Re: [PATCH v3 04/13] mm, compaction: move pageblock checks up from isolate_migratepages_range()

From: Vlastimil Babka
Date: Wed Jun 25 2014 - 04:59:30 EST

On 06/25/2014 02:53 AM, Joonsoo Kim wrote:
On Tue, Jun 24, 2014 at 05:42:50PM +0200, Vlastimil Babka wrote:
On 06/24/2014 10:33 AM, Joonsoo Kim wrote:
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. :)

I'm not sure if it's worth already. Where is the arbitrary range
adding overhead? I can only imagine that next_pageblock: label could
do a 'break;' instead of setting up next_capture_pfn, but that's
about it AFAICS.

In fact, there is just minor overhead, pfn_valid().
And isolate_freepages variants seems to do this correctly. :)

Someone could wonder why there are two isolate_migratepages variants
with arbitrary range compaction ability. IMHO, one
isolate_migratepage_xxx for pageblock range and two
isolate_migratepage_yyy/zzz for compaction and CMA is better
architecture.

OK, I will try. But we need to resolve the "where to test for page_zone() == cc->zone?" question.

And, One additional note. You can move update_pageblock_skip() to
isolate_migratepages() now.

Right, thanks.

Thanks.


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