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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 2 Dec 2022 15:51:05 -0800
Message-ID: <CALvZod5teb-tLz49PWny7+cV9pUQUom7k9+G6rXOhDH1tji9Lg@mail.gmail.com>
Subject: Re: [PATCH v3] mm: Add nodes= arg to memory.reclaim
To:     Mina Almasry <almasrymina@google.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Huang Ying <ying.huang@intel.com>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Yosry Ahmed <yosryahmed@google.com>, weixugc@google.com,
        fvdl@google.com, Michal Hocko <mhocko@suse.com>,
        bagasdotme@gmail.com, cgroups@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
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On Fri, Dec 2, 2022 at 2:37 PM Mina Almasry <almasrymina@google.com> wrote:
>
> The nodes= arg instructs the kernel to only scan the given nodes for
> proactive reclaim. For example use cases, consider a 2 tier memory system:
>
> nodes 0,1 -> top tier
> nodes 2,3 -> second tier
>
> $ echo "1m nodes=0" > memory.reclaim
>
> This instructs the kernel to attempt to reclaim 1m memory from node 0.
> Since node 0 is a top tier node, demotion will be attempted first. This
> is useful to direct proactive reclaim to specific nodes that are under
> pressure.
>
> $ echo "1m nodes=2,3" > memory.reclaim
>
> This instructs the kernel to attempt to reclaim 1m memory in the second tier,
> since this tier of memory has no demotion targets the memory will be
> reclaimed.
>
> $ echo "1m nodes=0,1" > memory.reclaim
>
> Instructs the kernel to reclaim memory from the top tier nodes, which can
> be desirable according to the userspace policy if there is pressure on
> the top tiers. Since these nodes have demotion targets, the kernel will
> attempt demotion first.
>
> Since commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg
> reclaim""), the proactive reclaim interface memory.reclaim does both
> reclaim and demotion. Reclaim and demotion incur different latency costs
> to the jobs in the cgroup. Demoted memory would still be addressable
> by the userspace at a higher latency, but reclaimed memory would need to
> incur a pagefault.
>
> The 'nodes' arg is useful to allow the userspace to control demotion
> and reclaim independently according to its policy: if the memory.reclaim
> is called on a node with demotion targets, it will attempt demotion first;
> if it is called on a node without demotion targets, it will only attempt
> reclaim.
>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
>

Acked-by: Shakeel Butt <shakeelb@google.com>