linux-kernel - Re: [PATCH 5/5] mm/vmscan: don't forcely shrink active anon lru list

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Date:   Fri, 22 Feb 2019 13:22:49 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Rik van Riel <riel@...riel.com>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 5/5] mm/vmscan: don't forcely shrink active anon lru list

On Fri, Feb 22, 2019 at 08:43:37PM +0300, Andrey Ryabinin wrote:
> shrink_node_memcg() always forcely shrink active anon list.
> This doesn't seem like correct behavior. If system/memcg has no swap, it's
> absolutely pointless to rebalance anon lru lists.
> And in case we did scan the active anon list above, it's unclear why would
> we need this additional force scan. If there are cases when we want more
> aggressive scan of the anon lru we should just change the scan target
> in get_scan_count() (and better explain such cases in the comments).
> 
> Remove this force shrink and let get_scan_count() to decide how
> much of active anon we want to shrink.

This change breaks the anon pre-aging.

The idea behind this is that the VM maintains a small batch of anon
reclaim candidates with recent access information. On every reclaim,
even when we just trim cache, which is the most common reclaim mode,
but also when we just swapped out some pages and shrunk the inactive
anon list, at the end of it we make sure that the list of potential
anon candidates is refilled for the next reclaim cycle.

The comments for this are above inactive_list_is_low() and the
age_active_anon() call from kswapd.

Re: no swap, you are correct. We should gate that rebalancing on
total_swap_pages, just like age_active_anon() does.