linux-kernel - Re: [patch 1/2] mm, compaction: kcompactd should not ignore pageblock skip

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Date:   Mon, 11 Sep 2017 14:13:36 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [patch 1/2] mm, compaction: kcompactd should not ignore pageblock
 skip

On Mon, 11 Sep 2017, Vlastimil Babka wrote:

> > A follow-up change will set the pageblock skip for this memory since it is 
> > never useful for either scanner.
> > """
> > 
> >> Also there's now a danger that in cases where there's no direct
> >> compaction happening (just kcompactd), nothing will ever call
> >> __reset_isolation_suitable().
> >>
> > 
> > I'm not sure that is helpful in a context where no high-order memory can 
> > call direct compaction that kcompactd needlessly scanning the same memory 
> > over and over is beneficial.
> 
> The point is that if it becomes beneficial again, we won't know as there
> will be still be skip bits.
> 

Why is kcompactd_do_work() not sometimes doing 
__reset_isolation_suitable() in the first place, if only to reset the 
per-zone migration and freeing scanner cached pfns?  It seems fragile to 
rely on other threads doing direct compaction to reset the per-zone state 
of compaction.