linux-kernel - Re: [PATCH V3 1/6] sched: idle: Add a weak arch_cpu_idle

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Message-ID: <20141110122954.GM3337@twins.programming.kicks-ass.net>
Date:	Mon, 10 Nov 2014 13:29:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	rjw@...ysocki.net, preeti@...ux.vnet.ibm.com,
	nicolas.pitre@...aro.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org,
	patches@...aro.org, lenb@...nel.org
Subject: Re: [PATCH V3 1/6] sched: idle: Add a weak arch_cpu_idle_poll
 function

On Fri, Nov 07, 2014 at 03:31:22PM +0100, Daniel Lezcano wrote:
> The poll function is called when a timer expired or if we force to poll when
> the cpu_idle_force_poll option is set.
> 
> The poll function does:
> 
>        local_irq_enable();
>        while (!tif_need_resched())
>                cpu_relax();
> 
> This default poll function suits for the x86 arch because its rep; nop;
> hardware power optimization. But on other archs, this optimization does not
> exists and we are not saving power. The arch specific bits may want to
> optimize this loop by adding their own optimization.

This doesn't make sense to me; should an arch not either implement an
actual idle driver or implement cpu_relax() properly, why allow for a
third weird option?
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