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[94.245.46.61]) by smtp.gmail.com with ESMTPSA id v9sm615235lfd.287.2020.12.10.11.24.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Dec 2020 11:24:01 -0800 (PST) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Thu, 10 Dec 2020 20:23:58 +0100 To: "Paul E. McKenney" Cc: joel@joelfernandes.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: Energy-efficiency options within RCU Message-ID: <20201210192358.GA2365@pc638.lan> References: <20201210183737.GA12900@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201210183737.GA12900@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org Hello, Paul. [Dropping CC] > Hello, Joel, > > In case you are -seriously- interested... ;-) > > Thanx, Paul > > rcu_nocbs= > > Adding a CPU to this list offloads RCU callback invocation from > that CPU's softirq handler to a kthread. In big.LITTLE systems, > this kthread can be placed on a LITTLE CPU, which has been > demonstrated to save significant energy in benchmarks. > http://www.rdrop.com/users/paulmck/realtime/paper/AMPenergy.2013.04.19a.pdf > I have checked our config. We do use rcu_nocbs=0-7 as kthreads but what i see those threads are not bound to 0-3 CPUs. In our case it is little cluster. I think i should check and run some test cases regarding power savings if i pin all threads to little cluster. > rcutree.rcu_idle_gp_delay= (Only CONFIG_RCU_FAST_NO_HZ=y kernels.) > > This defaults to four jiffies on the theory that grace periods > tend to last about that long. If grace periods tend to take > longer, then it makes a lot of sense to increase this. And maybe > battery-powered devices would rather have it be about 2x or 3x > the expected grace-period duration, who knows? > > I would keep it to a power of two, but the code should work with > other numbers. Except that I don't know that this has ever been > tested. ;-) > Same here. We do use it. -- Vlad Rezki