linux-kernel - Re: [PATCH v3 07/12] sched: test the cpu's capacity in wake affine

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Date:	Thu, 10 Jul 2014 13:06:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	linux@....linux.org.uk, linux-arm-kernel@...ts.infradead.org,
	preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com, efault@....de,
	nicolas.pitre@...aro.org, linaro-kernel@...ts.linaro.org,
	daniel.lezcano@...aro.org, dietmar.eggemann@....com
Subject: Re: [PATCH v3 07/12] sched: test the cpu's capacity in wake affine

On Mon, Jun 30, 2014 at 06:05:38PM +0200, Vincent Guittot wrote:
> Currently the task always wakes affine on this_cpu if the latter is idle.
> Before waking up the task on this_cpu, we check that this_cpu capacity is not
> significantly reduced because of RT tasks or irq activity.
> 
> Use case where the number of irq and/or the time spent under irq is important
> will take benefit of this because the task that is woken up by irq or softirq
> will not use the same CPU than irq (and softirq) but a idle one which share
> its cache.

The above, doesn't seem to explain:

> +	} else if (!(sd->flags & SD_SHARE_PKG_RESOURCES)) {
> +		this_eff_load = 0;
> +	}
> +
> +	balanced = this_eff_load <= prev_eff_load;

this. Why would you unconditionally allow wake_affine across cache
domains?
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