linux-kernel - Re: [PATCH 0/6] mm: make movable onlining suck less

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Date:   Thu, 6 Apr 2017 17:55:20 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Reza Arbab <arbab@...ux.vnet.ibm.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        Tang Chen <tangchen@...fujitsu.com>, qiuxishi@...wei.com,
        Kani Toshimitsu <toshi.kani@....com>, slaoub@...il.com,
        Joonsoo Kim <js1304@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Zhang Zhen <zhenzhang.zhang@...wei.com>,
        David Rientjes <rientjes@...gle.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dan Williams <dan.j.williams@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Lai Jiangshan <laijs@...fujitsu.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 0/6] mm: make movable onlining suck less

On Thu, Apr 06, 2017 at 06:21:55PM +0200, Michal Hocko wrote:
> > This was my first time using your git branch instead of applying the patches
> > from this thread to v4.11-rc5 myself.
> 
> OK, so this looks like another thing to resolve. I have seen this
> warning as well but I didn't consider it relevant because I had to tweak
> the code make the node go offline (removed check_and_unmap_cpu_on_node
> from try_offline_node) so I thought it was a fallout from there. 
> 
> But let's have a look. hotadd_new_pgdat does for an existing pgdat
> 		/* Reset the nr_zones, order and classzone_idx before reuse */
> 		pgdat->nr_zones = 0;
> 		pgdat->kswapd_order = 0;
> 		pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
> 
> so free_area_init_node absolutely has to hit this warning. This is not
> in the Linus tree because it is still in Andrew's mmotm coming from
> http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-vmscan-prevent-kswapd-sleeping-prematurely-due-to-mismatched-classzone_idx.patch
> 
> So yay, finally that doesn't come from me. Mel, I guess that either
> hotadd_new_pgdat should keep its kswapd_classzone_idx = 0 or the warning
> should be updated.
> 

Actually, it's obvious very quickly when I started the fix that updating
the warning would then trigger on normal boot. It's more appropriate to
let a hotadd of a new pgdat defer the initialisation of that field to
kswapd starting for the new node.

Can you try this? It's build/boot tested only, no hotplug testing.

---8<---
mm, vmscan: prevent kswapd sleeping prematurely due to mismatched classzone_idx -fix

The patch "mm, vmscan: prevent kswapd sleeping prematurely due to mismatched
classzone_idx" has different initial starting conditions when kswapd
is asleep. kswapd initialises it properly when it starts but the patch
initialises kswapd_classzone_idx early and trips on a warning in
free_area_init_node. This patch leaves the kswapd_classzone_idx as zero
and defers to kswapd to initialise it properly when it starts.

This is a fix to the mmotm patch
mm-vmscan-prevent-kswapd-sleeping-prematurely-due-to-mismatched-classzone_idx.patch

Signed-off-by: Mel Gorman <mgorman@...e.de>

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 2309a7fbec93..76d4745513ee 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1214,10 +1214,14 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
 
 		arch_refresh_nodedata(nid, pgdat);
 	} else {
-		/* Reset the nr_zones, order and classzone_idx before reuse */
+		/*
+		 * Reset the nr_zones, order and classzone_idx before reuse.
+		 * Note that kswapd will init kswapd_classzone_idx properly
+		 * when it starts in the near future.
+		 */
 		pgdat->nr_zones = 0;
 		pgdat->kswapd_order = 0;
-		pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
+		pgdat->kswapd_classzone_idx = 0;
 	}
 
 	/* we can use NODE_DATA(nid) from here */