Re: [PATCH 5/5] mm, compaction: more focused lru and pcplists draining

[Joonsoo Kim]

On Mon, Nov 03, 2014 at 09:12:33AM +0100, Vlastimil Babka wrote:
> On 10/27/2014 08:41 AM, Joonsoo Kim wrote:
> >On Tue, Oct 07, 2014 at 05:33:39PM +0200, Vlastimil Babka wrote:
> >>The goal of memory compaction is to create high-order freepages through page
> >>migration. Page migration however puts pages on the per-cpu lru_add cache,
> >>which is later flushed to per-cpu pcplists, and only after pcplists are
> >>drained the pages can actually merge. This can happen due to the per-cpu
> >>caches becoming full through further freeing, or explicitly.
> >>
> >>During direct compaction, it is useful to do the draining explicitly so that
> >>pages merge as soon as possible and compaction can detect success immediately
> >>and keep the latency impact at minimum. However the current implementation is
> >>far from ideal. Draining is done only in  __alloc_pages_direct_compact(),
> >>after all zones were already compacted, and the decisions to continue or stop
> >>compaction in individual zones was done without the last batch of migrations
> >>being merged. It is also missing the draining of lru_add cache before the
> >>pcplists.
> >>
> >>This patch moves the draining for direct compaction into compact_zone(). It
> >>adds the missing lru_cache draining and uses the newly introduced single zone
> >>pcplists draining to reduce overhead and avoid impact on unrelated zones.
> >>Draining is only performed when it can actually lead to merging of a page of
> >>desired order (passed by cc->order). This means it is only done when migration
> >>occurred in the previously scanned cc->order aligned block(s) and the
> >>migration scanner is now pointing to the next cc->order aligned block.
> >>
> >>The patch has been tested with stress-highalloc benchmark from mmtests.
> >>Although overal allocation success rates of the benchmark were not affected,
> >>the number of detected compaction successes has doubled. This suggests that
> >>allocations were previously successful due to implicit merging caused by
> >>background activity, making a later allocation attempt succeed immediately,
> >>but not attributing the success to compaction. Since stress-highalloc always
> >>tries to allocate almost the whole memory, it cannot show the improvement in
> >>its reported success rate metric. However after this patch, compaction should
> >>detect success and terminate earlier, reducing the direct compaction latencies
> >>in a real scenario.
> >>
> >>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 | 41 ++++++++++++++++++++++++++++++++++++++++-
> >>  mm/page_alloc.c |  4 ----
> >>  2 files changed, 40 insertions(+), 5 deletions(-)
> >>
> >>diff --git a/mm/compaction.c b/mm/compaction.c
> >>index 8fa888d..41b49d7 100644
> >>--- a/mm/compaction.c
> >>+++ b/mm/compaction.c
> >>@@ -1179,6 +1179,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
> >>  	while ((ret = compact_finished(zone, cc, migratetype)) ==
> >>  						COMPACT_CONTINUE) {
> >>  		int err;
> >>+		unsigned long last_migrated_pfn = 0;
> >
> >I think that this definition looks odd.
> >In every iteration, last_migrated_pfn is re-defined as 0.
> >Maybe, it is on outside of the loop.
> 
> Oops you're right, that's a mistake and it makes the code miss some
> of the drain points (a minority I think but anyway).
> 
> >>
> >>  		switch (isolate_migratepages(zone, cc)) {
> >>  		case ISOLATE_ABORT:
> >>@@ -1187,7 +1188,12 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
> >>  			cc->nr_migratepages = 0;
> >>  			goto out;
> >>  		case ISOLATE_NONE:
> >>-			continue;
> >>+			/*
> >>+			 * We haven't isolated and migrated anything, but
> >>+			 * there might still be unflushed migrations from
> >>+			 * previous cc->order aligned block.
> >>+			 */
> >>+			goto check_drain;
> >>  		case ISOLATE_SUCCESS:
> >>  			;
> >>  		}
> >>@@ -1212,6 +1218,39 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
> >>  				goto out;
> >>  			}
> >>  		}
> >>+
> >>+		/*
> >>+		 * Record where we have freed pages by migration and not yet
> >>+		 * flushed them to buddy allocator. Subtract 1, because often
> >>+		 * we finish a pageblock and migrate_pfn points to the first
> >>+		 * page* of the next one. In that case we want the drain below
> >>+		 * to happen immediately.
> >>+		 */
> >>+		if (!last_migrated_pfn)
> >>+			last_migrated_pfn = cc->migrate_pfn - 1;
> >
> >And, I wonder why last_migrated_pfn is set after isolate_migratepages().
> 
> Not sure I understand your question. With the mistake above, it
> cannot currently be set at the point isolate_migratepages() is
> called, so you might question the goto check_drain in the
> ISOLATE_NONE case, if that's what you are wondering about.
> 
> When I correct that, it might be set when COMPACT_CLUSTER_MAX pages
> are isolated and migrated the middle of a pageblock, and then the
> rest of the pageblock contains no pages that could be isolated, so
> the last isolate_migratepages() attempt in the pageblock returns
> with ISOLATE_NONE. Still there were some migrations that produced
> free pages that should be drained at that point.

To clarify my question, I attach psuedo code that I thought correct.

static int compact_zone()
{
        unsigned long last_migrated_pfn = 0;

        ...

        compaction_suitable();

        ...

        while (compact_finished()) {
                if (!last_migrated_pfn)
                        last_migrated_pfn = cc->migrate_pfn - 1;

                isolate_migratepages();
                switch case
                migrate_pages();
                ...

                check_drain: (at the end of loop)
                        do flush and reset last_migrated_pfn if needed
        }
}

We should record last_migrated_pfn before isolate_migratepages() and
then compare it with cc->migrate_pfn after isolate_migratepages() to
know if we moved away from the previous cc->order aligned block.
Am I missing something?

Thanks.
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