From mboxrd@z Thu Jan 1 00:00:00 1970 From: vincent.guittot@linaro.org (Vincent Guittot) Date: Wed, 4 Jun 2014 09:47:26 +0200 Subject: [PATCH v2 08/11] sched: get CPU's activity statistic In-Reply-To: <20140603155007.GZ30445@twins.programming.kicks-ass.net> References: <1400860385-14555-1-git-send-email-vincent.guittot@linaro.org> <1400860385-14555-9-git-send-email-vincent.guittot@linaro.org> <20140528121001.GI19967@e103034-lin> <20140528154703.GJ19967@e103034-lin> <20140603155007.GZ30445@twins.programming.kicks-ass.net> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 3 June 2014 17:50, Peter Zijlstra wrote: > On Wed, May 28, 2014 at 04:47:03PM +0100, Morten Rasmussen wrote: >> Since we may do periodic load-balance every 10 ms or so, we will perform >> a number of load-balances where runnable_avg_sum will mostly be >> reflecting the state of the world before a change (new task queued or >> moved a task to a different cpu). If you had have two tasks continuously >> on one cpu and your other cpu is idle, and you move one of the tasks to >> the other cpu, runnable_avg_sum will remain unchanged, 47742, on the >> first cpu while it starts from 0 on the other one. 10 ms later it will >> have increased a bit, 32 ms later it will be 47742/2, and 345 ms later >> it reaches 47742. In the mean time the cpu doesn't appear fully utilized >> and we might decide to put more tasks on it because we don't know if >> runnable_avg_sum represents a partially utilized cpu (for example a 50% >> task) or if it will continue to rise and eventually get to 47742. > > Ah, no, since we track per task, and update the per-cpu ones when we > migrate tasks, the per-cpu values should be instantly updated. > > If we were to increase per task storage, we might as well also track > running_avg not only runnable_avg. I agree that the removed running_avg should give more useful information about the the load of a CPU. The main issue with running_avg is that it's disturbed by other tasks (as point out previously). As a typical example, if we have 2 tasks with a load of 25% on 1 CPU, the unweighted runnable_load_avg will be in the range of [100% - 50%] depending of the parallelism of the runtime of the tasks whereas the reality is 50% and the use of running_avg will return this value Vincent