linux-kernel - Re: [PATCH V4 2/2] mm: add swapiness= arg to memory.reclaim

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Date:   Thu, 14 Dec 2023 10:28:01 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Dan Schatzberg <schatzberg.dan@...il.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Huan Yang <link@...o.com>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Jonathan Corbet <corbet@....net>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Yue Zhao <findns94@...il.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH V4 2/2] mm: add swapiness= arg to memory.reclaim

On Thu, Dec 14, 2023 at 10:22 AM Dan Schatzberg
<schatzberg.dan@...il.com> wrote:
>
> On Thu, Dec 14, 2023 at 09:38:55AM +0100, Michal Hocko wrote:
> > On Tue 12-12-23 17:38:03, Dan Schatzberg wrote:
> > > Allow proactive reclaimers to submit an additional swappiness=<val>
> > > argument to memory.reclaim. This overrides the global or per-memcg
> > > swappiness setting for that reclaim attempt.
> >
> > You are providing the usecase in the cover letter and Andrew usually
> > appends that to the first patch in the series. I think it would be
> > better to have the usecase described here.
> >
> > [...]
> > > @@ -1304,6 +1297,18 @@ PAGE_SIZE multiple when read back.
> > >     This means that the networking layer will not adapt based on
> > >     reclaim induced by memory.reclaim.
> > >
> > > +The following nested keys are defined.
> > > +
> > > +     ==========            ================================
> > > +     swappiness            Swappiness value to reclaim with
> > > +     ==========            ================================
> > > +
> > > +   Specifying a swappiness value instructs the kernel to perform
> > > +   the reclaim with that swappiness value. Note that this has the
> > > +   same semantics as the vm.swappiness sysctl - it sets the
> >
> > same semantics as vm.swappiness applied to memcg reclaim with all the
> > existing limitations and potential future extensions.
>
> Thanks, will make this change.
>
> >
> > > +   relative IO cost of reclaiming anon vs file memory but does
> > > +   not allow for reclaiming specific amounts of anon or file memory.
> > > +
> > >    memory.peak
> > >     A read-only single value file which exists on non-root
> > >     cgroups.
> >
> > The biggest problem with the implementation I can see, and others have
> > pointed out the same, is how fragile this is. You really have to check
> > the code and _every_ place which defines scan_control to learn that
> > mem_cgroup_shrink_node, reclaim_clean_pages_from_list,
> > reclaim_folio_list, lru_gen_seq_write, try_to_free_pages, balance_pgdat,
> > shrink_all_memory and __node_reclaim. I have only checked couple of
> > them, like direct reclaim and kswapd and none of them really sets up
> > swappiness to the default memcg nor global value. So you effectively end
> > up with swappiness == 0.
> >
> > While the review can point those out it is quite easy to break and you
> > will only learn about that very indirectly. I think it would be easier
> > to review and maintain if you go with a pointer that would fallback to
> > mem_cgroup_swappiness() if NULL which will be the case for every
> > existing reclaimer except memory.reclaim with swappiness value.
>
> I agree. My initial implementation used a pointer for this
> reason. I'll switch this back. Just to be clear - I still need to
> initialize scan_control.swappiness in all these other places right? It
> appears to mostly be stack-initialized which means it has
> indeterminate value, not necessarily zero.

My understanding is that in a partially initialized struct,
uninitialized members default to 0, but I am not a C expert :)