linux-kernel - Re: [PATCH V3] mm/compaction: correct deferral logic for proactive compaction

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Message-ID: <80a1a433-c520-4c73-61ce-55cf33739fc5@suse.cz>
Date:   Wed, 20 Jan 2021 12:04:45 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Rientjes <rientjes@...gle.com>,
        Charan Teja Reddy <charante@...eaurora.org>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com, khalid.aziz@...cle.com,
        ngupta@...ingupta.dev, vinmenon@...eaurora.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3] mm/compaction: correct deferral logic for proactive
 compaction

On 1/19/21 8:26 PM, David Rientjes wrote:
> On Mon, 18 Jan 2021, 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.
> 
> Isn't this an issue in deferred compaction as well?  It seems like 
> deferred compaction should check that work was actually performed before 
> deferring subsequent calls to compaction.

Direct compaction does, proactive not.

> In other words, I don't believe deferred compaction is intended to avoid 
> checks to determine if compaction is worth it; it should only defer 
> *additional* work that was not productive.

Yeah, that should be more optimal.

> Thoughts?
>