linux-kernel - Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation

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Message-ID: <87czif79k2.fsf@linux.ibm.com>
Date:   Mon, 21 Mar 2022 18:21:41 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Yu Zhao <yuzhao@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        page-reclaim@...gle.com, x86@...nel.org,
        Yu Zhao <yuzhao@...gle.com>, Brian Geffon <bgeffon@...gle.com>,
        Jan Alexander Steffens <heftig@...hlinux.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Steven Barrett <steven@...uorix.net>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Daniel Byrne <djbyrne@....edu>,
        Donald Carr <d@...os-reins.com>,
        Holger Hoffstätte 
        <holger@...lied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>,
        Shuang Zhai <szhai2@...rochester.edu>,
        Sofia Trinh <sofia.trinh@....works>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation

 +
> +static long get_nr_evictable(struct lruvec *lruvec, unsigned long max_seq,
> +			     unsigned long *min_seq, bool can_swap, bool *need_aging)
> +{
> +	int gen, type, zone;
> +	long old = 0;
> +	long young = 0;
> +	long total = 0;
> +	struct lru_gen_struct *lrugen = &lruvec->lrugen;
> +
> +	for (type = !can_swap; type < ANON_AND_FILE; type++) {
> +		unsigned long seq;
> +
> +		for (seq = min_seq[type]; seq <= max_seq; seq++) {
> +			long size = 0;
> +
> +			gen = lru_gen_from_seq(seq);
> +
> +			for (zone = 0; zone < MAX_NR_ZONES; zone++)
> +				size += READ_ONCE(lrugen->nr_pages[gen][type][zone]);
> +
> +			total += size;
> +			if (seq == max_seq)
> +				young += size;
> +			if (seq + MIN_NR_GENS == max_seq)
> +				old += size;
> +		}
> +	}
> +
> +	/* try to spread pages out across MIN_NR_GENS+1 generations */
> +	if (min_seq[LRU_GEN_FILE] + MIN_NR_GENS > max_seq)
> +		*need_aging = true;
> +	else if (min_seq[LRU_GEN_FILE] + MIN_NR_GENS < max_seq)
> +		*need_aging = false;

Can you explain/document the reason for the considering the below
conditions for ageing? 

> +	else if (young * MIN_NR_GENS > total)
> +		*need_aging = true;

Are we trying to consdier the case of more than half the total pages
young as needing ageing? If so should MIN_NR_GENS be 2 instead of using
that #define? Or 

> +	else if (old * (MIN_NR_GENS + 2) < total)
> +		*need_aging = true;

What is the significance of '+ 2' ? 

> +	else
> +		*need_aging = false;
> +
> +	return total > 0 ? total : 0;
> +}
> +