mm/compaction: cap compact_gap() at COMPACT_CLUSTER_MAX

compact_gap() returns 2 << order, which is used as watermark headroom in
__compaction_suitable() and as a threshold in kswapd reclaim decisions. 
The computed value scales exponentially by order.  For order-9 THP
allocations this evaluates to 1024 pages, but the compaction free
scanner's working set is bounded by COMPACT_CLUSTER_MAX (32 pages).  The
scanner stops isolating free pages once it matches the migration batch. 
The current gap over-reserves by 32x.

On fragmented production hosts, kswapd will try to reclaim up to the gap,
but it only reaches that threshold in 18% of attempts.  As a result,
reclaim continues in the majority of cases despite many lower-order free
pages being available.  The over-sized gap also causes 46% of order-9
compaction suitability checks to fail unnecessarily: the zone has
sufficient free pages for the scanner to operate, but not enough to clear
the inflated threshold.

Cap compact_gap() at COMPACT_CLUSTER_MAX so the watermark headroom
reflects the scanner's actual capacity.  This function is used by two key
heuristics.  The first is when kswapd can stop high-order reclaim and
downgrade to order-0 balancing, allowing kcompactd to be woken for the
original higher allocation order.  The second is zone suitability
checking, where the smaller gap allows compaction to start sooner.

Note that orders 0-4 are unaffected since their gap is already less than
or equal to COMPACT_CLUSTER_MAX.

A/B test on v6.13-based instagram production hosts (64GB, 60s
measurement):

Unpatched (43 hosts)
pgscan_kswapd (mean/host): ~1.6M
reclaim efficiency (steal/scan): 83.8%
per-compaction success (success/stall): 2.1%
THP success (alloc/alloc+fallback): 4.9%
forced lru_add_drain (mean/host): ~107K

Patched (59 hosts)
pgscan_kswapd (mean/host): ~449K
reclaim efficiency (steal/scan): 91.0%
per-compaction success (success/stall): 28.3%
THP success (alloc/alloc+fallback): 17.2%
forced lru_add_drain (mean/host): ~64K

Additional tests were also performed using a workload of similar shape and
based on mm-new at the time of testing.  Across three 60s runs, the patch
showed improvements consistent with the previous test: reduced kswapd
reclaim and fewer THP fault fallbacks.

Unpatched
kswapd_shrink_node downgrade to order-0 (mean): 0
thp_fault_fallback (mean): 1217
pgscan_kswapd (mean): 6328
pgsteal_kswapd (mean): 5657

Patched
kswapd_shrink_node downgrade to order-0 (mean): 28
thp_fault_fallback (mean): 738
pgscan_kswapd (mean): 3773
pgsteal_kswapd (mean): 3243

Link: https://lore.kernel.org/20260604061725.13800-1-jp.kobryn@linux.dev
Signed-off-by: JP Kobryn (Meta) <jp.kobryn@linux.dev>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index c829c48d1c71..f29ef0653546 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -2,6 +2,8 @@
 #ifndef _LINUX_COMPACTION_H
 #define _LINUX_COMPACTION_H
 
+#include <linux/swap.h>
+
 /*
  * Determines how hard direct compaction should try to succeed.
  * Lower value means higher priority, analogically to reclaim priority.
@@ -73,11 +75,9 @@ static inline unsigned long compact_gap(unsigned int order)
 	 * effectively limited by COMPACT_CLUSTER_MAX, as that's the maximum
 	 * that the migrate scanner can have isolated on migrate list, and free
 	 * scanner is only invoked when the number of isolated free pages is
-	 * lower than that. But it's not worth to complicate the formula here
-	 * as a bigger gap for higher orders than strictly necessary can also
-	 * improve chances of compaction success.
+	 * lower than that.
 	 */
-	return 2UL << order;
+	return min(2UL << order, COMPACT_CLUSTER_MAX);
 }
 
 static inline int current_is_kcompactd(void)