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[37.188.133.21]) by smtp.gmail.com with ESMTPSA id a8sm3198701wmc.20.2020.02.19.23.52.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 19 Feb 2020 23:52:19 -0800 (PST) Date: Thu, 20 Feb 2020 08:52:18 +0100 From: Michal Hocko To: Andrew Morton Cc: Wei Yang , linux-mm@kvack.org, linux-kernel@vger.kernel.org, rientjes@google.com Subject: Re: [Patch v4] mm/vmscan.c: remove cpu online notification for now Message-ID: <20200220075218.GA20509@dhcp22.suse.cz> References: <20200218224422.3407-1-richardw.yang@linux.intel.com> <20200219120810.c7677fa58594f5423549f59d@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200219120810.c7677fa58594f5423549f59d@linux-foundation.org> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed 19-02-20 12:08:10, Andrew Morton wrote: > On Wed, 19 Feb 2020 06:44:22 +0800 Wei Yang 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