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

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Message-ID: <20180313033519.GC13782@intel.com>
Date:   Tue, 13 Mar 2018 11:35:19 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, 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 Mon, Mar 12, 2018 at 10:32:32AM -0700, Dave Hansen wrote:
> 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?

I'll give it a try.

> 
> 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.