linux-kernel - Re: [PATCH v4 8/8] mm/swap: count a new anonymous page as a reclaim

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Message-ID: <20200323174257.GF204561@cmpxchg.org>
Date:   Mon, 23 Mar 2020 13:42:57 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     js1304@...il.com
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>, kernel-team@....com,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v4 8/8] mm/swap: count a new anonymous page as a
 reclaim_state's rotate

On Mon, Mar 23, 2020 at 02:52:12PM +0900, js1304@...il.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> reclaim_stat's rotate is used for controlling the ratio of scanning page
> 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 cause
> 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 regression
> 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 completely
> 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.

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/