linux-kernel - Re: [PATCH v7 09/12] mm: multigenerational LRU: runtime switch

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Date:   Tue, 8 Feb 2022 01:42:28 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Barry Song <21cnbao@...il.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>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michael Larabel <Michael@...haellarabel.com>,
        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,
        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>
Subject: Re: [PATCH v7 09/12] mm: multigenerational LRU: runtime switch

On Tue, Feb 08, 2022 at 01:18:59AM -0700, Yu Zhao wrote:
> Add /sys/kernel/mm/lru_gen/enabled as a runtime switch. Features that
> can be enabled or disabled include:
>   0x0001: the multigenerational LRU
>   0x0002: the page table walks, when arch_has_hw_pte_young() returns
>           true
>   0x0004: the use of the accessed bit in non-leaf PMD entries, when
>           CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG=y
>   [yYnN]: apply to all the features above
> E.g.,
>   echo y >/sys/kernel/mm/lru_gen/enabled
>   cat /sys/kernel/mm/lru_gen/enabled
>   0x0007
>   echo 5 >/sys/kernel/mm/lru_gen/enabled
>   cat /sys/kernel/mm/lru_gen/enabled
>   0x0005
> 
> NB: the page table walks happen on the scale of seconds under heavy
> memory pressure. Under such a condition, the mmap_lock contention is a
> lesser concern, compared with the LRU lock contention and the I/O
> congestion. So far the only well-known case of the mmap_lock
> contention is Android, due to Scudo [1] which allocates several
> thousand VMAs for merely a few hundred MBs. The SPF and the Maple Tree
> also have provided their own assessments [2][3]. However, if the page
> table walks do worsen the mmap_lock contention, the runtime switch can
> be used to disable this feature. In this case the multigenerational
> LRU will suffer a minor performance degradation, as shown previously.

Clarified the potential impact from the mmap_lock contention as
requested here:
https://lore.kernel.org/linux-mm/YdwQcl6D5Mbp9Z4h@dhcp22.suse.cz/

<snipped>

> +static void lru_gen_change_state(bool enable)
> +{
> +	static DEFINE_MUTEX(state_mutex);
> +
> +	struct mem_cgroup *memcg;
> +
> +	cgroup_lock();
> +	cpus_read_lock();
> +	get_online_mems();
> +	mutex_lock(&state_mutex);
> +
> +	if (enable == lru_gen_enabled())
> +		goto unlock;
> +
> +	if (enable)
> +		static_branch_enable_cpuslocked(&lru_gen_caps[LRU_GEN_CORE]);
> +	else
> +		static_branch_disable_cpuslocked(&lru_gen_caps[LRU_GEN_CORE]);

Fixed the lockdep warning for memory hotplug:
https://lore.kernel.org/linux-mm/87a6g0nczg.fsf@linux.ibm.com/

<snipped>