linux-kernel - Re: [PATCH v9 05/14] mm: multi-gen LRU: groundwork

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Date:   Tue, 15 Mar 2022 08:34:18 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     kernel@...ts.fedoraproject.org, kernel-team@...ts.ubuntu.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.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>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Kernel Page Reclaim v2 <page-reclaim@...gle.com>,
        "the arch/x86 maintainers" <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>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH v9 05/14] mm: multi-gen LRU: groundwork

Yu Zhao <yuzhao@...gle.com> writes:

> On Mon, Mar 14, 2022 at 2:09 AM Huang, Ying <ying.huang@...el.com> wrote:
>>
>> Hi, Yu,
>>
>> Yu Zhao <yuzhao@...gle.com> writes:
>> > diff --git a/mm/Kconfig b/mm/Kconfig
>> > index 3326ee3903f3..747ab1690bcf 100644
>> > --- a/mm/Kconfig
>> > +++ b/mm/Kconfig
>> > @@ -892,6 +892,16 @@ config ANON_VMA_NAME
>> >         area from being merged with adjacent virtual memory areas due to the
>> >         difference in their name.
>> >
>> > +# the multi-gen LRU {
>> > +config LRU_GEN
>> > +     bool "Multi-Gen LRU"
>> > +     depends on MMU
>> > +     # the following options can use up the spare bits in page flags
>> > +     depends on !MAXSMP && (64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP)
>>
>> LRU_GEN depends on !MAXSMP.  So, What is the maximum NR_CPUS supported
>> by LRU_GEN?
>
> LRU_GEN doesn't really care about NR_CPUS. IOW, it doesn't impose a
> max number. The dependency is with NODES_SHIFT selected by MAXSMP:
>     default "10" if MAXSMP
> This combined with LAST_CPUPID_SHIFT can exhaust the spare bits in page flags.

>From the following code snippets from page-flags-layout.h,
LAST_CPUPID_SHIFT is related to NR_CPUS instead of NODES_SHIFT.

#define LAST__PID_SHIFT 8
#define LAST__PID_MASK  ((1 << LAST__PID_SHIFT)-1)

#define LAST__CPU_SHIFT NR_CPUS_BITS
#define LAST__CPU_MASK  ((1 << LAST__CPU_SHIFT)-1)

#define LAST_CPUPID_SHIFT (LAST__PID_SHIFT+LAST__CPU_SHIFT)

Best Regards,
Huang, Ying

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