linux-kernel - Re: [patch 1/2] mm, compaction: kcompactd should not ignore pageblock skip

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Message-ID: <5d578461-0982-f719-3a04-b2f3552dc7cc@suse.cz>
Date:   Wed, 23 Aug 2017 10:23:49 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [patch 1/2] mm, compaction: kcompactd should not ignore pageblock
 skip

On 08/16/2017 01:39 AM, David Rientjes wrote:
> Kcompactd is needlessly ignoring pageblock skip information.  It is doing
> MIGRATE_SYNC_LIGHT compaction, which is no more powerful than
> MIGRATE_SYNC compaction.
> 
> If compaction recently failed to isolate memory from a set of pageblocks,
> there is nothing to indicate that kcompactd will be able to do so, or
> that it is beneficial from attempting to isolate memory.
> 
> Use the pageblock skip hint to avoid rescanning pageblocks needlessly
> until that information is reset.
> 
> Signed-off-by: David Rientjes <rientjes@...gle.com>

It would be much better if patches like this were accompanied by some
numbers.

Also there's now a danger that in cases where there's no direct
compaction happening (just kcompactd), nothing will ever call
__reset_isolation_suitable().

> ---
>  mm/compaction.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1927,9 +1927,8 @@ static void kcompactd_do_work(pg_data_t *pgdat)
>  		.total_free_scanned = 0,
>  		.classzone_idx = pgdat->kcompactd_classzone_idx,
>  		.mode = MIGRATE_SYNC_LIGHT,
> -		.ignore_skip_hint = true,
> +		.ignore_skip_hint = false,
>  		.gfp_mask = GFP_KERNEL,
> -
>  	};
>  	trace_mm_compaction_kcompactd_wake(pgdat->node_id, cc.order,
>  							cc.classzone_idx);
>