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

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Message-ID: <5460C9A6.6080103@linux.vnet.ibm.com>
Date:	Mon, 10 Nov 2014 19:50:22 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
CC:	rjw@...ysocki.net, 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

Hi Peter,

On 11/10/2014 05:59 PM, Peter Zijlstra wrote:
> 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?
> 

The previous version of this patch simply invoked cpu_idle_loop() for
cases where latency_req was 0. This would have changed the behavior
on PowerPC wherein earlier the 0th idle index was returned which is also
a polling loop but differs from cpu_idle_loop() in two ways:

a. It polls at a relatively lower power state than cpu_relax().
b. We set certain registers to indicate that the cpu is idle.

Hence for all such cases wherein the cpu is required to poll while idle
(only for cases such as force poll, broadcast ipi to arrive soon and
latency_req = 0), we should be able to call into cpuidle_idle_loop()
only if the cpuidle driver's 0th idle state has an exit_latency > 0.
(The 0th idle state is expected to be a polling loop with
exit_latency = 0).

If otherwise, it would mean the driver has an optimized polling loop
when idle. But instead of adding in the logic of checking the
exit_latency, we thought it would be simpler to call into an arch
defined polling idle loop under the above circumstances. If that is no
better we could fall back to cpuidle_idle_loop().

Regards
Preeti U Murthy

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