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

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Date:	Thu, 15 Dec 2011 11:08:30 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Vincent Guittot <vincent.guittot@...aro.org>
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 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..

Then again, its probably easier to keep update_group_power on this_cpu
than to allow a remote update of your cpu_power.

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