Re: [patch 1/2] mm, compaction: kcompactd should not ignore pageblock skip

On Wed, 23 Aug 2017, Vlastimil Babka wrote:

On 08/16/2017 01:39 AM, David Rientjes wrote:

quoted

Kcompactd is needlessly ignoring pageblock skip information.  It is doing
MIGRATE_SYNC_LIGHT compaction, which is no more powerful than
MIGRATE_SYNC compaction.

If compaction recently failed to isolate memory from a set of pageblocks,
there is nothing to indicate that kcompactd will be able to do so, or
that it is beneficial from attempting to isolate memory.

Use the pageblock skip hint to avoid rescanning pageblocks needlessly
until that information is reset.

Signed-off-by: David Rientjes <rientjes@google.com>

It would be much better if patches like this were accompanied by some
numbers.

The numbers were from https://marc.info/?l=linux-mm&m=150231232707999 
where very large amounts (>90% of system RAM) were hugetlb pages.  We can 
supplement this changelog with the following if it helps:

"""
Currently, kcompactd ignores pageblock skip information that can become 
useful if it is known that memory should not be considered by both the 
migration and freeing scanners.  Abundant hugetlb memory is a good example 
of memory that is needlessly (and expensively) scanned since the hugepage 
order normally matches the pageblock order.

For example, on a sysctl with very large amounts of memory reserved by the 
hugetlb subsystem:

compact_migrate_scanned 2931254031294 
compact_free_scanned    102707804816705 
compact_isolated        1309145254 

Kcompactd ends up successfully isolating ~0.0012% of memory that is 
scans (the above does not involve direct compaction).

A follow-up change will set the pageblock skip for this memory since it is 
never useful for either scanner.
"""

Also there's now a danger that in cases where there's no direct
compaction happening (just kcompactd), nothing will ever call
__reset_isolation_suitable().

I'm not sure that is helpful in a context where no high-order memory can 
call direct compaction that kcompactd needlessly scanning the same memory 
over and over is beneficial.

quoted

---
 mm/compaction.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
--- a/mm/compaction.c
+++ b/mm/compaction.c

@@ -1927,9 +1927,8 @@ static void kcompactd_do_work(pg_data_t *pgdat)
 		.total_free_scanned = 0,
 		.classzone_idx = pgdat->kcompactd_classzone_idx,
 		.mode = MIGRATE_SYNC_LIGHT,
-		.ignore_skip_hint = true,
+		.ignore_skip_hint = false,
 		.gfp_mask = GFP_KERNEL,
-
 	};
 	trace_mm_compaction_kcompactd_wake(pgdat->node_id, cc.order,
 							cc.classzone_idx);

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