linux-kernel - Re: [PATCH V4 2/3] tick/cpuidle: Initialize hrtimer mode of broadcast

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Date:	Tue, 11 Feb 2014 23:01:18 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	linux-pm@...r.kernel.org, peterz@...radead.org,
	benh@...nel.crashing.org, rafael.j.wysocki@...el.com,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	mingo@...nel.org, deepthi@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, fweisbec@...il.com, paulus@...ba.org,
	srivatsa.bhat@...ux.vnet.ibm.com, svaidy@...ux.vnet.ibm.com
Subject: Re: [PATCH V4 2/3] tick/cpuidle: Initialize hrtimer mode of broadcast

On 02/11/2014 04:58 PM, Thomas Gleixner wrote:
> On Tue, 11 Feb 2014, Daniel Lezcano wrote:
>> On 02/07/2014 09:06 AM, Preeti U Murthy wrote:
>> Setting the smp affinity on the earliest timer should be handled automatically
>> with the CLOCK_EVT_FEAT_DYNIRQ flag. Did you look at using this flag ?
>
> How should this flag help? Not at all, because the hrtimer based
> broadcast device cannot assign affinities.
>
>> Another comment is the overall approach. We enter the cpuidle idle framework
>> with a specific state to go to and it is the tick framework telling us we
>> mustn't go to this state. IMO the logic is wrong, the decision to not enter
>> this state should be moved somewhere else.
>>
>> Why don't you create a cpuidle driver with the shallow idle states assigned to
>> a cpu (let's say cpu0) and another one with all the deeper idle states for the
>> rest of the cpus ? Using the multiple cpuidle driver support makes it
>> possible. The timer won't be moving around and a cpu will be dedicated to act
>> as the broadcast timer.
>>
>> Wouldn't make sense and be less intrusive than the patchset you proposed ?
>
> How do you arm the broadcast timer on CPU0 from CPU1? You can't!
>
> You cannot access the cpu local timer on a different cpu. So you would
> have to send an IPI over to CPU0 so that it can reevaluate and
> schedule the broadcast. That's even more backwards than telling the
> cpuidle code that the CPU is not in a state to go deep.

Indeed :)

Thanks for the clarification.

   -- Daniel

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