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

[Michal Hocko]

On Thu 06-04-17 17:55:20, Mel Gorman wrote:
> 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.

It will start during the online phase which is later than this physical
memory hotadd.

> 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@xxxxxxx>

Yes, that is what I would expect. Feel free to add
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

if this is routed as a separate patch. Although I expect Andrew will
fold it into the original patch.
 
> 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 */
> 
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-- 
Michal Hocko
SUSE Labs