linux-kernel - Re: [PATCH Resend v6] sched: fix wrong rq's runnable

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Date:	Fri, 19 Apr 2013 13:11:22 +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 11:21, Mike Galbraith <efault@....de> wrote:
> On Fri, 2013-04-19 at 10:50 +0200, Vincent Guittot wrote:
>> On 19 April 2013 10:14, Mike Galbraith <efault@....de> wrote:
>> > On Fri, 2013-04-19 at 09:49 +0200, Vincent Guittot wrote:
>> >> 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.
>> >
>> > If it only scares fair class tasks away from the periodic rt load, that
>> > seems like a benefit to me, not a liability.  If we really really need
>>
>> I'm not sure that such behavior that is only based on erroneous value,
>> is good one.
>>
>> > perfect load numbers, fine, we have to eat some cycles, but when I look
>> > at it, it looks like one of those "Perfect is the enemy of good" things.
>>
>> The target is not perfect number but good enough to be usable. The
>> systctl_migration_cost threshold is good for idle balancing but can
>> generates wrong load value
>
> But again, why do we care?  To be able to mix rt and fair loads and
> still make pretty mixed load utilization numbers?  Paying a general case

If runnable_avg_sum can be wrong, it becomes unusable and all the
stuff around becomes useless.

> fast path price to make strange (to me) load utilization numbers pretty
> is not very attractive.  If you muck about with rt classes, you need to
> have a good reason for doing that.  If you do have a good reason, you
> also allocated all resources, including CPU, so don't need the kernel to

Some tasks have responsiveness constraints so they use rt class but
they also live with cfs tasks.

Vincent

> balance the load for you.  Paying any fast path price to make the kernel
> balance a mixed rt/fair load just seems fundamentally wrong to me.
>
> -Mike
>
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