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From:   Yang Shi <shy828301@gmail.com>
Date:   Thu, 20 Feb 2020 09:03:52 -0800
Message-ID: <CAHbLzkr9B1EdNuxPqRF5PrsoO+ZaHdJBydQiMRAEjgq9uRgpMQ@mail.gmail.com>
Subject: Re: [Patch v4] mm/vmscan.c: remove cpu online notification for now
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Wei Yang <richardw.yang@linux.intel.com>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Rientjes <rientjes@google.com>
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On Wed, Feb 19, 2020 at 11:52 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Wed 19-02-20 12:08:10, Andrew Morton wrote:
> > On Wed, 19 Feb 2020 06:44:22 +0800 Wei Yang <richardw.yang@linux.intel.com> wrote:
> >
> > > kswapd kernel thread starts either with a CPU affinity set to the full
> > > cpu mask of its target node or without any affinity at all if the node
> > > is CPUless. There is a cpu hotplug callback (kswapd_cpu_online) that
> > > implements an elaborate way to update this mask when a cpu is onlined.
> > >
> > > It is not really clear whether there is any actual benefit from this
> > > scheme. Completely CPU-less NUMA nodes rarely gain a new CPU during
> > > runtime.
> >
> > This is the case across all platforms, all architectures, all users for
> > the next N years?  I'm surprised that we know this with sufficient
> > confidence.  Can you explain how you came to make this assertion?
>
> CPUless NUMA nodes are quite rare - mostly ppc with crippled LPARs.
> I am not aware those would dynamically get CPUs for those nodes later in
> the runtime. Maybe they do but we would like to learn about that. A
> missing cpu mask is not going cause any fatal problems anyway.

Persistent memory nodes are CPUless nodes. But, I don't think they
would get any CPU online later in the runtime.

>
> As the changelog states the callback can be reintroduced with a sign of
> testing and usecase description. I prefer we drop this code in the mean
> time as the benefit is not really clear or testable.
>
> > > Drop the code for that reason. If there is a real usecase then
> > > we can resurrect and simplify the code.
>
> --
> Michal Hocko
> SUSE Labs
>