[PATCH] Re: kswapd stuck using 100% CPU

[Rik van Riel <riel@redhat.com>]

On Sat, 24 Mar 2012 13:03:53 +1100
Anton Blanchard <anton@samba.org> wrote:

> 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

Are you running without CONFIG_COMPACTION enabled by any chance?

Because if you do, the stub function compaction_suitable will always
return COMPACT_SKIPPED:
 
> I haven't had time to bisect but I did notice we were looping here:
> 
> +++ 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

The patch below should fix it.

-----

Only test compaction_suitable if the kernel is built with CONFIG_COMPACTION,
otherwise the stub compaction_suitable function will always return
COMPACT_SKIPPED and send kswapd into an infinite loop.

Signed-off-by: Rik van Riel <riel@redhat.com>
Reported-by: Anton Blanchard <anton@samba.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7658fd6..33c332b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2946,7 +2946,8 @@ out:
 				continue;
 
 			/* Would compaction fail due to lack of free memory? */
-			if (compaction_suitable(zone, order) == COMPACT_SKIPPED)
+			if (COMPACTION_BUILD &&
+			    compaction_suitable(zone, order) == COMPACT_SKIPPED)
 				goto loop_again;
 
 			/* Confirm the zone is balanced for order-0 */

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