On Mon 26-09-16 18:20:24, Vlastimil Babka wrote:
> The compaction_zonelist_suitable() function tries to determine if compaction
> will be able to proceed after sufficient reclaim, i.e. whether there are
> enough reclaimable pages to provide enough order-0 freepages for compaction.
>
> This addition of reclaimable pages to the free pages works well for the order-0
> watermark check, but in the fragmentation index check we only consider truly
> free pages. Thus we can get fragindex value close to 0 which indicates failure
> do to lack of memory, and wrongly decide that compaction won't be suitable even
> after reclaim.
>
> Instead of trying to somehow adjust fragindex for reclaimable pages, let's just
> skip it from compaction_zonelist_suitable().
Yes this makes a lot of sense to me. The fragidx should be mostly about
a balance between reclaim and the compaction so it shouldn't be used for
fail or retry decisions.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/compaction.c | 35 ++++++++++++++++++-----------------
> 1 file changed, 18 insertions(+), 17 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 86d4d0bbfc7c..5ff7f801c345 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1379,7 +1379,6 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
> int classzone_idx,
> unsigned long wmark_target)
> {
> - int fragindex;
> unsigned long watermark;
>
> if (is_via_compact_memory(order))
> @@ -1415,6 +1414,18 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
> ALLOC_CMA, wmark_target))
> return COMPACT_SKIPPED;
>
> + return COMPACT_CONTINUE;
> +}
> +
> +enum compact_result compaction_suitable(struct zone *zone, int order,
> + unsigned int alloc_flags,
> + int classzone_idx)
> +{
> + enum compact_result ret;
> + int fragindex;
> +
> + ret = __compaction_suitable(zone, order, alloc_flags, classzone_idx,
> + zone_page_state(zone, NR_FREE_PAGES));
> /*
> * fragmentation index determines if allocation failures are due to
> * low memory or external fragmentation
> @@ -1426,21 +1437,12 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
> *
> * Only compact if a failure would be due to fragmentation.
> */
> - fragindex = fragmentation_index(zone, order);
> - if (fragindex >= 0 && fragindex <= sysctl_extfrag_threshold)
> - return COMPACT_NOT_SUITABLE_ZONE;
> -
> - return COMPACT_CONTINUE;
> -}
> -
> -enum compact_result compaction_suitable(struct zone *zone, int order,
> - unsigned int alloc_flags,
> - int classzone_idx)
> -{
> - enum compact_result ret;
> + if (ret == COMPACT_CONTINUE) {
> + fragindex = fragmentation_index(zone, order);
> + if (fragindex >= 0 && fragindex <= sysctl_extfrag_threshold)
> + return COMPACT_NOT_SUITABLE_ZONE;
> + }
>
> - ret = __compaction_suitable(zone, order, alloc_flags, classzone_idx,
> - zone_page_state(zone, NR_FREE_PAGES));
> trace_mm_compaction_suitable(zone, order, ret);
> if (ret == COMPACT_NOT_SUITABLE_ZONE)
> ret = COMPACT_SKIPPED;
> @@ -1473,8 +1475,7 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
> available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
> compact_result = __compaction_suitable(zone, order, alloc_flags,
> ac_classzone_idx(ac), available);
> - if (compact_result != COMPACT_SKIPPED &&
> - compact_result != COMPACT_NOT_SUITABLE_ZONE)
> + if (compact_result != COMPACT_SKIPPED)
> return true;
> }
>
> --
> 2.10.0
--
Michal Hocko
SUSE Labs
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