Re: [PATCH -v4 8/9] migrate_pages: batch flushing TLB

[Zi Yan]

On 6 Feb 2023, at 1:33, Huang Ying wrote:

> 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).
>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Bharata B Rao <bharata@xxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: haoxin <xhao@xxxxxxxxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> ---
>  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 9378fa2ad4a5..ca6e2ff02a09 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1230,7 +1230,7 @@ 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;
>  	}
>
> @@ -1781,6 +1781,8 @@ 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:

Maybe a comment:
/* Flush TLBs for all the unmapped pages */

> +	try_to_unmap_flush();
> +
>  	retry = 1;
>  	for (pass = 0;
>  	     pass < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);
> 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

Everything else looks good to me. Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

--
Best Regards,
Yan, Zi

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