linux-kernel - Re: [PATCH 0/1] Add swappiness argument to memory.reclaim

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Date:   Thu, 30 Nov 2023 16:37:13 -0500
From:   Dan Schatzberg <schatzberg.dan@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Yosry Ahmed <yosryahmed@...gle.com>, Huan Yang <link@...o.com>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Huang Ying <ying.huang@...el.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Peter Xu <peterx@...hat.com>,
        "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        Yue Zhao <findns94@...il.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 0/1] Add swappiness argument to memory.reclaim

On Thu, Nov 30, 2023 at 12:30:27PM -0800, Shakeel Butt wrote:
> On Thu, Nov 30, 2023 at 11:47 AM Dan Schatzberg
> <schatzberg.dan@...il.com> wrote:
> >
> > On Thu, Nov 30, 2023 at 11:56:42AM -0500, Johannes Weiner wrote:
> > > [...]
> > > So I wouldn't say it's merely a reclaim hint. It controls a very
> > > concrete and influential factor in VM decision making. And since the
> > > global swappiness is long-established ABI, I don't expect its meaning
> > > to change significantly any time soon.
> >
> > I want to add to this last point. While swappiness does not have
> > terribly well-defined semantics - it is the (only?) existing mechanism
> > to control balance between anon and file reclaim. I'm merely
> > advocating for the ability to adjust swappiness during proactive
> > reclaim separately from reactive reclaim. To what degree the behavior
> > and semantics of swappiness change is a bit orthogonal here.
> 
> Let me ask my question in this chain as it might have been missed:
> 
> Whatever the semantics of swappiness are (including the edge cases
> like no swap, file_is_tiny, trim cache), should the reclaim code treat
> the global swappiness and user-provided swappiness differently?

I can't think of any reason why we would want swappiness interpreted
differently if it's provided at proactive reclaim time vs
globally. Did you have something in mind here?