kswapd stuck using 100% CPU

[Anton Blanchard <anton@samba.org>]

Hi,

I booted the latest git today on a ppc64 box. When I pushed it into
swap I noticed both kswapd's were using 100% CPU and the soft lockup
detector suggested it was stuck in balance_pgdat:

BUG: soft lockup - CPU#7 stuck for 23s! [kswapd1:359]
Call Trace:
[c00000000015e190] .balance_pgdat+0x150/0x940 
[c00000000015eb2c] .kswapd+0x1ac/0x490
[c00000000009edbc] .kthread+0xbc/0xd0
[c00000000002142c] .kernel_thread+0x54/0x70

I haven't had time to bisect but I did notice we were looping here:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7658fd6..c92bad2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2945,9 +2959,11 @@ out:
 			if (zone->all_unreclaimable && priority != DEF_PRIORITY)
 				continue;
 
+#if 0
 			/* Would compaction fail due to lack of free memory? */
 			if (compaction_suitable(zone, order) == COMPACT_SKIPPED)
 				goto loop_again;
+#endif
 
 			/* Confirm the zone is balanced for order-0 */
 			if (!zone_watermark_ok(zone, 0,


After commenting it out the box is happy again.

Anton

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