On Fri, Jun 20, 2014 at 05:49:34PM +0200, Vlastimil Babka wrote:
> isolate_migratepages_range() is the main function of the compaction scanner,
> called either on a single pageblock by isolate_migratepages() during regular
> compaction, or on an arbitrary range by CMA's __alloc_contig_migrate_range().
> It currently perfoms two pageblock-wide compaction suitability checks, and
> because of the CMA callpath, it tracks if it crossed a pageblock boundary in
> order to repeat those checks.
>
> However, closer inspection shows that those checks are always true for CMA:
> - isolation_suitable() is true because CMA sets cc->ignore_skip_hint to true
> - migrate_async_suitable() check is skipped because CMA uses sync compaction
>
> We can therefore move the checks to isolate_migratepages(), reducing variables
> and simplifying isolate_migratepages_range(). The update_pageblock_skip()
> function also no longer needs set_unsuitable parameter.
>
> Furthermore, going back to compact_zone() and compact_finished() when pageblock
> is unsuitable is wasteful - the checks are meant to skip pageblocks quickly.
> The patch therefore also introduces a simple loop into isolate_migratepages()
> so that it does not return immediately on pageblock checks, but keeps going
> until isolate_migratepages_range() gets called once. Similarily to
> isolate_freepages(), the function periodically checks if it needs to reschedule
> or abort async compaction.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> mm/compaction.c | 112 +++++++++++++++++++++++++++++---------------------------
> 1 file changed, 59 insertions(+), 53 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 3064a7f..ebe30c9 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -132,7 +132,7 @@ void reset_isolation_suitable(pg_data_t *pgdat)
> */
> static void update_pageblock_skip(struct compact_control *cc,
> struct page *page, unsigned long nr_isolated,
> - bool set_unsuitable, bool migrate_scanner)
> + bool migrate_scanner)
> {
> struct zone *zone = cc->zone;
> unsigned long pfn;
> @@ -146,12 +146,7 @@ static void update_pageblock_skip(struct compact_control *cc,
> if (nr_isolated)
> return;
>
> - /*
> - * Only skip pageblocks when all forms of compaction will be known to
> - * fail in the near future.
> - */
> - if (set_unsuitable)
> - set_pageblock_skip(page);
> + set_pageblock_skip(page);
>
> pfn = page_to_pfn(page);
>
> @@ -180,7 +175,7 @@ static inline bool isolation_suitable(struct compact_control *cc,
>
> static void update_pageblock_skip(struct compact_control *cc,
> struct page *page, unsigned long nr_isolated,
> - bool set_unsuitable, bool migrate_scanner)
> + bool migrate_scanner)
> {
> }
> #endif /* CONFIG_COMPACTION */
> @@ -345,8 +340,7 @@ isolate_fail:
>
> /* Update the pageblock-skip if the whole pageblock was scanned */
> if (blockpfn == end_pfn)
> - update_pageblock_skip(cc, valid_page, total_isolated, true,
> - false);
> + update_pageblock_skip(cc, valid_page, total_isolated, false);
>
> count_compact_events(COMPACTFREE_SCANNED, nr_scanned);
> if (total_isolated)
> @@ -474,14 +468,12 @@ unsigned long
> isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> unsigned long low_pfn, unsigned long end_pfn, bool unevictable)
> {
> - unsigned long last_pageblock_nr = 0, pageblock_nr;
> unsigned long nr_scanned = 0, nr_isolated = 0;
> struct list_head *migratelist = &cc->migratepages;
> struct lruvec *lruvec;
> unsigned long flags;
> bool locked = false;
> struct page *page = NULL, *valid_page = NULL;
> - bool set_unsuitable = true;
> const isolate_mode_t mode = (cc->mode == MIGRATE_ASYNC ?
> ISOLATE_ASYNC_MIGRATE : 0) |
> (unevictable ? ISOLATE_UNEVICTABLE : 0);
> @@ -545,28 +537,6 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> if (!valid_page)
> valid_page = page;
>
> - /* If isolation recently failed, do not retry */
> - pageblock_nr = low_pfn >> pageblock_order;
> - if (last_pageblock_nr != pageblock_nr) {
> - int mt;
> -
> - last_pageblock_nr = pageblock_nr;
> - if (!isolation_suitable(cc, page))
> - goto next_pageblock;
> -
> - /*
> - * For async migration, also only scan in MOVABLE
> - * blocks. Async migration is optimistic to see if
> - * the minimum amount of work satisfies the allocation
> - */
> - mt = get_pageblock_migratetype(page);
> - if (cc->mode == MIGRATE_ASYNC &&
> - !migrate_async_suitable(mt)) {
> - set_unsuitable = false;
> - goto next_pageblock;
> - }
> - }
> -
> /*
> * Skip if free. page_order cannot be used without zone->lock
> * as nothing prevents parallel allocations or buddy merging.
> @@ -668,8 +638,7 @@ next_pageblock:
> * if the whole pageblock was scanned without isolating any page.
> */
> if (low_pfn == end_pfn)
> - update_pageblock_skip(cc, valid_page, nr_isolated,
> - set_unsuitable, true);
> + update_pageblock_skip(cc, valid_page, nr_isolated, true);
>
> trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);
>
> @@ -840,34 +809,74 @@ typedef enum {
> } isolate_migrate_t;
>
> /*
> - * Isolate all pages that can be migrated from the block pointed to by
> - * the migrate scanner within compact_control.
> + * Isolate all pages that can be migrated from the first suitable block,
> + * starting at the block pointed to by the migrate scanner pfn within
> + * compact_control.
> */
> static isolate_migrate_t isolate_migratepages(struct zone *zone,
> struct compact_control *cc)
> {
> unsigned long low_pfn, end_pfn;
> + struct page *page;
>
> - /* Do not scan outside zone boundaries */
> - low_pfn = max(cc->migrate_pfn, zone->zone_start_pfn);
> + /* Start at where we last stopped, or beginning of the zone */
> + low_pfn = cc->migrate_pfn;
>
> /* Only scan within a pageblock boundary */
> end_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages);
>
> - /* Do not cross the free scanner or scan within a memory hole */
> - if (end_pfn > cc->free_pfn || !pfn_valid(low_pfn)) {
> - cc->migrate_pfn = end_pfn;
> - return ISOLATE_NONE;
> - }
> + /*
> + * Iterate over whole pageblocks until we find the first suitable.
> + * Do not cross the free scanner.
> + */
> + for (; end_pfn <= cc->free_pfn;
> + low_pfn = end_pfn, end_pfn += pageblock_nr_pages) {
> +
> + /*
> + * This can potentially iterate a massively long zone with
> + * many pageblocks unsuitable, so periodically check if we
> + * need to schedule, or even abort async compaction.
> + */
> + if (!(low_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages))
> + && compact_should_abort(cc))
> + break;
>
> - /* Perform the isolation */
> - low_pfn = isolate_migratepages_range(zone, cc, low_pfn, end_pfn, false);
> - if (!low_pfn || cc->contended)
> - return ISOLATE_ABORT;
> + /* Do not scan within a memory hole */
> + if (!pfn_valid(low_pfn))
> + continue;
> +
> + page = pfn_to_page(low_pfn);
> + /* If isolation recently failed, do not retry */
> + if (!isolation_suitable(cc, page))
> + continue;
>
> + /*
> + * For async compaction, also only scan in MOVABLE blocks.
> + * Async compaction is optimistic to see if the minimum amount
> + * of work satisfies the allocation.
> + */
> + if (cc->mode == MIGRATE_ASYNC &&
> + !migrate_async_suitable(get_pageblock_migratetype(page)))
> + continue;
> +
> + /* Perform the isolation */
> + low_pfn = isolate_migratepages_range(zone, cc, low_pfn,
> + end_pfn, false);
> + if (!low_pfn || cc->contended)
> + return ISOLATE_ABORT;
> +
> + /*
> + * Either we isolated something and proceed with migration. Or
> + * we failed and compact_zone should decide if we should
> + * continue or not.
> + */
> + break;
> + }
> +
> + /* Record where migration scanner will be restarted */
If we make isolate_migratepages* interface like as isolate_freepages*,
we can get more clean and micro optimized code. Because
isolate_migratepages_range() can handle arbitrary range and this patch
make isolate_migratepages() also handle arbitrary range, there would
be some redundant codes. :)
Thanks.
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