linux-kernel - Re: [Patch v4] mm/vmscan.c: remove cpu online notification for now

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Message-ID: <20200220075218.GA20509@dhcp22.suse.cz>
Date:   Thu, 20 Feb 2020 08:52:18 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Wei Yang <richardw.yang@...ux.intel.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, rientjes@...gle.com
Subject: Re: [Patch v4] mm/vmscan.c: remove cpu online notification for now

On Wed 19-02-20 12:08:10, Andrew Morton wrote:
> On Wed, 19 Feb 2020 06:44:22 +0800 Wei Yang <richardw.yang@...ux.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.

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