linux-kernel - Re: [PATCH 7/8] mm, Remove cold parameter from free_hot_cold

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Date:   Thu, 19 Oct 2017 15:12:33 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 7/8] mm, Remove cold parameter from free_hot_cold_page*

On 10/18/2017 09:59 AM, Mel Gorman wrote:
> Most callers users of free_hot_cold_page claim the pages being released are
> cache hot. The exception is the page reclaim paths where it is likely that
> enough pages will be freed in the near future that the per-cpu lists are
> going to be recycled and the cache hotness information is lost.

Maybe it would make sense for reclaim to skip pcplists? (out of scope of
this series, of course).

> As no one
> really cares about the hotness of pages being released to the allocator,
> just ditch the parameter.
> 
> The APIs are renamed to indicate that it's no longer about hot/cold pages. It
> should also be less confusing as there are subtle differences between them.
> __free_pages drops a reference and frees a page when the refcount reaches
> zero. free_hot_cold_page handled pages whose refcount was already zero
> which is non-obvious from the name. free_unref_page should be more obvious.
> 
> No performance impact is expected as the overhead is marginal. The parameter
> is removed simply because it is a bit stupid to have a useless parameter
> copied everywhere.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

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

A comment below, though.

...
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 167e163cf733..13582efc57a0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2590,7 +2590,7 @@ void mark_free_pages(struct zone *zone)
>  }
>  #endif /* CONFIG_PM */
>  
> -static bool free_hot_cold_page_prepare(struct page *page, unsigned long pfn)
> +static bool free_unref_page_prepare(struct page *page, unsigned long pfn)
>  {
>  	int migratetype;
>  
> @@ -2602,8 +2602,7 @@ static bool free_hot_cold_page_prepare(struct page *page, unsigned long pfn)
>  	return true;
>  }
>  
> -static void free_hot_cold_page_commit(struct page *page, unsigned long pfn,
> -				bool cold)
> +static void free_unref_page_commit(struct page *page, unsigned long pfn)
>  {
>  	struct zone *zone = page_zone(page);
>  	struct per_cpu_pages *pcp;
> @@ -2628,10 +2627,7 @@ static void free_hot_cold_page_commit(struct page *page, unsigned long pfn,
>  	}
>  
>  	pcp = &this_cpu_ptr(zone->pageset)->pcp;
> -	if (!cold)
> -		list_add(&page->lru, &pcp->lists[migratetype]);
> -	else
> -		list_add_tail(&page->lru, &pcp->lists[migratetype]);
> +	list_add_tail(&page->lru, &pcp->lists[migratetype]);

Did you intentionally use the cold version here? Patch 8/8 uses the hot
version in __rmqueue_pcplist() and that makes more sense to me. It
should be either negligible or better, not worse.

>  	pcp->count++;
>  	if (pcp->count >= pcp->high) {
>  		unsigned long batch = READ_ONCE(pcp->batch);