Re: [PATCH v5 05/14] mm, compaction: move pageblock checks up from isolate_migratepages_range()
From: David Rientjes
Date: Mon Jul 28 2014 - 20:29:49 EST
On Mon, 28 Jul 2014, 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 compaction-specific checks to isolate_migratepages()
> and simplify isolate_migratepages_range(). Furthermore, we can mimic the
> freepage scanner family of functions, which has isolate_freepages_block()
> function called both by compaction from isolate_freepages() and by CMA from
> isolate_freepages_range(), where each use-case adds own specific glue code.
> This allows further code simplification.
>
> Therefore, we rename isolate_migratepages_range() to isolate_freepages_block()
s/isolate_freepages_block/isolate_migratepages_block/
I read your commit description before looking at the patch and was very
nervous about the direction you were going if that was true :) I'm
relieved to see it was just a typo.
> and limit its functionality to a single pageblock (or its subset). For CMA,
> a new different isolate_migratepages_range() is created as a CMA-specific
> wrapper for the _block() function. The checks specific to compaction are moved
> to isolate_migratepages(). As part of the unification of these two families of
> functions, we remove the redundant zone parameter where applicable, since zone
> pointer is already passed in cc->zone.
>
> Furthermore, going back to compact_zone() and compact_finished() when pageblock
> is found unsuitable (now by isolate_migratepages()) 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 failed 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>
> Acked-by: 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 | 234 +++++++++++++++++++++++++++++++++-----------------------
> mm/internal.h | 4 +-
> mm/page_alloc.c | 3 +-
> 3 files changed, 140 insertions(+), 101 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 0a871e5..bac6e37 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)
> @@ -451,40 +445,34 @@ static bool too_many_isolated(struct zone *zone)
> }
>
> /**
> - * isolate_migratepages_range() - isolate all migrate-able pages in range.
> - * @zone: Zone pages are in.
> + * isolate_migratepages_block() - isolate all migrate-able pages within
> + * a single pageblock
> * @cc: Compaction control structure.
> - * @low_pfn: The first PFN of the range.
> - * @end_pfn: The one-past-the-last PFN of the range.
> - * @unevictable: true if it allows to isolate unevictable pages
> + * @low_pfn: The first PFN to isolate
> + * @end_pfn: The one-past-the-last PFN to isolate, within same pageblock
> + * @isolate_mode: Isolation mode to be used.
> *
> * Isolate all pages that can be migrated from the range specified by
> - * [low_pfn, end_pfn). Returns zero if there is a fatal signal
> - * pending), otherwise PFN of the first page that was not scanned
> - * (which may be both less, equal to or more then end_pfn).
> - *
> - * Assumes that cc->migratepages is empty and cc->nr_migratepages is
> - * zero.
> + * [low_pfn, end_pfn). The range is expected to be within same pageblock.
> + * Returns zero if there is a fatal signal pending, otherwise PFN of the
> + * first page that was not scanned (which may be both less, equal to or more
> + * than end_pfn).
> *
> - * Apart from cc->migratepages and cc->nr_migratetypes this function
> - * does not modify any cc's fields, in particular it does not modify
> - * (or read for that matter) cc->migrate_pfn.
> + * The pages are isolated on cc->migratepages list (not required to be empty),
> + * and cc->nr_migratepages is updated accordingly. The cc->migrate_pfn field
> + * is neither read nor updated.
> */
> -unsigned long
> -isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> - unsigned long low_pfn, unsigned long end_pfn, bool unevictable)
> +static unsigned long
> +isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> + unsigned long end_pfn, isolate_mode_t isolate_mode)
> {
> - unsigned long last_pageblock_nr = 0, pageblock_nr;
> + struct zone *zone = cc->zone;
> 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);
>
> /*
> * Ensure that there are not too many pages isolated from the LRU
> @@ -515,19 +503,6 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> }
> }
>
> - /*
> - * migrate_pfn does not necessarily start aligned to a
> - * pageblock. Ensure that pfn_valid is called when moving
> - * into a new MAX_ORDER_NR_PAGES range in case of large
> - * memory holes within the zone
> - */
> - if ((low_pfn & (MAX_ORDER_NR_PAGES - 1)) == 0) {
> - if (!pfn_valid(low_pfn)) {
> - low_pfn += MAX_ORDER_NR_PAGES - 1;
> - continue;
> - }
> - }
> -
> if (!pfn_valid_within(low_pfn))
> continue;
> nr_scanned++;
> @@ -545,28 +520,6 @@ 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.
> @@ -601,8 +554,11 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> */
> if (PageTransHuge(page)) {
> if (!locked)
> - goto next_pageblock;
> - low_pfn += (1 << compound_order(page)) - 1;
> + low_pfn = ALIGN(low_pfn + 1,
> + pageblock_nr_pages) - 1;
> + else
> + low_pfn += (1 << compound_order(page)) - 1;
> +
Hmm, any reason not to always advance and align low_pfn to
pageblock_nr_pages? I don't see how pageblock_order > HPAGE_PMD_ORDER
would make sense if encountering thp.
> continue;
> }
>
> @@ -632,7 +588,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> lruvec = mem_cgroup_page_lruvec(page, zone);
>
> /* Try isolate the page */
> - if (__isolate_lru_page(page, mode) != 0)
> + if (__isolate_lru_page(page, isolate_mode) != 0)
> continue;
>
> VM_BUG_ON_PAGE(PageTransCompound(page), page);
> @@ -651,11 +607,6 @@ isolate_success:
> ++low_pfn;
> break;
> }
> -
> - continue;
> -
> -next_pageblock:
> - low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1;
> }
>
> acct_isolated(zone, locked, cc);
> @@ -668,8 +619,7 @@ 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);
>
> @@ -680,15 +630,61 @@ next_pageblock:
> return low_pfn;
> }
>
> +/**
> + * isolate_migratepages_range() - isolate migrate-able pages in a PFN range
> + * @start_pfn: The first PFN to start isolating.
> + * @end_pfn: The one-past-last PFN.
Need to specify @cc?
> + *
> + * Returns zero if isolation fails fatally due to e.g. pending signal.
> + * Otherwise, function returns one-past-the-last PFN of isolated page
> + * (which may be greater than end_pfn if end fell in a middle of a THP page).
> + */
> +unsigned long
> +isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
> + unsigned long end_pfn)
> +{
> + unsigned long pfn, block_end_pfn;
> +
> + /* Scan block by block. First and last block may be incomplete */
> + pfn = start_pfn;
> + block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
> +
> + for (; pfn < end_pfn; pfn = block_end_pfn,
> + block_end_pfn += pageblock_nr_pages) {
> +
> + block_end_pfn = min(block_end_pfn, end_pfn);
> +
> + /* Skip whole pageblock in case of a memory hole */
> + if (!pfn_valid(pfn))
> + continue;
> +
> + pfn = isolate_migratepages_block(cc, pfn, block_end_pfn,
> + ISOLATE_UNEVICTABLE);
> +
> + /*
> + * In case of fatal failure, release everything that might
> + * have been isolated in the previous iteration, and signal
> + * the failure back to caller.
> + */
> + if (!pfn) {
> + putback_movable_pages(&cc->migratepages);
> + cc->nr_migratepages = 0;
> + break;
> + }
> + }
> +
> + return pfn;
> +}
> +
> #endif /* CONFIG_COMPACTION || CONFIG_CMA */
> #ifdef CONFIG_COMPACTION
> /*
> * Based on information in the current compact_control, find blocks
> * suitable for isolating free pages from and then isolate them.
> */
> -static void isolate_freepages(struct zone *zone,
> - struct compact_control *cc)
> +static void isolate_freepages(struct compact_control *cc)
> {
> + struct zone *zone = cc->zone;
> struct page *page;
> unsigned long block_start_pfn; /* start of current pageblock */
> unsigned long block_end_pfn; /* end of current pageblock */
> @@ -806,7 +802,7 @@ static struct page *compaction_alloc(struct page *migratepage,
> */
> if (list_empty(&cc->freepages)) {
> if (!cc->contended)
> - isolate_freepages(cc->zone, cc);
> + isolate_freepages(cc);
>
> if (list_empty(&cc->freepages))
> return NULL;
> @@ -840,34 +836,81 @@ 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;
> + const isolate_mode_t isolate_mode =
> + (cc->mode == MIGRATE_ASYNC ? ISOLATE_ASYNC_MIGRATE : 0);
>
> - /* 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 as
> + * initialized by compact_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;
> +
> + /* Skip whole pageblock in case of 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_block(cc, low_pfn, end_pfn,
> + isolate_mode);
Hmm, why would we want to unconditionally set pageblock_skip if no pages
could be isolated from a pageblock when
isolate_mode == ISOLATE_ASYNC_MIGRATE? It seems like it erroneously skip
pageblocks for cases when isolate_mode == 0.
> +
> + if (!low_pfn || cc->contended)
> + return ISOLATE_ABORT;
>
> - /* 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 */
> cc->migrate_pfn = low_pfn;
>
> - return ISOLATE_SUCCESS;
> + return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
> }
>
> static int compact_finished(struct zone *zone,
> @@ -1040,9 +1083,6 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
> ;
> }
>
> - if (!cc->nr_migratepages)
> - continue;
> -
> err = migrate_pages(&cc->migratepages, compaction_alloc,
> compaction_free, (unsigned long)cc, cc->mode,
> MR_COMPACTION);
> diff --git a/mm/internal.h b/mm/internal.h
> index a1b651b..5a0738f 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -154,8 +154,8 @@ unsigned long
> isolate_freepages_range(struct compact_control *cc,
> unsigned long start_pfn, unsigned long end_pfn);
> unsigned long
> -isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> - unsigned long low_pfn, unsigned long end_pfn, bool unevictable);
> +isolate_migratepages_range(struct compact_control *cc,
> + unsigned long low_pfn, unsigned long end_pfn);
>
> #endif
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 91191fb..f424752 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6292,8 +6292,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>
> if (list_empty(&cc->migratepages)) {
> cc->nr_migratepages = 0;
> - pfn = isolate_migratepages_range(cc->zone, cc,
> - pfn, end, true);
> + pfn = isolate_migratepages_range(cc, pfn, end);
> if (!pfn) {
> ret = -EINTR;
> break;
--
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