Date:   Mon, 29 Jun 2020 12:10:53 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan.kim@...il.com>,
        Michal Hocko <mhocko@...e.com>, kernel-team@....com,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH for v5.8 1/3] mm: workingset: age nonresident information
 alongside anonymous pages

On 6/16/20 8:16 AM, js1304@...il.com wrote:
> From: Johannes Weiner <hannes@...xchg.org>
> 
> After ("mm: workingset: let cache workingset challenge anon fix"), we

This could now be updated to:
After commit 34e58cac6d8f ("mm: workingset: let cache workingset challenge
anon"), we ...

> compare refault distances to active_file + anon. But age of the
> non-resident information is only driven by the file LRU. As a result,
> we may overestimate the recency of any incoming refaults and activate
> them too eagerly, causing unnecessary LRU churn in certain situations.
> 
> Make anon aging drive nonresident age as well to address that.

Fixes: 34e58cac6d8f ("mm: workingset: let cache workingset challenge anon")

> Reported-by: Joonsoo Kim <js1304@...il.com>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  include/linux/mmzone.h |  4 ++--
>  include/linux/swap.h   |  1 +
>  mm/vmscan.c            |  3 +++
>  mm/workingset.c        | 46 +++++++++++++++++++++++++++-------------------
>  4 files changed, 33 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index c4c37fd..f6f8849 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -257,8 +257,8 @@ struct lruvec {
>  	 */
>  	unsigned long			anon_cost;
>  	unsigned long			file_cost;
> -	/* Evictions & activations on the inactive file list */
> -	atomic_long_t			inactive_age;
> +	/* Non-resident age, driven by LRU movement */
> +	atomic_long_t			nonresident_age;
>  	/* Refaults at the time of last reclaim cycle */
>  	unsigned long			refaults;
>  	/* Various lruvec state flags (enum lruvec_flags) */
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 4c5974b..5b3216b 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -313,6 +313,7 @@ struct vma_swap_readahead {
>  };
>  
>  /* linux/mm/workingset.c */
> +void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages);
>  void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg);
>  void workingset_refault(struct page *page, void *shadow);
>  void workingset_activation(struct page *page);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b6d8432..749d239 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -904,6 +904,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page,
>  		__delete_from_swap_cache(page, swap);
>  		xa_unlock_irqrestore(&mapping->i_pages, flags);
>  		put_swap_page(page, swap);
> +		workingset_eviction(page, target_memcg);
>  	} else {
>  		void (*freepage)(struct page *);
>  		void *shadow = NULL;
> @@ -1884,6 +1885,8 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
>  				list_add(&page->lru, &pages_to_free);
>  		} else {
>  			nr_moved += nr_pages;
> +			if (PageActive(page))
> +				workingset_age_nonresident(lruvec, nr_pages);
>  		}
>  	}
>  
> diff --git a/mm/workingset.c b/mm/workingset.c
> index d481ea4..50b7937 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -156,8 +156,8 @@
>   *
>   *		Implementation
>   *
> - * For each node's file LRU lists, a counter for inactive evictions
> - * and activations is maintained (node->inactive_age).
> + * For each node's LRU lists, a counter for inactive evictions and
> + * activations is maintained (node->nonresident_age).
>   *
>   * On eviction, a snapshot of this counter (along with some bits to
>   * identify the node) is stored in the now empty page cache
> @@ -213,7 +213,17 @@ static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
>  	*workingsetp = workingset;
>  }
>  
> -static void advance_inactive_age(struct mem_cgroup *memcg, pg_data_t *pgdat)
> +/**
> + * workingset_age_nonresident - age non-resident entries as LRU ages
> + * @memcg: the lruvec that was aged
> + * @nr_pages: the number of pages to count
> + *
> + * As in-memory pages are aged, non-resident pages need to be aged as
> + * well, in order for the refault distances later on to be comparable
> + * to the in-memory dimensions. This function allows reclaim and LRU
> + * operations to drive the non-resident aging along in parallel.
> + */
> +void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages)
>  {
>  	/*
>  	 * Reclaiming a cgroup means reclaiming all its children in a
> @@ -227,11 +237,8 @@ static void advance_inactive_age(struct mem_cgroup *memcg, pg_data_t *pgdat)
>  	 * the root cgroup's, age as well.
>  	 */
>  	do {
> -		struct lruvec *lruvec;
> -
> -		lruvec = mem_cgroup_lruvec(memcg, pgdat);
> -		atomic_long_inc(&lruvec->inactive_age);
> -	} while (memcg && (memcg = parent_mem_cgroup(memcg)));
> +		atomic_long_add(nr_pages, &lruvec->nonresident_age);
> +	} while ((lruvec = parent_lruvec(lruvec)));
>  }
>  
>  /**
> @@ -254,12 +261,11 @@ void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg)
>  	VM_BUG_ON_PAGE(page_count(page), page);
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
>  
> -	advance_inactive_age(page_memcg(page), pgdat);
> -
>  	lruvec = mem_cgroup_lruvec(target_memcg, pgdat);
> +	workingset_age_nonresident(lruvec, hpage_nr_pages(page));
>  	/* XXX: target_memcg can be NULL, go through lruvec */
>  	memcgid = mem_cgroup_id(lruvec_memcg(lruvec));
> -	eviction = atomic_long_read(&lruvec->inactive_age);
> +	eviction = atomic_long_read(&lruvec->nonresident_age);
>  	return pack_shadow(memcgid, pgdat, eviction, PageWorkingset(page));
>  }
>  
> @@ -309,20 +315,20 @@ void workingset_refault(struct page *page, void *shadow)
>  	if (!mem_cgroup_disabled() && !eviction_memcg)
>  		goto out;
>  	eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
> -	refault = atomic_long_read(&eviction_lruvec->inactive_age);
> +	refault = atomic_long_read(&eviction_lruvec->nonresident_age);
>  
>  	/*
>  	 * Calculate the refault distance
>  	 *
>  	 * The unsigned subtraction here gives an accurate distance
> -	 * across inactive_age overflows in most cases. There is a
> +	 * across nonresident_age overflows in most cases. There is a
>  	 * special case: usually, shadow entries have a short lifetime
>  	 * and are either refaulted or reclaimed along with the inode
>  	 * before they get too old.  But it is not impossible for the
> -	 * inactive_age to lap a shadow entry in the field, which can
> -	 * then result in a false small refault distance, leading to a
> -	 * false activation should this old entry actually refault
> -	 * again.  However, earlier kernels used to deactivate
> +	 * nonresident_age to lap a shadow entry in the field, which
> +	 * can then result in a false small refault distance, leading
> +	 * to a false activation should this old entry actually
> +	 * refault again.  However, earlier kernels used to deactivate
>  	 * unconditionally with *every* reclaim invocation for the
>  	 * longest time, so the occasional inappropriate activation
>  	 * leading to pressure on the active list is not a problem.
> @@ -359,7 +365,7 @@ void workingset_refault(struct page *page, void *shadow)
>  		goto out;
>  
>  	SetPageActive(page);
> -	advance_inactive_age(memcg, pgdat);
> +	workingset_age_nonresident(lruvec, hpage_nr_pages(page));
>  	inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE);
>  
>  	/* Page was active prior to eviction */
> @@ -382,6 +388,7 @@ void workingset_refault(struct page *page, void *shadow)
>  void workingset_activation(struct page *page)
>  {
>  	struct mem_cgroup *memcg;
> +	struct lruvec *lruvec;
>  
>  	rcu_read_lock();
>  	/*
> @@ -394,7 +401,8 @@ void workingset_activation(struct page *page)
>  	memcg = page_memcg_rcu(page);
>  	if (!mem_cgroup_disabled() && !memcg)
>  		goto out;
> -	advance_inactive_age(memcg, page_pgdat(page));
> +	lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
> +	workingset_age_nonresident(lruvec, hpage_nr_pages(page));
>  out:
>  	rcu_read_unlock();
>  }
> 

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