From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: AG47ELspVd8dfWE/rbDSVZTa4lPwsyL/FCGZMd1wg0E5LiZg7lyuvbv11SdcMiATioRLTJNF0ikI ARC-Seal: i=1; a=rsa-sha256; t=1521647704; cv=none; d=google.com; s=arc-20160816; b=C660/dt67H22PTuSCo7p8uW+XZCaxFyYcfy7HhXaNljdYqp/9R/rVN8ERhlaFjM7Jf 1x8hMH02WPX4a438Y/e/u8Xh92VQoqP+lHOPBwNxEGDtn+MOrtYTuGSsysRl580K3IV7 SS7L6GJ1W6UZOJ1+TEk11jsY+l25eTAg5lkaQ5L7s8JJgSriXdXeu1/A5Pqlyds22Bl0 b5oVzrxpZ084g1vPa8gKfWGI/nvvCAoAvh2TotD89wYOtEssMf0WfO1+9NQaq2dZy0Jb 4MagMcRC8Psmh8yBE73lz0buyyhi4N7oKhvXSxrsGvNPGXD58uZHxKnQsRTIod8fu3Qd /T5g== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=user-agent:in-reply-to:content-disposition:mime-version:references :message-id:subject:cc:to:from:date:arc-authentication-results; bh=fnfoXzZ8rmuZfSLiZNVpLdaxFMa5rBMtyYFwnLOJlG8=; b=QORd5SItcqHQexj9y1dalZxhIk8CLxKPVWx0Cq6MG3AfTocN7xZETzKi3cZdbwL3ev gJufZoJYs+1giquZhRyWFDYoNB3KDAJA/0IuqSubc1TMIBXxq8UfKnz449nIYdb1ix1Z 2mbmCG5VkKLd90HvWtSMyKhg3ktiniwit5o2AFXMav+J8Amywk3nNTig4LM8vFhhQjWU 1gzGbR69oYEKvfH+dVp0/Nv/0Lmyil7zXd8a1lM2dCdCxEHfvRYOyFA+827IqlAoYhLw kMJOeXXqNJxLUsQO4WfDFfh4d468rM2/mG5ZLX1ui5qZPe2/3VDBgmz47137zC8eCkWf ukiA== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of patrick.bellasi@arm.com designates 217.140.101.70 as permitted sender) smtp.mailfrom=patrick.bellasi@arm.com Authentication-Results: mx.google.com; spf=pass (google.com: domain of patrick.bellasi@arm.com designates 217.140.101.70 as permitted sender) smtp.mailfrom=patrick.bellasi@arm.com Date: Wed, 21 Mar 2018 15:54:58 +0000 From: Patrick Bellasi To: Quentin Perret Cc: Dietmar Eggemann , linux-kernel@vger.kernel.org, Peter Zijlstra , Thara Gopinath , linux-pm@vger.kernel.org, Morten Rasmussen , Chris Redpath , Valentin Schneider , "Rafael J . Wysocki" , Greg Kroah-Hartman , Vincent Guittot , Viresh Kumar , Todd Kjos , Joel Fernandes Subject: Re: [RFC PATCH 4/6] sched/fair: Introduce an energy estimation helper function Message-ID: <20180321155458.GD13951@e110439-lin> References: <20180320094312.24081-1-dietmar.eggemann@arm.com> <20180320094312.24081-5-dietmar.eggemann@arm.com> <20180321123921.GB13951@e110439-lin> <20180321142630.GB2168@queper01-VirtualBox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180321142630.GB2168@queper01-VirtualBox> User-Agent: Mutt/1.5.24 (2015-08-30) X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1595449339672655930?= X-GMAIL-MSGID: =?utf-8?q?1595563262664732353?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On 21-Mar 14:26, Quentin Perret wrote: > On Wednesday 21 Mar 2018 at 12:39:21 (+0000), Patrick Bellasi wrote: > > On 20-Mar 09:43, Dietmar Eggemann wrote: > > > From: Quentin Perret > > > > [...] > > > > > +static unsigned long compute_energy(struct task_struct *p, int dst_cpu) > > > +{ > > > + unsigned long util, fdom_max_util; > > > + struct capacity_state *cs; > > > + unsigned long energy = 0; > > > + struct freq_domain *fdom; > > > + int cpu; > > > + > > > + for_each_freq_domain(fdom) { > > > + fdom_max_util = 0; > > > + for_each_cpu_and(cpu, &(fdom->span), cpu_online_mask) { > > > + util = cpu_util_next(cpu, p, dst_cpu); > > > > Would be nice to find a way to cache all these util and reuse them > > below... even just to ensure data consistency between the "cs" > > computation and its usage... > > So actually, what I can do is add something like > > fdom_tot_util += util; > > to this loop and compute > > energy = cs->power * fdom_tot_util / cs->cap; > > only once, instead of having the second loop to compute the energy. We don't > have to scale the util for each and every CPU since they share the same > cap state. That would save some divisions and ensure the consistency > between the selection of the cap state and the associated energy > computation. What do you think ? Right, would say that under the hypothesis the we are in the same frequency domain (and we are because of fdom->span), that's basically doing: sum_i(P_x * U_i / C_x) => P_x / C_x * sum_i(U_i) Where (C_x, P_x) are the EM reported capacity and power for the expected frequency domain OPP. > Or maybe you were talking about consistency between several consecutive > calls to compute_energy() ? Nope, the above +1 > > > + fdom_max_util = max(util, fdom_max_util); > > > + } > > > + > > > + /* > > > + * Here we assume that the capacity states of CPUs belonging to > > > + * the same frequency domains are shared. Hence, we look at the > > > + * capacity state of the first CPU and re-use it for all. > > > + */ > > > + cpu = cpumask_first(&(fdom->span)); > > > + cs = find_cap_state(cpu, fdom_max_util); > > ^^^^ > > > > The above code could theoretically return NULL, although likely EAS is > > completely disabled if em->nb_cap_states == 0, right? > > That's right. sched_energy_present cannot be enabled with > em->nb_cap_states == 0, and compute_energy() is never called without > sched_energy_present in the proposed implementation. > > > > > If that's the case then, in the previous function, you can certainly > > avoid the initialization of *cs and maybe also add an explicit: > > > > BUG_ON(em->nb_cap_states == 0); > > > > which helps even just as "in code documentation". > > > > But, I'm not sure if maintainers like BUG_ON in scheduler code :) > > Yes, I'm not sure about the BUG_ON either :). FWIW, there are already some BUG_ON in fair.c... thus, if they can pinpoint a specific bug in case of errors, they should be acceptable ? > I agree that it would be nice to document somewhere that > compute_energy() is unsafe to call without sched_energy_present. > I can simply add a proper doc comment to this function actually. > Would that work ? Right, it's just that _maybe_ an explicit BUG_ON is improving the documentation by making more explicit the error on testing ? Thus, I would probably add both... but Peter will tell you for sure ;) -- #include Patrick Bellasi