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From:   Joonsoo Kim <js1304@gmail.com>
Date:   Tue, 24 Mar 2020 15:28:36 +0900
Message-ID: <CAAmzW4Mts-EbsYGWbdJyAR01dsoVNR4OYbRDCxfp_eD_p4fD0w@mail.gmail.com>
Subject: Re: [PATCH v4 8/8] mm/swap: count a new anonymous page as a
 reclaim_state's rotate
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        Hugh Dickins <hughd@google.com>,
        Minchan Kim <minchan@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>, kernel-team@lge.com,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
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2020=EB=85=84 3=EC=9B=94 24=EC=9D=BC (=ED=99=94) =EC=98=A4=EC=A0=84 2:43, J=
ohannes Weiner <hannes@cmpxchg.org>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:
>
> On Mon, Mar 23, 2020 at 02:52:12PM +0900, js1304@gmail.com wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >
> > reclaim_stat's rotate is used for controlling the ratio of scanning pag=
e
> > between file and anonymous LRU. All new anonymous pages are counted
> > for rotate before the patch, protecting anonymous pages on active LRU, =
and,
> > it makes that reclaim on anonymous LRU is less happened than file LRU.
> >
> > Now, situation is changed. all new anonymous pages are not added
> > to the active LRU so rotate would be far less than before. It will caus=
e
> > that reclaim on anonymous LRU happens more and it would result in bad
> > effect on some system that is optimized for previous setting.
> >
> > Therefore, this patch counts a new anonymous page as a reclaim_state's
> > rotate. Although it is non-logical to add this count to
> > the reclaim_state's rotate in current algorithm, reducing the regressio=
n
> > would be more important.
> >
> > I found this regression on kernel-build test and it is roughly 2~5%
> > performance degradation. With this workaround, performance is completel=
y
> > restored.
> >
> > v2: fix a bug that reuses the rotate value for previous page
>
> I agree with the rationale, but the magic bit in the page->lru list
> pointers seems pretty ugly.

Yes, pretty ugly.

> I wrote a patch a few years ago that split lru_add_pvecs into an add
> and a putback component. This was to avoid unintentional balancing
> effects of LRU isolations, but I think you can benefit from that
> cleanup here as well. Would you mind taking a look at it and maybe
> take it up into your series?
>
> https://lore.kernel.org/patchwork/patch/685708/

Thanks for pointing that.
I will remove the magic bit and imitate your patch to solve the problem
of this patch.

Thanks.