linux-kernel - Re: [RFC PATCH v4 06/40] mm: Demarcate and maintain pageblocks in region-order in the zones' freelists

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Message-ID: <5244B22C.9020503@sr71.net>
Date:	Thu, 26 Sep 2013 15:16:12 -0700
From:	Dave Hansen <dave@...1.net>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	akpm@...ux-foundation.org, mgorman@...e.de, hannes@...xchg.org,
	tony.luck@...el.com, matthew.garrett@...ula.com, riel@...hat.com,
	arjan@...ux.intel.com, srinivas.pandruvada@...ux.intel.com,
	willy@...ux.intel.com, kamezawa.hiroyu@...fujitsu.com,
	lenb@...nel.org, rjw@...k.pl
CC:	gargankita@...il.com, paulmck@...ux.vnet.ibm.com,
	svaidy@...ux.vnet.ibm.com, andi@...stfloor.org,
	isimatu.yasuaki@...fujitsu.com, santosh.shilimkar@...com,
	kosaki.motohiro@...il.com, linux-pm@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 06/40] mm: Demarcate and maintain pageblocks in
 region-order in the zones' freelists

On 09/25/2013 04:14 PM, Srivatsa S. Bhat wrote:
> @@ -605,16 +713,22 @@ static inline void __free_one_page(struct page *page,
>  		buddy_idx = __find_buddy_index(combined_idx, order + 1);
>  		higher_buddy = higher_page + (buddy_idx - combined_idx);
>  		if (page_is_buddy(higher_page, higher_buddy, order + 1)) {
> -			list_add_tail(&page->lru,
> -				&zone->free_area[order].free_list[migratetype].list);
> +
> +			/*
> +			 * Implementing an add_to_freelist_tail() won't be
> +			 * very useful because both of them (almost) add to
> +			 * the tail within the region. So we could potentially
> +			 * switch off this entire "is next-higher buddy free?"
> +			 * logic when memory regions are used.
> +			 */
> +			add_to_freelist(page, &area->free_list[migratetype]);
>  			goto out;
>  		}
>  	}

Commit 6dda9d55b says that this had some discrete performance gains.
It's a bummer that this deoptimizes it, and I think that (expected)
performance degredation at least needs to be referenced _somewhere_.

I also find it very hard to take code seriously which stuff like this:

> +#ifdef CONFIG_DEBUG_PAGEALLOC
> +		WARN(region->nr_free == 0, "%s: nr_free messed up\n", __func__);
> +#endif

nine times.
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