Messages in this thread | | | Date | Tue, 20 Nov 2012 14:04:41 -0500 | From | Johannes Weiner <> | Subject | kswapd endless loop for compaction |
| |
Hi guys,
while testing a 3.7-rc5ish kernel, I noticed that kswapd can drop into a busy spin state without doing reclaim. printk-style debugging told me that this happens when the distance between a zone's high watermark and its low watermark is less than two huge pages (DMA zone).
1. The first loop in balance_pgdat() over the zones finds all zones to be above their high watermark and only does goto out (all_zones_ok).
2. pgdat_balanced() at the out: label also just checks the high watermark, so the node is considered balanced and the order is not reduced.
3. In the `if (order)' block after it, compaction_suitable() checks if the zone's low watermark + twice the huge page size is okay, which it's not necessarily in a small zone, and so COMPACT_SKIPPED makes it it go back to loop_again:.
This will go on until somebody else allocates and breaches the high watermark and then hopefully goes on to reclaim the zone above low watermark + 2 * THP.
I'm not really sure what the correct solution is. Should we modify the zone_watermark_ok() checks in balance_pgdat() to take into account the higher watermark requirements for reclaim on behalf of compaction? Change the check in compaction_suitable() / not use it directly?
Thanks, Johannes
| |