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

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Message-ID: <CAOUHufbUyTL-xx1RBovfSh+CrK7mBHGOouvyQBjy1RX1f3dkzw@mail.gmail.com>
Date:   Tue, 19 Apr 2022 18:39:17 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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, Apr 12, 2022 at 1:06 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Apr 11, 2022 at 07:16:15PM -0700, Andrew Morton wrote:
>
> > > +{
> > > +   int gen;
> > > +   unsigned long old_flags, new_flags;
> > > +
> > > +   do {
> > > +           new_flags = old_flags = READ_ONCE(folio->flags);
> > > +           if (!(new_flags & LRU_GEN_MASK))
> > > +                   return false;
> > > +
> > > +           VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
> > > +           VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio);
> > > +
> > > +           gen = ((new_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
> > > +
> > > +           new_flags &= ~LRU_GEN_MASK;
> > > +           /* for shrink_page_list() */
> > > +           if (reclaiming)
> > > +                   new_flags &= ~(BIT(PG_referenced) | BIT(PG_reclaim));
> > > +           else if (lru_gen_is_active(lruvec, gen))
> > > +                   new_flags |= BIT(PG_active);
> > > +   } while (cmpxchg(&folio->flags, old_flags, new_flags) != old_flags);
>
> Also; please use the form:
>
>         unsigned long new, old = READ_ONCE(folio->flags);
>
>         do {
>                 new = /* something */ old;
>         } while (!try_cmpxchg(&folio->flags, &old, new))

Sweet. Thanks.

A related question: if I pass new = old to try_cmpxchg(), does it know
that and avoid an unnecessary atomic op?