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[46.139.12.213]) by smtp.gmail.com with ESMTPSA id i125-v6sm83636wmd.23.2018.09.10.23.22.51 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 10 Sep 2018 23:22:51 -0700 (PDT) Date: Tue, 11 Sep 2018 08:22:49 +0200 From: Ingo Molnar To: Peter Zijlstra Cc: Dietmar Eggemann , Ingo Molnar , Fengguang Wu , Li Zhijian , linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched/fair: Disable LB_BIAS by default Message-ID: <20180911062249.GD72017@gmail.com> References: <20180809135753.21077-1-dietmar.eggemann@arm.com> <20180910141205.GM24106@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180910141205.GM24106@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Thu, Aug 09, 2018 at 02:57:53PM +0100, Dietmar Eggemann wrote: > > LB_BIAS allows the adjustment on how conservative load should be > > balanced. > > > It is very likely that LB_BIAS' influence on load balancing can be > > neglected (see test results below). This is further supported by: > > > > (1) Weighted CPU load today is by itself a decayed average value (PELT) > > (cfs_rq->avg->runnable_load_avg) and not the instantaneous load > > (rq->load.weight) it was when LB_BIAS was introduced. > > > > (2) Sd imbalance_pct is used for CPU_NEWLY_IDLE and CPU_NOT_IDLE (relate > > to sd's newidle and busy idx) in find_busiest_group() when comparing > > busiest and local avg load to make load balancing even more > > conservative. > > > > (3) The sd forkexec and newidle idx are always set to 0 so there is no > > adjustment on how conservatively load balancing is done here. > > > > (4) Affine wakeup based on weight (wake_affine_weight()) will not be > > impacted since the sd wake idx is always set to 0. > > > > Let's disable LB_BIAS by default for a few kernel releases to make sure > > that no workload and no scheduler topology is affected. The benefit of > > being able to remove the LB_BIAS dependency from source_load() and > > target_load() is that the entire rq->cpu_load[idx] code could be removed > > in this case. > > Certainly worth a try; as I've written somewhere in a comment; it would > be very nice to get rid of that load tracking crud. > > And it is trivial to revert if something does show up. > > Ingo, what do you think? Ack, I'm very much in favor of reducing complexity. Thanks, Ingo