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

[David Rientjes <rientjes@google.com>]

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?

Thanks!