From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx110.postini.com [74.125.245.110]) by kanga.kvack.org (Postfix) with SMTP id ACD2D6B0072 for ; Wed, 19 Dec 2012 18:17:11 -0500 (EST) Date: Thu, 20 Dec 2012 00:17:07 +0100 From: Zlatko Calusic MIME-Version: 1.0 Message-ID: <50D24AF3.1050809@iskon.hr> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: [PATCH] mm: do not sleep in balance_pgdat if there's no i/o congestion Sender: owner-linux-mm@kvack.org List-ID: To: Linus Torvalds , Andrew Morton , Mel Gorman , Hugh Dickins Cc: linux-mm , Linux Kernel Mailing List On a 4GB RAM machine, where Normal zone is much smaller than DMA32 zone, the Normal zone gets fragmented in time. This requires relatively more pressure in balance_pgdat to get the zone above the required watermark. Unfortunately, the congestion_wait() call in there slows it down for a completely wrong reason, expecting that there's a lot of writeback/swapout, even when there's none (much more common). After a few days, when fragmentation progresses, this flawed logic translates to a very high CPU iowait times, even though there's no I/O congestion at all. If THP is enabled, the problem occurs sooner, but I was able to see it even on !THP kernels, just by giving it a bit more time to occur. The proper way to deal with this is to not wait, unless there's congestion. Thanks to Mel Gorman, we already have the function that perfectly fits the job. The patch was tested on a machine which nicely revealed the problem after only 1 day of uptime, and it's been working great. --- mm/vmscan.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index b7ed376..4588d1d 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2546,7 +2546,7 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining, static unsigned long balance_pgdat(pg_data_t *pgdat, int order, int *classzone_idx) { - int all_zones_ok; + struct zone *unbalanced_zone; unsigned long balanced; int i; int end_zone = 0; /* Inclusive. 0 = ZONE_DMA */ @@ -2580,7 +2580,7 @@ loop_again: unsigned long lru_pages = 0; int has_under_min_watermark_zone = 0; - all_zones_ok = 1; + unbalanced_zone = NULL; balanced = 0; /* @@ -2719,7 +2719,7 @@ loop_again: } if (!zone_balanced(zone, testorder, 0, end_zone)) { - all_zones_ok = 0; + unbalanced_zone = zone; /* * We are still under min water mark. This * means that we have a GFP_ATOMIC allocation @@ -2752,7 +2752,7 @@ loop_again: pfmemalloc_watermark_ok(pgdat)) wake_up(&pgdat->pfmemalloc_wait); - if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx))) + if (!unbalanced_zone || (order && pgdat_balanced(pgdat, balanced, *classzone_idx))) break; /* kswapd: all done */ /* * OK, kswapd is getting into trouble. Take a nap, then take @@ -2762,7 +2762,7 @@ loop_again: if (has_under_min_watermark_zone) count_vm_event(KSWAPD_SKIP_CONGESTION_WAIT); else - congestion_wait(BLK_RW_ASYNC, HZ/10); + wait_iff_congested(unbalanced_zone, BLK_RW_ASYNC, HZ/10); } /* @@ -2781,7 +2781,7 @@ out: * high-order: Balanced zones must make up at least 25% of the node * for the node to be balanced */ - if (!(all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))) { + if (unbalanced_zone && (!order || !pgdat_balanced(pgdat, balanced, *classzone_idx))) { cond_resched(); try_to_freeze(); -- 1.7.10.4 -- Zlatko -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org