linux-kernel - Re: [PATCH] mm: workingset: ignore slab memory size when calculating shadows pressure

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Message-Id: <20200903211059.7dc9530e6d988eaeefe53cf7@linux-foundation.org>
Date:   Thu, 3 Sep 2020 21:10:59 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Roman Gushchin <guro@...com>
Cc:     <linux-mm@...ck.org>, =Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, <kernel-team@...com>,
        <linux-kernel@...r.kernel.org>, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm: workingset: ignore slab memory size when
 calculating shadows pressure

On Thu, 3 Sep 2020 16:00:55 -0700 Roman Gushchin <guro@...com> wrote:

> In the memcg case count_shadow_nodes() sums the number of pages in lru
> lists and the amount of slab memory (reclaimable and non-reclaimable)
> as a baseline for the allowed number of shadow entries.
> 
> It seems to be a good analogy for the !memcg case, where
> node_present_pages() is used. However, it's not quite true, as there
> two problems:
> 
> 1) Due to slab reparenting introduced by commit fb2f2b0adb98 ("mm:
> memcg/slab: reparent memcg kmem_caches on cgroup removal") local
> per-lruvec slab counters might be inaccurate on non-leaf levels.
> It's the only place where local slab counters are used.
> 
> 2) Shadow nodes by themselves are backed by slabs. So there is a loop
> dependency: the more shadow entries are there, the less pressure the
> kernel applies to reclaim them.
> 
> Fortunately, there is a simple way to solve both problems: slab
> counters shouldn't be taken into the account by count_shadow_nodes().
> 
> ...
>
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -495,10 +495,6 @@ static unsigned long count_shadow_nodes(struct shrinker *shrinker,
>  		for (pages = 0, i = 0; i < NR_LRU_LISTS; i++)
>  			pages += lruvec_page_state_local(lruvec,
>  							 NR_LRU_BASE + i);
> -		pages += lruvec_page_state_local(
> -			lruvec, NR_SLAB_RECLAIMABLE_B) >> PAGE_SHIFT;
> -		pages += lruvec_page_state_local(
> -			lruvec, NR_SLAB_UNRECLAIMABLE_B) >> PAGE_SHIFT;
>  	} else
>  #endif
>  		pages = node_present_pages(sc->nid);

Did this have any observable runtime effects?