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Subject: Re: [PATCH V3] mm/compaction: correct deferral logic for proactive
 compaction
To:     Charan Teja Reddy <charante@codeaurora.org>,
        akpm@linux-foundation.org, mhocko@suse.com, khalid.aziz@oracle.com,
        ngupta@nitingupta.dev, vinmenon@codeaurora.org
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <1610989938-31374-1-git-send-email-charante@codeaurora.org>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <ae24ee80-e540-b4dd-616e-2f3e38a52e26@suse.cz>
Date:   Mon, 25 Jan 2021 16:56:58 +0100
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On 1/18/21 6:12 PM, Charan Teja Reddy wrote:
> should_proactive_compact_node() returns true when sum of the
> weighted fragmentation score of all the zones in the node is greater
> than the wmark_high of compaction, which then triggers the proactive
> compaction that operates on the individual zones of the node. But
> proactive compaction runs on the zone only when its weighted
> fragmentation score is greater than wmark_low(=wmark_high - 10).
> 
> This means that the sum of the weighted fragmentation scores of all the
> zones can exceed the wmark_high but individual weighted fragmentation
> zone scores can still be less than wmark_low which makes the unnecessary
> trigger of the proactive compaction only to return doing nothing.
> 
> Issue with the return of proactive compaction with out even trying is
> its deferral. It is simply deferred for 1 << COMPACT_MAX_DEFER_SHIFT if
> the scores across the proactive compaction is same, thinking that
> compaction didn't make any progress but in reality it didn't even try.
> With the delay between successive retries for proactive compaction is
> 500msec, it can result into the deferral for ~30sec with out even trying
> the proactive compaction.
> 
> Test scenario is that: compaction_proactiveness=50 thus the wmark_low =
> 50 and wmark_high = 60. System have 2 zones(Normal and Movable) with
> sizes 5GB and 6GB respectively. After opening some apps on the android,
> the weighted fragmentation scores of these zones are 47 and 49
> respectively. Since the sum of these fragmentation scores are above the
> wmark_high which triggers the proactive compaction and there since the
> individual zones weighted fragmentation scores are below wmark_low, it
> returns without trying the proactive compaction. As a result the
> weighted fragmentation scores of the zones are still 47 and 49 which
> makes the existing logic to defer the compaction thinking that
> noprogress is made across the compaction.
> 
> Fix this by checking just zone fragmentation score, not the weighted, in
> __compact_finished() and use the zones weighted fragmentation score in
> fragmentation_score_node(). In the test case above, If the weighted
> average of is above wmark_high, then individual score (not adjusted) of
> atleast one zone has to be above wmark_high. Thus it avoids the
> unnecessary trigger and deferrals of the proactive compaction.
> 
> Fix-suggested-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
> ---
> 
> Changes in V3: Addressed suggestions from Vlastimil
> 
> Changes in V2: https://lore.kernel.org/patchwork/patch/1366862/
> 
> Changes in V1: https://lore.kernel.org/patchwork/patch/1364646/

Andrew, I've noticed that v1 is still in mmotm [1] together with v3, which
doesn't make sense together, please drop [1]. Thanks

[1]
https://www.ozlabs.org/~akpm/mmots/broken-out/mm-compaction-return-proper-state-in-should_proactive_compact_node.patch

>  mm/compaction.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e5acb97..ccddb3a 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1925,20 +1925,28 @@ static bool kswapd_is_running(pg_data_t *pgdat)
>  
>  /*
>   * A zone's fragmentation score is the external fragmentation wrt to the
> - * COMPACTION_HPAGE_ORDER scaled by the zone's size. It returns a value
> - * in the range [0, 100].
> + * COMPACTION_HPAGE_ORDER. It returns a value in the range [0, 100].
> + */
> +static unsigned int fragmentation_score_zone(struct zone *zone)
> +{
> +	return extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
> +}
> +
> +/*
> + * A weighted zone's fragmentation score is the external fragmentation
> + * wrt to the COMPACTION_HPAGE_ORDER scaled by the zone's size. It
> + * returns a value in the range [0, 100].
>   *
>   * The scaling factor ensures that proactive compaction focuses on larger
>   * zones like ZONE_NORMAL, rather than smaller, specialized zones like
>   * ZONE_DMA32. For smaller zones, the score value remains close to zero,
>   * and thus never exceeds the high threshold for proactive compaction.
>   */
> -static unsigned int fragmentation_score_zone(struct zone *zone)
> +static unsigned int fragmentation_score_zone_weighted(struct zone *zone)
>  {
>  	unsigned long score;
>  
> -	score = zone->present_pages *
> -			extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
> +	score = zone->present_pages * fragmentation_score_zone(zone);
>  	return div64_ul(score, zone->zone_pgdat->node_present_pages + 1);
>  }
>  
> @@ -1958,7 +1966,7 @@ static unsigned int fragmentation_score_node(pg_data_t *pgdat)
>  		struct zone *zone;
>  
>  		zone = &pgdat->node_zones[zoneid];
> -		score += fragmentation_score_zone(zone);
> +		score += fragmentation_score_zone_weighted(zone);
>  	}
>  
>  	return score;
>