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

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Date:	Thu, 15 Dec 2011 14:36:34 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org, linaro-dev@...ts.linaro.org,
	patches@...aro.org, Ingo Molnar <mingo@...e.hu>,
	Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [RFC] sched: Ensure cpu_power periodic update

On 15 December 2011 11:08, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Mon, 2011-12-12 at 20:21 +0100, Vincent Guittot wrote:
>> With a lot of small tasks, the softirq sched is nearly never called
>> when no_hz is enable. In this case the load_balance is mainly called with
>> the newly_idle mode which doesn't update the cpu_power.
>> Add a next_update field which ensure a maximum update period when
>> there is short activity
>
>> +     if (local_group) {
>> +             if (idle != CPU_NEWLY_IDLE) {
>> +                     if (balance_cpu != this_cpu) {
>> +                             *balance = 0;
>> +                             return;
>> +                     }
>> +                     update_group_power(sd, this_cpu);
>> +             } else if (time_after_eq(jiffies, group->sgp->next_update))
>> +                     update_group_power(sd, this_cpu);
>>       }
>
> Hmm, I would have expected it to be called from the NOHZ balancing path
> instead of the new_idle path. Your changelog fails to mentions any
> considerations on this..
>

As we are not lucky, the small tasks are mainly running between ticks
and  the timer interrupt doesn't fire which implies that both
rebalance_domain of the cpu and nohz_balancer_kick are not called. We
have a lot of call to idle_balance() when cpus become idle and very
few calls to rebalance or nohz_idle_balance. If some tasks are rt
tasks, the cpu_power should be updated regularly to reflect current
use of the cpu by rt scheduler.

I'm using cyclictest to easily reproduce the problem on my dual cortex-A9

> 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.

> So I'm not opposed to this patch, I'd just like a little extra
> clarification.

Vincent
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