Re: [patch] mm, compaction: determine isolation mode only once

[Vlastimil Babka <vbabka@suse.cz>]

On 03/07/2014 01:01 PM, David Rientjes wrote:
> The conditions that control the isolation mode in
> isolate_migratepages_range() do not change during the iteration, so
> extract them out and only define the value once.
>
> This actually does have an effect, gcc doesn't optimize it itself because
> of cc->sync.
>
> Signed-off-by: David Rientjes <rientjes@google.com>

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

> ---
>   mm/compaction.c | 9 ++-------
>   1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -454,12 +454,13 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>   	unsigned long last_pageblock_nr = 0, pageblock_nr;
>   	unsigned long nr_scanned = 0, nr_isolated = 0;
>   	struct list_head *migratelist = &cc->migratepages;
> -	isolate_mode_t mode = 0;
>   	struct lruvec *lruvec;
>   	unsigned long flags;
>   	bool locked = false;
>   	struct page *page = NULL, *valid_page = NULL;
>   	bool skipped_async_unsuitable = false;
> +	const isolate_mode_t mode = (!cc->sync ? ISOLATE_ASYNC_MIGRATE : 0) |
> +				    (unevictable ? ISOLATE_UNEVICTABLE : 0);
>
>   	/*
>   	 * Ensure that there are not too many pages isolated from the LRU
> @@ -592,12 +593,6 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>   			continue;
>   		}
>
> -		if (!cc->sync)
> -			mode |= ISOLATE_ASYNC_MIGRATE;
> -
> -		if (unevictable)
> -			mode |= ISOLATE_UNEVICTABLE;
> -
>   		lruvec = mem_cgroup_page_lruvec(page, zone);
>
>   		/* Try isolate the page */
>

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