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
(nit-picking) s/perfoms/performs/
> 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.
This looks to me a good direction.
One thing below ...
> 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
...
> @@ -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);
Can we move (page_zone != zone) check here as isolate_freepages() does?
Thanks,
Naoya Horiguchi
> + /* 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 */
> cc->migrate_pfn = low_pfn;
>
> - return ISOLATE_SUCCESS;
> + return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
> }
>
> static int compact_finished(struct zone *zone,
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