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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
