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Date:   Mon, 3 Aug 2020 10:05:29 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Charan Teja Reddy <charante@...eaurora.org>,
        akpm@...ux-foundation.org, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, vinmenon@...eaurora.org
Subject: Re: [PATCH] mm, memory_hotplug: update pcp lists everytime onlining a
 memory block

On 02.08.20 14:54, Charan Teja Reddy wrote:
> When onlining a first memory block in a zone, pcp lists are not updated
> thus pcp struct will have the default setting of ->high = 0,->batch = 1.
> This means till the second memory block in a zone(if it have) is onlined
> the pcp lists of this zone will not contain any pages because pcp's
> ->count is always greater than ->high thus free_pcppages_bulk() is
> called to free batch size(=1) pages every time system wants to add a
> page to the pcp list through free_unref_page(). To put this in a word,
> system is not using benefits offered by the pcp lists when there is a
> single onlineable memory block in a zone. Correct this by always
> updating the pcp lists when memory block is onlined.

I guess such setups are rare ... but I can imagine it being the case
with virtio-mem in the future ... not 100% if this performance
optimization is really relevant in practice ... how did you identify this?

> 
> Signed-off-by: Charan Teja Reddy <charante@...eaurora.org>
> ---
>  mm/memory_hotplug.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index dcdf327..7f62d69 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -854,8 +854,7 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
>  	node_states_set_node(nid, &arg);
>  	if (need_zonelists_rebuild)
>  		build_all_zonelists(NULL);
> -	else
> -		zone_pcp_update(zone);
> +	zone_pcp_update(zone);
>  
>  	init_per_zone_wmark_min();
>  
> 

Does, in general, look sane to me.

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb

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