Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To:     Yu Zhao <yuzhao@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andi Kleen <ak@linux.intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
        Jesse Barnes <jsbarnes@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Michael Larabel <Michael@michaellarabel.com>,
        Michal Hocko <mhocko@kernel.org>,
        Mike Rapoport <rppt@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Will Deacon <will@kernel.org>,
        Ying Huang <ying.huang@intel.com>,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        page-reclaim@google.com, x86@kernel.org,
        Yu Zhao <yuzhao@google.com>, Brian Geffon <bgeffon@google.com>,
        Jan Alexander Steffens <heftig@archlinux.org>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Steven Barrett <steven@liquorix.net>,
        Suleiman Souhlal <suleiman@google.com>,
        Daniel Byrne <djbyrne@mtu.edu>,
        Donald Carr <d@chaos-reins.com>,
        Holger =?utf-8?Q?Hoffst=C3=A4tte?= 
        <holger@applied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@yandex.ru>,
        Shuang Zhai <szhai2@cs.rochester.edu>,
        Sofia Trinh <sofia.trinh@edi.works>,
        Vaibhav Jain <vaibhav@linux.ibm.com>
Subject: Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation
In-Reply-To: <20220309021230.721028-7-yuzhao@google.com>
References: <20220309021230.721028-1-yuzhao@google.com>
 <20220309021230.721028-7-yuzhao@google.com>
Date:   Mon, 21 Mar 2022 18:21:41 +0530
Message-ID: <87czif79k2.fsf@linux.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

 +
> +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;
> +}
> +