linux-kernel - Re: [RFC] sched: Ensure cpu

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Date:	Fri, 16 Dec 2011 09:23:58 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	"patches@...aro.org" <patches@...aro.org>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>
Subject: Re: [RFC] sched: Ensure cpu_power periodic update

On 16 December 2011 01:58, Suresh Siddha <suresh.b.siddha@...el.com> wrote:
> On Thu, 2011-12-15 at 05:36 -0800, Vincent Guittot wrote:
>> I'm using cyclictest to easily reproduce the problem on my dual cortex-A9
>
> So does the cyclictest itself exhibit the problem or running cyclictest
> with another workload showed the problem? In other words, what numbers
> of the workload did you see change with this patch?
>

Using a cyclictest -q -t 5 -D 4 on my dual cortex-A9 shows the fact
that the softirqs timer and sched are not called very often and the
cpu_power is nearly never updated.

I have also used the following sequence :

cyclictest -q -t 5 -D 4 &
sleep 2
cyclictest -q -t 3 --affinity=0 -p 99 -D 2

The cpu_power of cpu0 should start to decrease when the rt threads are
started. Without the patch, we must wait for the next sched softirq
for starting to update the cpu_power and we have no guarantee of the
maximum interval. With the patch, the cpu_power is updated regularly
using the balance_interval value.

Vincent

>>
>> > Then again, its probably easier to keep update_group_power on this_cpu
>> > than to allow a remote update of your cpu_power.
>> >
>>
>> This additional path for updating the cpu_power will only be used by
>> this_cpu because it is called by idle_balance. But we still have a
>> call to update_group_power by a remote cpu when nohz_idle_balance is
>> called.
>
> As Vincent mentioned, the current mainline kernel already updates the
> remote cpu's group_power in the nohz idle load balancing patch.
>
> Also with all the recent nohz idle load balancing using kick, on a
> dual-core system there may not be any nohz idle load balancing if
> multiple tasks wakeup, run for short time and go back to idle before the
> next tick. We rely on the wakeup balance to get it right in this case.
>
> thanks,
> suresh
>
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