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SubjectRe: [PATCH 20/25] mm, compaction: Reduce unnecessary skipping of migration target scanner
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On 1/4/19 1:50 PM, Mel Gorman wrote:
> The fast isolation of pages can move the scanner faster than is necessary
> depending on the contents of the free list. This patch will only allow
> the fast isolation to initialise the scanner and advance it slowly. The
> primary means of moving the scanner forward is via the linear scanner
> to reduce the likelihood the migration source/target scanners meet
> prematurely triggering a rescan.

Maybe I've seen enough code today and need to stop, but AFAICS the description
here doesn't match the actual code changes? What I see are some cleanups, and a
change in free scanner that will set pageblock skip bit after a pageblock has
been scanned, even if there were pages isolated, while previously it would set
the skip bit only if nothing was isolated.

> 4.20.0 4.20.0
> noresched-v2r15 slowfree-v2r15
> Amean fault-both-1 0.00 ( 0.00%) 0.00 * 0.00%*
> Amean fault-both-3 2736.50 ( 0.00%) 2512.53 ( 8.18%)
> Amean fault-both-5 4133.70 ( 0.00%) 4159.43 ( -0.62%)
> Amean fault-both-7 5738.61 ( 0.00%) 5950.15 ( -3.69%)
> Amean fault-both-12 9392.82 ( 0.00%) 8674.38 ( 7.65%)
> Amean fault-both-18 13257.15 ( 0.00%) 12850.79 ( 3.07%)
> Amean fault-both-24 16859.44 ( 0.00%) 17242.86 ( -2.27%)
> Amean fault-both-30 16249.30 ( 0.00%) 19404.18 * -19.42%*
> Amean fault-both-32 14904.71 ( 0.00%) 16200.79 ( -8.70%)
>
> The impact to latency, success rates and scan rates is marginal but
> avoiding unnecessary restarts is important. It helps later patches that
> are more careful about how pageblocks are treated as earlier iterations
> of those patches hit corner cases where the restarts were punishing and
> very visible.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
> mm/compaction.c | 27 ++++++++++-----------------
> 1 file changed, 10 insertions(+), 17 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 75eb0d40d4d7..6c5552c6d8f9 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -324,10 +324,9 @@ static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
> * future. The information is later cleared by __reset_isolation_suitable().
> */
> static void update_pageblock_skip(struct compact_control *cc,
> - struct page *page, unsigned long nr_isolated)
> + struct page *page, unsigned long pfn)
> {
> struct zone *zone = cc->zone;
> - unsigned long pfn;
>
> if (cc->no_set_skip_hint)
> return;
> @@ -335,13 +334,8 @@ static void update_pageblock_skip(struct compact_control *cc,
> if (!page)
> return;
>
> - if (nr_isolated)
> - return;
> -
> set_pageblock_skip(page);
>
> - pfn = page_to_pfn(page);
> -
> /* Update where async and sync compaction should restart */
> if (pfn < zone->compact_cached_free_pfn)
> zone->compact_cached_free_pfn = pfn;
> @@ -359,7 +353,7 @@ static inline bool pageblock_skip_persistent(struct page *page)
> }
>
> static inline void update_pageblock_skip(struct compact_control *cc,
> - struct page *page, unsigned long nr_isolated)
> + struct page *page, unsigned long pfn)
> {
> }
>
> @@ -450,7 +444,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> bool strict)
> {
> int nr_scanned = 0, total_isolated = 0;
> - struct page *cursor, *valid_page = NULL;
> + struct page *cursor;
> unsigned long flags = 0;
> bool locked = false;
> unsigned long blockpfn = *start_pfn;
> @@ -477,9 +471,6 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> if (!pfn_valid_within(blockpfn))
> goto isolate_fail;
>
> - if (!valid_page)
> - valid_page = page;
> -
> /*
> * For compound pages such as THP and hugetlbfs, we can save
> * potentially a lot of iterations if we skip them at once.
> @@ -576,10 +567,6 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> if (strict && blockpfn < end_pfn)
> total_isolated = 0;
>
> - /* Update the pageblock-skip if the whole pageblock was scanned */
> - if (blockpfn == end_pfn)
> - update_pageblock_skip(cc, valid_page, total_isolated);
> -
> cc->total_free_scanned += nr_scanned;
> if (total_isolated)
> count_compact_events(COMPACTISOLATED, total_isolated);
> @@ -1295,8 +1282,10 @@ fast_isolate_freepages(struct compact_control *cc)
> }
> }
>
> - if (highest && highest > cc->zone->compact_cached_free_pfn)
> + if (highest && highest >= cc->zone->compact_cached_free_pfn) {
> + highest -= pageblock_nr_pages;
> cc->zone->compact_cached_free_pfn = highest;
> + }
>
> cc->total_free_scanned += nr_scanned;
> if (!page)
> @@ -1376,6 +1365,10 @@ static void isolate_freepages(struct compact_control *cc)
> isolate_freepages_block(cc, &isolate_start_pfn, block_end_pfn,
> freelist, false);
>
> + /* Update the skip hint if the full pageblock was scanned */
> + if (isolate_start_pfn == block_end_pfn)
> + update_pageblock_skip(cc, page, block_start_pfn);
> +
> /* Are enough freepages isolated? */
> if (cc->nr_freepages >= cc->nr_migratepages) {
> if (isolate_start_pfn >= block_end_pfn) {
>

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