Re: [PATCH v3 02/13] mm, compaction: defer each zone individually instead of preferred zone

[Zhang Yanfei]

On 06/20/2014 11:49 PM, Vlastimil Babka wrote:
> When direct sync compaction is often unsuccessful, it may become deferred for
> some time to avoid further useless attempts, both sync and async. Successful
> high-order allocations un-defer compaction, while further unsuccessful
> compaction attempts prolong the copmaction deferred period.
> 
> Currently the checking and setting deferred status is performed only on the
> preferred zone of the allocation that invoked direct compaction. But compaction
> itself is attempted on all eligible zones in the zonelist, so the behavior is
> suboptimal and may lead both to scenarios where 1) compaction is attempted
> uselessly, or 2) where it's not attempted despite good chances of succeeding,
> as shown on the examples below:
> 
> 1) A direct compaction with Normal preferred zone failed and set deferred
>    compaction for the Normal zone. Another unrelated direct compaction with
>    DMA32 as preferred zone will attempt to compact DMA32 zone even though
>    the first compaction attempt also included DMA32 zone.
> 
>    In another scenario, compaction with Normal preferred zone failed to compact
>    Normal zone, but succeeded in the DMA32 zone, so it will not defer
>    compaction. In the next attempt, it will try Normal zone which will fail
>    again, instead of skipping Normal zone and trying DMA32 directly.
> 
> 2) Kswapd will balance DMA32 zone and reset defer status based on watermarks
>    looking good. A direct compaction with preferred Normal zone will skip
>    compaction of all zones including DMA32 because Normal was still deferred.
>    The allocation might have succeeded in DMA32, but won't.
> 
> This patch makes compaction deferring work on individual zone basis instead of
> preferred zone. For each zone, it checks compaction_deferred() to decide if the
> zone should be skipped. If watermarks fail after compacting the zone,
> defer_compaction() is called. The zone where watermarks passed can still be
> deferred when the allocation attempt is unsuccessful. When allocation is
> successful, compaction_defer_reset() is called for the zone containing the
> allocated page. This approach should approximate calling defer_compaction()
> only on zones where compaction was attempted and did not yield allocated page.
> There might be corner cases but that is inevitable as long as the decision
> to stop compacting dues not guarantee that a page will be allocated.
> 
> During testing on a two-node machine with a single very small Normal zone on
> node 1, this patch has improved success rates in stress-highalloc mmtests
> benchmark. The success here were previously made worse by commit 3a025760fc
> ("mm: page_alloc: spill to remote nodes before waking kswapd") as kswapd was
> no longer resetting often enough the deferred compaction for the Normal zone,
> and DMA32 zones on both nodes were thus not considered for compaction.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>

Really good.

Reviewed-by: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>

> ---
>  include/linux/compaction.h |  6 ++++--
>  mm/compaction.c            | 29 ++++++++++++++++++++++++-----
>  mm/page_alloc.c            | 33 ++++++++++++++++++---------------
>  3 files changed, 46 insertions(+), 22 deletions(-)
> 
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index 01e3132..76f9beb 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -22,7 +22,8 @@ extern int sysctl_extfrag_handler(struct ctl_table *table, int write,
>  extern int fragmentation_index(struct zone *zone, unsigned int order);
>  extern unsigned long try_to_compact_pages(struct zonelist *zonelist,
>  			int order, gfp_t gfp_mask, nodemask_t *mask,
> -			enum migrate_mode mode, bool *contended);
> +			enum migrate_mode mode, bool *contended, bool *deferred,
> +			struct zone **candidate_zone);
>  extern void compact_pgdat(pg_data_t *pgdat, int order);
>  extern void reset_isolation_suitable(pg_data_t *pgdat);
>  extern unsigned long compaction_suitable(struct zone *zone, int order);
> @@ -91,7 +92,8 @@ static inline bool compaction_restarting(struct zone *zone, int order)
>  #else
>  static inline unsigned long try_to_compact_pages(struct zonelist *zonelist,
>  			int order, gfp_t gfp_mask, nodemask_t *nodemask,
> -			enum migrate_mode mode, bool *contended)
> +			enum migrate_mode mode, bool *contended, bool *deferred,
> +			struct zone **candidate_zone)
>  {
>  	return COMPACT_CONTINUE;
>  }
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 5175019..7c491d0 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1122,13 +1122,15 @@ int sysctl_extfrag_threshold = 500;
>   * @nodemask: The allowed nodes to allocate from
>   * @mode: The migration mode for async, sync light, or sync migration
>   * @contended: Return value that is true if compaction was aborted due to lock contention
> - * @page: Optionally capture a free page of the requested order during compaction
> + * @deferred: Return value that is true if compaction was deferred in all zones
> + * @candidate_zone: Return the zone where we think allocation should succeed
>   *
>   * This is the main entry point for direct page compaction.
>   */
>  unsigned long try_to_compact_pages(struct zonelist *zonelist,
>  			int order, gfp_t gfp_mask, nodemask_t *nodemask,
> -			enum migrate_mode mode, bool *contended)
> +			enum migrate_mode mode, bool *contended, bool *deferred,
> +			struct zone **candidate_zone)
>  {
>  	enum zone_type high_zoneidx = gfp_zone(gfp_mask);
>  	int may_enter_fs = gfp_mask & __GFP_FS;
> @@ -1142,8 +1144,7 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
>  	if (!order || !may_enter_fs || !may_perform_io)
>  		return rc;
>  
> -	count_compact_event(COMPACTSTALL);
> -
> +	*deferred = true;
>  #ifdef CONFIG_CMA
>  	if (allocflags_to_migratetype(gfp_mask) == MIGRATE_MOVABLE)
>  		alloc_flags |= ALLOC_CMA;
> @@ -1153,16 +1154,34 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
>  								nodemask) {
>  		int status;
>  
> +		if (compaction_deferred(zone, order))
> +			continue;
> +
> +		*deferred = false;
> +
>  		status = compact_zone_order(zone, order, gfp_mask, mode,
>  						contended);
>  		rc = max(status, rc);
>  
>  		/* If a normal allocation would succeed, stop compacting */
>  		if (zone_watermark_ok(zone, order, low_wmark_pages(zone), 0,
> -				      alloc_flags))
> +				      alloc_flags)) {
> +			*candidate_zone = zone;
>  			break;
> +		} else if (mode != MIGRATE_ASYNC) {
> +			/*
> +			 * We think that allocation won't succeed in this zone
> +			 * so we defer compaction there. If it ends up
> +			 * succeeding after all, it will be reset.
> +			 */
> +			defer_compaction(zone, order);
> +		}
>  	}
>  
> +	/* If at least one zone wasn't deferred, we count a compaction stall */
> +	if (!*deferred)
> +		count_compact_event(COMPACTSTALL);
> +
>  	return rc;
>  }
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ee92384..6593f79 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2238,18 +2238,17 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  	bool *contended_compaction, bool *deferred_compaction,
>  	unsigned long *did_some_progress)
>  {
> -	if (!order)
> -		return NULL;
> +	struct zone *last_compact_zone = NULL;
>  
> -	if (compaction_deferred(preferred_zone, order)) {
> -		*deferred_compaction = true;
> +	if (!order)
>  		return NULL;
> -	}
>  
>  	current->flags |= PF_MEMALLOC;
>  	*did_some_progress = try_to_compact_pages(zonelist, order, gfp_mask,
>  						nodemask, mode,
> -						contended_compaction);
> +						contended_compaction,
> +						deferred_compaction,
> +						&last_compact_zone);
>  	current->flags &= ~PF_MEMALLOC;
>  
>  	if (*did_some_progress != COMPACT_SKIPPED) {
> @@ -2263,27 +2262,31 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  				order, zonelist, high_zoneidx,
>  				alloc_flags & ~ALLOC_NO_WATERMARKS,
>  				preferred_zone, classzone_idx, migratetype);
> +
>  		if (page) {
> -			preferred_zone->compact_blockskip_flush = false;
> -			compaction_defer_reset(preferred_zone, order, true);
> +			struct zone *zone = page_zone(page);
> +
> +			zone->compact_blockskip_flush = false;
> +			compaction_defer_reset(zone, order, true);
>  			count_vm_event(COMPACTSUCCESS);
>  			return page;
>  		}
>  
>  		/*
> +		 * last_compact_zone is where try_to_compact_pages thought
> +		 * allocation should succeed, so it did not defer compaction.
> +		 * But now we know that it didn't succeed, so we do the defer.
> +		 */
> +		if (last_compact_zone && mode != MIGRATE_ASYNC)
> +			defer_compaction(last_compact_zone, order);
> +
> +		/*
>  		 * It's bad if compaction run occurs and fails.
>  		 * The most likely reason is that pages exist,
>  		 * but not enough to satisfy watermarks.
>  		 */
>  		count_vm_event(COMPACTFAIL);
>  
> -		/*
> -		 * As async compaction considers a subset of pageblocks, only
> -		 * defer if the failure was a sync compaction failure.
> -		 */
> -		if (mode != MIGRATE_ASYNC)
> -			defer_compaction(preferred_zone, order);
> -
>  		cond_resched();
>  	}
>  
> 


-- 
Thanks.
Zhang Yanfei
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