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Re: [PATCH v2 17/18] mm, vmscan: make compaction_ready() more accurate and readable

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On Tue 31-05-16 15:08:17, Vlastimil Babka wrote:
> The compaction_ready() is used during direct reclaim for costly order
> allocations to skip reclaim for zones where compaction should be attempted
> instead. It's combining the standard compaction_suitable() check with its own
> watermark check based on high watermark with extra gap, and the result is
> confusing at best.
> 
> This patch attempts to better structure and document the checks involved.
> First, compaction_suitable() can determine that the allocation should either
> succeed already, or that compaction doesn't have enough free pages to proceed.
> The third possibility is that compaction has enough free pages, but we still
> decide to reclaim first - unless we are already above the high watermark with
> gap.  This does not mean that the reclaim will actually reach this watermark
> during single attempt, this is rather an over-reclaim protection. So document
> the code as such. The check for compaction_deferred() is removed completely, as
> it in fact had no proper role here.
> 
> The result after this patch is mainly a less confusing code. We also skip some
> over-reclaim in cases where the allocation should already succed.

Yes this is indeed more understandable

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

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/vmscan.c | 49 +++++++++++++++++++++++--------------------------
>  1 file changed, 23 insertions(+), 26 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 00034ec9229b..640d2e615c36 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2456,40 +2456,37 @@ static bool shrink_zone(struct zone *zone, struct scan_control *sc,
>  }
>  
>  /*
> - * Returns true if compaction should go ahead for a high-order request, or
> - * the high-order allocation would succeed without compaction.
> + * Returns true if compaction should go ahead for a costly-order request, or
> + * the allocation would already succeed without compaction. Return false if we
> + * should reclaim first.
>   */
>  static inline bool compaction_ready(struct zone *zone, int order, int classzone_idx)
>  {
> -	unsigned long balance_gap, watermark;
> -	bool watermark_ok;
> +	unsigned long watermark;
> +	enum compact_result suitable;
>  
> -	/*
> -	 * Compaction takes time to run and there are potentially other
> -	 * callers using the pages just freed. Continue reclaiming until
> -	 * there is a buffer of free pages available to give compaction
> -	 * a reasonable chance of completing and allocating the page
> -	 */
> -	balance_gap = min(low_wmark_pages(zone), DIV_ROUND_UP(
> -			zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
> -	watermark = high_wmark_pages(zone) + balance_gap + compact_gap(order);
> -	watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, classzone_idx);
> -
> -	/*
> -	 * If compaction is deferred, reclaim up to a point where
> -	 * compaction will have a chance of success when re-enabled
> -	 */
> -	if (compaction_deferred(zone, order))
> -		return watermark_ok;
> +	suitable = compaction_suitable(zone, order, 0, classzone_idx);
> +	if (suitable == COMPACT_PARTIAL)
> +		/* Allocation should succeed already. Don't reclaim. */
> +		return true;
> +	if (suitable == COMPACT_SKIPPED)
> +		/* Compaction cannot yet proceed. Do reclaim. */
> +		return false;
>  
>  	/*
> -	 * If compaction is not ready to start and allocation is not likely
> -	 * to succeed without it, then keep reclaiming.
> +	 * Compaction is already possible, but it takes time to run and there
> +	 * are potentially other callers using the pages just freed. So proceed
> +	 * with reclaim to make a buffer of free pages available to give
> +	 * compaction a reasonable chance of completing and allocating the page.
> +	 * Note that we won't actually reclaim the whole buffer in one attempt
> +	 * as the target watermark in should_continue_reclaim() is lower. But if
> +	 * we are already above the high+gap watermark, don't reclaim at all.
>  	 */
> -	if (compaction_suitable(zone, order, 0, classzone_idx) == COMPACT_SKIPPED)
> -		return false;
> +	watermark = high_wmark_pages(zone) + compact_gap(order);
> +	watermark += min(low_wmark_pages(zone), DIV_ROUND_UP(
> +			zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
>  
> -	return watermark_ok;
> +	return zone_watermark_ok_safe(zone, 0, watermark, classzone_idx);
>  }
>  
>  /*
> -- 
> 2.8.3

-- 
Michal Hocko
SUSE Labs

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[PATCH v2 00/18] make direct compaction more deterministic Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:10 +0200
  [PATCH v2 02/18] mm, page_alloc: set alloc_flags only once in slowpath Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:10 +0200
  [PATCH v2 15/18] mm, compaction: use proper alloc_flags in __compaction_suitable() Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:10 +0200
  [PATCH v2 14/18] mm, compaction: create compact_gap wrapper Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:10 +0200
    Re: [PATCH v2 14/18] mm, compaction: create compact_gap wrapper Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:10 +0200
  [PATCH v2 17/18] mm, vmscan: make compaction_ready() more accurate and readable Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:10 +0200
    Re: [PATCH v2 17/18] mm, vmscan: make compaction_ready() more  accurate and readable Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:20 +0200
  [PATCH v2 04/18] mm, page_alloc: restructure direct compaction handling in slowpath Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 03/18] mm, page_alloc: don't retry initial attempt in slowpath Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 03/18] mm, page_alloc: don't retry initial attempt in  slowpath Michal Hocko <mhocko@kernel.org> - 2016-06-01 15:30 +0200
      Re: [PATCH v2 03/18] mm, page_alloc: don't retry initial attempt in  slowpath Vlastimil Babka <vbabka@suse.cz> - 2016-06-01 17:00 +0200
  [PATCH v2 05/18] mm, page_alloc: make THP-specific decisions more generic Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 13/18] mm, compaction: use correct watermark when checking allocation success Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 13/18] mm, compaction: use correct watermark when  checking allocation success Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:00 +0200
  [PATCH v2 06/18] mm, thp: remove __GFP_NORETRY from khugepaged and madvised allocations Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 06/18] mm, thp: remove __GFP_NORETRY from khugepaged  and madvised allocations Michal Hocko <mhocko@kernel.org> - 2016-06-01 15:40 +0200
  [PATCH v2 12/18] mm, compaction: more reliably increase direct compaction priority Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 12/18] mm, compaction: more reliably increase direct  compaction priority Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:00 +0200
  [PATCH v2 11/18] mm, compaction: add the ultimate direct compaction priority Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 10/18] mm, compaction: cleanup unused functions Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 10/18] mm, compaction: cleanup unused functions Michal Hocko <mhocko@kernel.org> - 2016-06-01 15:50 +0200
  [PATCH v2 01/18] mm, compaction: don't isolate PageWriteback pages in MIGRATE_SYNC_LIGHT mode Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 09/18] mm, compaction: make whole_zone flag ignore cached scanner positions Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 18/18] mm, vmscan: use proper classzone_idx in should_continue_reclaim() Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 18/18] mm, vmscan: use proper classzone_idx in  should_continue_reclaim() Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:30 +0200
      Re: [PATCH v2 18/18] mm, vmscan: use proper classzone_idx in  should_continue_reclaim() Vlastimil Babka <vbabka@suse.cz> - 2016-06-01 17:20 +0200
        Re: [PATCH v2 18/18] mm, vmscan: use proper classzone_idx in  should_continue_reclaim() Michal Hocko <mhocko@kernel.org> - 2016-06-01 17:50 +0200
  [PATCH v2 08/18] mm, compaction: simplify contended compaction handling Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 16/18] mm, compaction: require only min watermarks for non-costly orders Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 16/18] mm, compaction: require only min watermarks for  non-costly orders Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:10 +0200
  [PATCH v2 07/18] mm, compaction: introduce direct compaction priority Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200

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