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Date: Fri, 19 Apr 2013 09:49:19 +0200 From: Vincent Guittot <vincent.guittot@...aro.org> To: Mike Galbraith <efault@....de> Cc: linux-kernel <linux-kernel@...r.kernel.org>, "linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, Frédéric Weisbecker <fweisbec@...il.com> Subject: Re: [PATCH Resend v6] sched: fix wrong rq's runnable_avg update with rt tasks On 19 April 2013 06:30, Mike Galbraith <efault@....de> wrote: > On Thu, 2013-04-18 at 18:34 +0200, Vincent Guittot wrote: >> The current update of the rq's load can be erroneous when RT tasks are >> involved >> >> The update of the load of a rq that becomes idle, is done only if the avg_idle >> is less than sysctl_sched_migration_cost. If RT tasks and short idle duration >> alternate, the runnable_avg will not be updated correctly and the time will be >> accounted as idle time when a CFS task wakes up. >> >> A new idle_enter function is called when the next task is the idle function >> so the elapsed time will be accounted as run time in the load of the rq, >> whatever the average idle time is. The function update_rq_runnable_avg is >> removed from idle_balance. >> >> When a RT task is scheduled on an idle CPU, the update of the rq's load is >> not done when the rq exit idle state because CFS's functions are not >> called. Then, the idle_balance, which is called just before entering the >> idle function, updates the rq's load and makes the assumption that the >> elapsed time since the last update, was only running time. >> >> As a consequence, the rq's load of a CPU that only runs a periodic RT task, >> is close to LOAD_AVG_MAX whatever the running duration of the RT task is. > > Why do we care what rq's load says, if the only thing running is a > periodic RT task? I _think_ I recall that stuff being put under the cfs scheduler will use a wrong rq load the next time it wants to schedule a task > throttle specifically to not waste cycles doing that on every > microscopic idle. yes but this lead to the wrong computation of runnable_avg_sum. To be more precise, we only need to call __update_entity_runnable_avg, __update_tg_runnable_avg is not mandatory in this step. > > Seems to me when scheduling an rt task, you want to do as little other > than switching to/from the rt task as possible. I don't let rt tasks do > idle balancing either, their job isn't to balance fair class on the way > out the door, it's to get off/onto the cpu ASAP, and do rt work. I agree but the patch is not about balancing fair task but keep coherent runnable value Vincent > > -Mike > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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