linux-kernel - Re: [RFC PATCH 2/3] mm, compaction: use MIN_COMPACT_COSTLY

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Date:   Thu, 1 Aug 2019 13:33:02 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Hillf Danton <hdanton@...a.com>, Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 2/3] mm, compaction: use MIN_COMPACT_COSTLY_PRIORITY
 everywhere for costly orders

On 8/1/19 6:01 AM, Vlastimil Babka wrote:
> Could you try testing the patch below instead? It should hopefully
> eliminate the stalls. If it makes hugepage allocation give up too early,
> we'll know we have to involve __GFP_RETRY_MAYFAIL in allowing the
> MIN_COMPACT_PRIORITY priority. Thanks!

Thanks.  This patch does eliminate the stalls I was seeing.

In my testing, there is little difference in how many hugetlb pages are
allocated.  It does not appear to be giving up/failing too early.  But,
this is only with __GFP_RETRY_MAYFAIL.  The real concern would with THP
requests.  Any suggestions on how to test that?

-- 
Mike Kravetz

> ----8<----
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index 9569e7c786d3..b8bfe8d5d2e9 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -129,11 +129,7 @@ static inline bool compaction_failed(enum compact_result result)
>  	return false;
>  }
>  
> -/*
> - * Compaction  has backed off for some reason. It might be throttling or
> - * lock contention. Retrying is still worthwhile.
> - */
> -static inline bool compaction_withdrawn(enum compact_result result)
> +static inline bool compaction_needs_reclaim(enum compact_result result)
>  {
>  	/*
>  	 * Compaction backed off due to watermark checks for order-0
> @@ -142,6 +138,15 @@ static inline bool compaction_withdrawn(enum compact_result result)
>  	if (result == COMPACT_SKIPPED)
>  		return true;
>  
> +	return false;
> +}
> +
> +/*
> + * Compaction  has backed off for some reason. It might be throttling or
> + * lock contention. Retrying is still worthwhile.
> + */
> +static inline bool compaction_withdrawn(enum compact_result result)
> +{
>  	/*
>  	 * If compaction is deferred for high-order allocations, it is
>  	 * because sync compaction recently failed. If this is the case
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 272c6de1bf4e..3dfce1f79112 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3965,6 +3965,11 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  	if (compaction_failed(compact_result))
>  		goto check_priority;
>  
> +	if (compaction_needs_reclaim(compact_result)) {
> +		ret = compaction_zonelist_suitable(ac, order, alloc_flags);
> +		goto out;
> +	}
> +
>  	/*
>  	 * make sure the compaction wasn't deferred or didn't bail out early
>  	 * due to locks contention before we declare that we should give up.
> @@ -3972,8 +3977,7 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  	 * compaction.
>  	 */
>  	if (compaction_withdrawn(compact_result)) {
> -		ret = compaction_zonelist_suitable(ac, order, alloc_flags);
> -		goto out;
> +		goto check_priority;
>  	}
>  
>  	/*
>