Re: [PATCH 12/14] mm, oom: protect !costly allocations some more

[Joonsoo Kim]

On Wed, Apr 20, 2016 at 03:47:25PM -0400, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> should_reclaim_retry will give up retries for higher order allocations
> if none of the eligible zones has any requested or higher order pages
> available even if we pass the watermak check for order-0. This is done
> because there is no guarantee that the reclaimable and currently free
> pages will form the required order.
> 
> This can, however, lead to situations were the high-order request (e.g.
> order-2 required for the stack allocation during fork) will trigger
> OOM too early - e.g. after the first reclaim/compaction round. Such a
> system would have to be highly fragmented and there is no guarantee
> further reclaim/compaction attempts would help but at least make sure
> that the compaction was active before we go OOM and keep retrying even
> if should_reclaim_retry tells us to oom if
> 	- the last compaction round backed off or
> 	- we haven't completed at least MAX_COMPACT_RETRIES active
> 	  compaction rounds.
> 
> The first rule ensures that the very last attempt for compaction
> was not ignored while the second guarantees that the compaction has done
> some work. Multiple retries might be needed to prevent occasional
> pigggy backing of other contexts to steal the compacted pages before
> the current context manages to retry to allocate them.
> 
> compaction_failed() is taken as a final word from the compaction that
> the retry doesn't make much sense. We have to be careful though because
> the first compaction round is MIGRATE_ASYNC which is rather weak as it
> ignores pages under writeback and gives up too easily in other
> situations. We therefore have to make sure that MIGRATE_SYNC_LIGHT mode
> has been used before we give up. With this logic in place we do not have
> to increase the migration mode unconditionally and rather do it only if
> the compaction failed for the weaker mode. A nice side effect is that
> the stronger migration mode is used only when really needed so this has
> a potential of smaller latencies in some cases.
> 
> Please note that the compaction doesn't tell us much about how
> successful it was when returning compaction_made_progress so we just
> have to blindly trust that another retry is worthwhile and cap the
> number to something reasonable to guarantee a convergence.
> 
> If the given number of successful retries is not sufficient for a
> reasonable workloads we should focus on the collected compaction
> tracepoints data and try to address the issue in the compaction code.
> If this is not feasible we can increase the retries limit.
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  mm/page_alloc.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 77 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3b78936eca70..bb4df1be0d43 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2939,6 +2939,13 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
>  	return page;
>  }
>  
> +
> +/*
> + * Maximum number of compaction retries wit a progress before OOM
> + * killer is consider as the only way to move forward.
> + */
> +#define MAX_COMPACT_RETRIES 16
> +
>  #ifdef CONFIG_COMPACTION
>  /* Try memory compaction for high-order allocations before reclaim */
>  static struct page *
> @@ -3006,6 +3013,43 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  
>  	return NULL;
>  }
> +
> +static inline bool
> +should_compact_retry(unsigned int order, enum compact_result compact_result,
> +		     enum migrate_mode *migrate_mode,
> +		     int compaction_retries)
> +{
> +	if (!order)
> +		return false;
> +
> +	/*
> +	 * compaction considers all the zone as desperately out of memory
> +	 * so it doesn't really make much sense to retry except when the
> +	 * failure could be caused by weak migration mode.
> +	 */
> +	if (compaction_failed(compact_result)) {

IIUC, this compaction_failed() means that at least one zone is
compacted and failed. This is not same with your assumption in the
comment. If compaction is done and failed on ZONE_DMA, it would be
premature decision.

> +		if (*migrate_mode == MIGRATE_ASYNC) {
> +			*migrate_mode = MIGRATE_SYNC_LIGHT;
> +			return true;
> +		}
> +		return false;
> +	}
> +
> +	/*
> +	 * !costly allocations are really important and we have to make sure
> +	 * the compaction wasn't deferred or didn't bail out early due to locks
> +	 * contention before we go OOM. Still cap the reclaim retry loops with
> +	 * progress to prevent from looping forever and potential trashing.
> +	 */
> +	if (order <= PAGE_ALLOC_COSTLY_ORDER) {
> +		if (compaction_withdrawn(compact_result))
> +			return true;
> +		if (compaction_retries <= MAX_COMPACT_RETRIES)
> +			return true;
> +	}
> +
> +	return false;
> +}
>  #else
>  static inline struct page *
>  __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> @@ -3014,6 +3058,14 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  {
>  	return NULL;
>  }
> +
> +static inline bool
> +should_compact_retry(unsigned int order, enum compact_result compact_result,
> +		     enum migrate_mode *migrate_mode,
> +		     int compaction_retries)
> +{
> +	return false;
> +}
>  #endif /* CONFIG_COMPACTION */
>  
>  /* Perform direct synchronous page reclaim */
> @@ -3260,6 +3312,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	unsigned long did_some_progress;
>  	enum migrate_mode migration_mode = MIGRATE_ASYNC;
>  	enum compact_result compact_result;
> +	int compaction_retries = 0;
>  	int no_progress_loops = 0;
>  
>  	/*
> @@ -3371,13 +3424,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  			 compaction_failed(compact_result)))
>  		goto nopage;
>  
> -	/*
> -	 * It can become very expensive to allocate transparent hugepages at
> -	 * fault, so use asynchronous memory compaction for THP unless it is
> -	 * khugepaged trying to collapse.
> -	 */
> -	if (!is_thp_gfp_mask(gfp_mask) || (current->flags & PF_KTHREAD))
> -		migration_mode = MIGRATE_SYNC_LIGHT;
> +	if (order && compaction_made_progress(compact_result))
> +		compaction_retries++;
>  
>  	/* Try direct reclaim and then allocating */
>  	page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac,
> @@ -3408,6 +3456,17 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  				 no_progress_loops))
>  		goto retry;
>  
> +	/*
> +	 * It doesn't make any sense to retry for the compaction if the order-0
> +	 * reclaim is not able to make any progress because the current
> +	 * implementation of the compaction depends on the sufficient amount
> +	 * of free memory (see __compaction_suitable)
> +	 */
> +	if (did_some_progress > 0 &&
> +			should_compact_retry(order, compact_result,
> +				&migration_mode, compaction_retries))

Checking did_some_progress on each round have subtle corner case. Think
about following situation.

round, compaction, did_some_progress, compaction
0, defer, 1
0, defer, 1
0, defer, 1
0, defer, 1
0, defer, 0

In this case, compaction has enough chance to succeed since freepages
increase, but, compaction will not be triggered.

Thanks.

> +		goto retry;
> +
>  	/* Reclaim has failed us, start killing things */
>  	page = __alloc_pages_may_oom(gfp_mask, order, ac, &did_some_progress);
>  	if (page)
> @@ -3421,10 +3480,18 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  
>  noretry:
>  	/*
> -	 * High-order allocations do not necessarily loop after
> -	 * direct reclaim and reclaim/compaction depends on compaction
> -	 * being called after reclaim so call directly if necessary
> +	 * High-order allocations do not necessarily loop after direct reclaim
> +	 * and reclaim/compaction depends on compaction being called after
> +	 * reclaim so call directly if necessary.
> +	 * It can become very expensive to allocate transparent hugepages at
> +	 * fault, so use asynchronous memory compaction for THP unless it is
> +	 * khugepaged trying to collapse. All other requests should tolerate
> +	 * at least light sync migration.
>  	 */
> +	if (is_thp_gfp_mask(gfp_mask) && !(current->flags & PF_KTHREAD))
> +		migration_mode = MIGRATE_ASYNC;
> +	else
> +		migration_mode = MIGRATE_SYNC_LIGHT;
>  	page = __alloc_pages_direct_compact(gfp_mask, order, alloc_flags,
>  					    ac, migration_mode,
>  					    &compact_result);
> -- 
> 2.8.0.rc3
> 
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