linux-kernel - Re: [PATCH v10 05/14] mm: multi-gen LRU: groundwork

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Message-Id: <20220426164241.99e6a283c371ed75fa5c12a0@linux-foundation.org>
Date:   Tue, 26 Apr 2022 16:42:41 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Stephen Rothwell <sfr@...hwell.id.au>,
        Linux-MM <linux-mm@...ck.org>, 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>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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 <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.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 v10 05/14] mm: multi-gen LRU: groundwork

On Tue, 26 Apr 2022 16:39:07 -0600 Yu Zhao <yuzhao@...gle.com> wrote:

> On Mon, Apr 11, 2022 at 8:16 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> >
> > On Wed,  6 Apr 2022 21:15:17 -0600 Yu Zhao <yuzhao@...gle.com> wrote:
> >
> > > Evictable pages are divided into multiple generations for each lruvec.
> > > The youngest generation number is stored in lrugen->max_seq for both
> > > anon and file types as they are aged on an equal footing. The oldest
> > > generation numbers are stored in lrugen->min_seq[] separately for anon
> > > and file types as clean file pages can be evicted regardless of swap
> > > constraints. These three variables are monotonically increasing.
> > >
> > > ...
> > >
> > > +static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio, bool reclaiming)
> >
> > There's a lot of function inlining here.  Fortunately the compiler will
> > ignore it all, because some of it looks wrong.  Please review (and
> > remeasure!).  If inlining is reqlly justified, use __always_inline, and
> > document the reasons for doing so.
> 
> I totally expect modern compilers to make better decisions than I do.
> And personally, I'd never use __always_inline; instead, I'd strongly
> recommend FDO/LTO.

My (badly expressed) point is that there's a lot of inlining of large
functions here.

For example, lru_gen_add_folio() is huge and has 4(?) call sites.  This
may well produce slower code due to the icache footprint.

Experiment: moving lru_gen_del_folio() into mm/vmscan.c shrinks that
file's .text from 80612 bytes to 78956.

I tend to think that out-of-line regular old C functions should be the
default and that the code should be inlined only when a clear benefit
is demonstrable, or has at least been seriously thought about.

> > > --- a/mm/Kconfig
> > > +++ b/mm/Kconfig
> > > @@ -909,6 +909,14 @@ config ANON_VMA_NAME
> > >         area from being merged with adjacent virtual memory areas due to the
> > >         difference in their name.
> > >
> > > +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)
> > > +     help
> > > +       A high performance LRU implementation to overcommit memory.
> > > +
> > >  source "mm/damon/Kconfig"
> >
> > This is a problem.  I had to jump through hoops just to be able to
> > compile-test this.  Turns out I had to figure out how to disable
> > MAXSMP.
> >
> > Can we please figure out a way to ensure that more testers are at least
> > compile testing this?  Allnoconfig, defconfig, allyesconfig, allmodconfig.
> >
> > Also, I suggest that we actually make MGLRU the default while in linux-next.
> 
> The !MAXSMP is to work around [1], which I haven't had the time to
> fix. That BUILD_BUG_ON() shouldn't assert sizeof(struct page) == 64
> since the true size depends on WANT_PAGE_VIRTUAL as well as
> LAST_CPUPID_NOT_IN_PAGE_FLAGS. My plan is here [2].
> 
> [1] https://lore.kernel.org/r/20190905154603.10349-4-aneesh.kumar@linux.ibm.com/
> [2] https://lore.kernel.org/r/Ygl1Gf+ATBuI%2Fm2q@google.com/

OK, thanks.  This is fairly urgent for -next and -rc inclusion.  If
practically nobody is compiling the feature then practically nobody is
testing it.  Let's come up with a way to improves the expected coverage
by a lot.