Re: [PATCH 3/4] mm: compaction: Update pageblock skip when first migration candidate is not at the start

Vlastimil Babka
2023-05-25T13:37:53Z

On 5/15/23 13:33, Mel Gorman wrote:
> isolate_migratepages_block should mark a pageblock as skip if scanning
> started on an aligned pageblock boundary but it only updates the skip
> flag if the first migration candidate is also aligned. Tracing during
> a compaction stress load (mmtests: workload-usemem-stress-numa-compact)
> that many pageblocks are not marked skip causing excessive scanning of
> blocks that had been recently checked. Update pageblock skip based on
> "valid_page" which is set if scanning started on a pageblock boundary.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

I wonder if this has an unintended side-effect that if we resume
isolate_migratepages_block() of a partially compacted pageblock to finish
it, test_and_set_skip() will now tell us to abort, because we already set
the skip bit in the previous call. This would include the
cc->finish_pageblock rescan cases.

So unless I miss something that already prevents that, I agree we should not
tie setting the skip bit to pageblock_aligned(pfn), but maybe if we are not
pageblock aligned, we should ignore the already-set skip bit, as it was most
likely being set by us in the previous iteration and should not prevent us
from finishing the pageblock?

> ---
> mm/compaction.c | 15 +++++----------
> 1 file changed, 5 insertions(+), 10 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index accc6568091a..d7be990b1d60 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -392,18 +392,14 @@ void reset_isolation_suitable(pg_data_t *pgdat)
> * Sets the pageblock skip bit if it was clear. Note that this is a hint as
> * locks are not required for read/writers. Returns true if it was already set.
> */
> -static bool test_and_set_skip(struct compact_control *cc, struct page *page,
> - unsigned long pfn)
> +static bool test_and_set_skip(struct compact_control *cc, struct page *page)
> {
> bool skip;
>
> - /* Do no update if skip hint is being ignored */
> + /* Do not update if skip hint is being ignored */
> if (cc->ignore_skip_hint)
> return false;
>
> - if (!pageblock_aligned(pfn))
> - return false;
> -
> skip = get_pageblock_skip(page);
> if (!skip && !cc->no_set_skip_hint)
> set_pageblock_skip(page);
> @@ -470,8 +466,7 @@ static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
> {
> }
>
> -static bool test_and_set_skip(struct compact_control *cc, struct page *page,
> - unsigned long pfn)
> +static bool test_and_set_skip(struct compact_control *cc, struct page *page)
> {
> return false;
> }
> @@ -1075,9 +1070,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> lruvec_memcg_debug(lruvec, page_folio(page));
>
> /* Try get exclusive access under lock */
> - if (!skip_updated) {
> + if (!skip_updated && valid_page) {
> skip_updated = true;
> - if (test_and_set_skip(cc, page, low_pfn))
> + if (test_and_set_skip(cc, valid_page))
> goto isolate_abort;
> }
>