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

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Date:   Wed, 10 Oct 2018 17:30:11 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Arun KS <arunks@...eaurora.org>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        akpm@...ux-foundation.org, dan.j.williams@...el.com,
        mhocko@...e.com, iamjoonsoo.kim@....com,
        gregkh@...uxfoundation.org, osalvador@...e.de, malat@...ian.org,
        kirill.shutemov@...ux.intel.com, jrdr.linux@...il.com,
        yasu.isimatu@...il.com, mgorman@...hsingularity.net,
        aaron.lu@...el.com, devel@...uxdriverproject.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xen-devel@...ts.xenproject.org
Cc:     vatsa@...eaurora.org, vinmenon@...eaurora.org, getarunks@...il.com
Subject: Re: [PATCH v5 1/2] memory_hotplug: Free pages as higher order

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@...eaurora.org>

[...]

> @@ -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.