linux-kernel - Re: [PATCH v4 3/3 update] mm/free_pcppages_bulk: prefetch buddy while not holding lock

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Message-ID: <988ce376-bdc4-0989-5133-612bfa3f7c45@intel.com>
Date:   Mon, 12 Mar 2018 10:32:32 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Aaron Lu <aaron.lu@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Huang Ying <ying.huang@...el.com>,
        Kemi Wang <kemi.wang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v4 3/3 update] mm/free_pcppages_bulk: prefetch buddy while
 not holding lock

On 03/09/2018 12:24 AM, Aaron Lu wrote:
> +			/*
> +			 * We are going to put the page back to the global
> +			 * pool, prefetch its buddy to speed up later access
> +			 * under zone->lock. It is believed the overhead of
> +			 * an additional test and calculating buddy_pfn here
> +			 * can be offset by reduced memory latency later. To
> +			 * avoid excessive prefetching due to large count, only
> +			 * prefetch buddy for the last pcp->batch nr of pages.
> +			 */
> +			if (count > pcp->batch)
> +				continue;
> +			pfn = page_to_pfn(page);
> +			buddy_pfn = __find_buddy_pfn(pfn, 0);
> +			buddy = page + (buddy_pfn - pfn);
> +			prefetch(buddy);

FWIW, I think this needs to go into a helper function.  Is that possible?

There's too much logic happening here.  Also, 'count' going from
batch_size->0 is totally non-obvious from the patch context.  It makes
this hunk look totally wrong by itself.