang, Ying" <ying.huang@intel.com>
Tested-by: default avatarXin Hao <xhao@linux.alibaba.com>
Reviewed-by: default avatarZi Yan <ziy@nvidia.com>
Reviewed-by: default avatarXin Hao <xhao@linux.alibaba.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: Minchan Kim <minchan@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent ebe75e47

mm/migrate.c

+4 −1
Original line number Diff line number Diff line
@@ -1248,7 +1248,7 @@ 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;

	}
@@ -1806,6 +1806,9 @@ 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:

	/* Flush TLBs for all unmapped folios */

	try_to_unmap_flush();



	retry = 1;

	for (pass = 0;

	     pass < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);

mm/rmap.c

+17 −3
Original line number Diff line number Diff line
@@ -1952,8 +1952,22 @@ 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. */

			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. */

		if (pte_dirty(pteval))
@@ -2124,10 +2138,10 @@ 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) &&