Date:   Mon, 18 Jan 2021 18:11:01 +0530
From:   Charan Teja Kalla <charante@...eaurora.org>
To:     Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org,
        mhocko@...e.com, khalid.aziz@...cle.com, ngupta@...ingupta.dev,
        vinmenon@...eaurora.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] mm/compaction: correct deferral logic for proactive
 compaction

Thanks Vlasitmil!!

On 1/18/2021 6:07 PM, Vlastimil Babka wrote:
> On 1/18/21 1:20 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@...e.cz>
>> Signed-off-by: Charan Teja Reddy <charante@...eaurora.org>
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 
> But I would move fragmentation_score_zone() above
> fragmentation_score_zone_weighted(), so fragmentation_score_zone_weighted() can
> call fragmentation_score_zone() instead of having two places with
> extfrag_for_order(...).

Yes, this suggestion makes the code cleaner. I will raise V3 for this.
Thanks.

> 
> Thanks.
> 
>> ---
>>
>> Changes in V2: Addressed comments from vlastimil
>>
>> Changes in V1: https://lore.kernel.org/patchwork/patch/1364646/
>>
>>  mm/compaction.c | 19 ++++++++++++++-----
>>  1 file changed, 14 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index e5acb97..1b98427 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1924,16 +1924,16 @@ 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].
>> + * 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;
>>  
>> @@ -1943,6 +1943,15 @@ static unsigned int fragmentation_score_zone(struct zone *zone)
>>  }
>>  
>>  /*
>> + * A zone's fragmentation score is the external fragmentation wrt to the
>> + * 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);
>> +}
>> +
>> +/*
>>   * The per-node proactive (background) compaction process is started by its
>>   * corresponding kcompactd thread when the node's fragmentation score
>>   * exceeds the high threshold. The compaction process remains active till
>> @@ -1958,7 +1967,7 @@ 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;
>>
> 

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