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

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

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@suse.cz>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Michal Nazarewicz <mina86@mina86.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: David Rientjes <rientjes@google.com>
> ---
>  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.

>  
>  		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().

Thanks.

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