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Subject: Re: [PATCH V3] mm/compaction: correct deferral logic for proactive
 compaction
To:     David Rientjes <rientjes@google.com>,
        Vlastimil Babka <vbabka@suse.cz>
Cc:     akpm@linux-foundation.org, mhocko@suse.com, khalid.aziz@oracle.com,
        ngupta@nitingupta.dev, vinmenon@codeaurora.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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From:   Charan Teja Kalla <charante@codeaurora.org>
Message-ID: <7ce8f4f1-0d83-bb85-e516-33071d85ad3a@codeaurora.org>
Date:   Wed, 27 Jan 2021 21:17:58 +0530
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On 1/25/2021 4:24 AM, David Rientjes wrote:
> On Wed, 20 Jan 2021, Vlastimil Babka wrote:
> 
>> 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.
>>
> 
> Charan, is this something you'd like to follow up on, or should I take a 
> look instead?
> 

Sure David. Happy to follow up on this. Thanks!

> Thanks!
> 

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