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Subject: Re: [PATCH v5 2/2] mm/page_alloc: remove software prefetching in
 __free_pages_core
To:     Arun KS <arunks@codeaurora.org>, kys@microsoft.com,
        haiyangz@microsoft.com, sthemmin@microsoft.com,
        boris.ostrovsky@oracle.com, jgross@suse.com,
        akpm@linux-foundation.org, dan.j.williams@intel.com,
        mhocko@suse.com, iamjoonsoo.kim@lge.com,
        gregkh@linuxfoundation.org, osalvador@suse.de, malat@debian.org,
        kirill.shutemov@linux.intel.com, jrdr.linux@gmail.com,
        yasu.isimatu@gmail.com, mgorman@techsingularity.net,
        aaron.lu@intel.com, devel@linuxdriverproject.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        xen-devel@lists.xenproject.org
Cc:     vatsa@codeaurora.org, vinmenon@codeaurora.org, getarunks@gmail.com
References: <1538727006-5727-1-git-send-email-arunks@codeaurora.org>
 <1538727006-5727-2-git-send-email-arunks@codeaurora.org>
From:   Vlastimil Babka <vbabka@suse.cz>
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Message-ID: <e8afbe5e-06b1-f179-6292-1a5967432849@suse.cz>
Date:   Wed, 10 Oct 2018 18:36:52 +0200
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On 10/5/18 10:10 AM, Arun KS wrote:
> They not only increase the code footprint, they actually make things
> slower rather than faster. Remove them as contemporary hardware doesn't
> need any hint.
> 
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Arun KS <arunks@codeaurora.org>

Yeah, a tight loop with fixed stride is a trivial case for hw prefetcher.

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_alloc.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7ab5274..90db431 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1258,14 +1258,10 @@ void __free_pages_core(struct page *page, unsigned int order)
>  	struct page *p = page;
>  	unsigned int loop;
>  
> -	prefetchw(p);
> -	for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
> -		prefetchw(p + 1);
> +	for (loop = 0; loop < nr_pages ; loop++, p++) {
>  		__ClearPageReserved(p);
>  		set_page_count(p, 0);
>  	}
> -	__ClearPageReserved(p);
> -	set_page_count(p, 0);
>  
>  	page_zone(page)->managed_pages += nr_pages;
>  	set_page_refcounted(page);
>