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

[Vlastimil Babka]

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.

> Thanks.

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