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Re: [Xen-devel] [PATCH v5 1/2] memory_hotplug: Free pages as higher order


  • To: Arun KS <arunks@xxxxxxxxxxxxxx>, kys@xxxxxxxxxxxxx, haiyangz@xxxxxxxxxxxxx, sthemmin@xxxxxxxxxxxxx, boris.ostrovsky@xxxxxxxxxx, jgross@xxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, dan.j.williams@xxxxxxxxx, mhocko@xxxxxxxx, iamjoonsoo.kim@xxxxxxx, gregkh@xxxxxxxxxxxxxxxxxxx, osalvador@xxxxxxx, malat@xxxxxxxxxx, kirill.shutemov@xxxxxxxxxxxxxxx, jrdr.linux@xxxxxxxxx, yasu.isimatu@xxxxxxxxx, mgorman@xxxxxxxxxxxxxxxxxxx, aaron.lu@xxxxxxxxx, devel@xxxxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
  • From: Vlastimil Babka <vbabka@xxxxxxx>
  • Date: Wed, 10 Oct 2018 17:30:11 +0200
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  • Cc: vinmenon@xxxxxxxxxxxxxx, vatsa@xxxxxxxxxxxxxx, getarunks@xxxxxxxxx
  • Delivery-date: Wed, 10 Oct 2018 15:33:08 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 10/5/18 10:10 AM, Arun KS wrote:
> When free pages are done with higher order, time spend on
> coalescing pages by buddy allocator can be reduced. With
> section size of 256MB, hot add latency of a single section
> shows improvement from 50-60 ms to less than 1 ms, hence
> improving the hot add latency by 60%. Modify external
> providers of online callback to align with the change.
> 
> Signed-off-by: Arun KS <arunks@xxxxxxxxxxxxxx>

[...]

> @@ -655,26 +655,44 @@ void __online_page_free(struct page *page)
>  }
>  EXPORT_SYMBOL_GPL(__online_page_free);
>  
> -static void generic_online_page(struct page *page)
> +static int generic_online_page(struct page *page, unsigned int order)
>  {
> -     __online_page_set_limits(page);

This is now not called anymore, although the xen/hv variants still do
it. The function seems empty these days, maybe remove it as a followup
cleanup?

> -     __online_page_increment_counters(page);
> -     __online_page_free(page);
> +     __free_pages_core(page, order);
> +     totalram_pages += (1UL << order);
> +#ifdef CONFIG_HIGHMEM
> +     if (PageHighMem(page))
> +             totalhigh_pages += (1UL << order);
> +#endif

__online_page_increment_counters() would have used
adjust_managed_page_count() which would do the changes under
managed_page_count_lock. Are we safe without the lock? If yes, there
should perhaps be a comment explaining why.

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