linux-kernel - Re: [PATCH 1/2] PPC: powernv: remove redundant cpuidle_idle

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Message-ID: <52F84B40.8090403@linux.vnet.ibm.com>
Date:	Mon, 10 Feb 2014 09:15:04 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>
CC:	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/2] PPC: powernv: remove redundant cpuidle_idle_call()

Hi Peter,

On 02/07/2014 06:11 PM, Peter Zijlstra wrote:
> On Fri, Feb 07, 2014 at 05:11:26PM +0530, Preeti U Murthy wrote:
>> But observe the idle state "snooze" on powerpc. The power that this idle
>> state saves is through the lowering of the thread priority of the CPU.
>> After it lowers the thread priority, it is done. It cannot
>> "wait_for_interrupts". It will exit my_idle(). It is now upto the
>> generic idle loop to increase the thread priority if the need_resched
>> flag is set. Only an interrupt routine can increase the thread priority.
>> Else we will need to do it explicitly. And in such states which have a
>> polling nature, the cpu will not receive a reschedule IPI.
>>
>> That is why in the snooze_loop() we poll on need_resched. If it is set
>> we up the priority of the thread using HMT_MEDIUM() and then exit the
>> my_idle() loop. In case of interrupts, the priority gets automatically
>> increased.
> 
> You can poll without setting TS_POLLING/TIF_POLLING_NRFLAGS just fine
> and get the IPI if that is what you want.
> 
> Depending on how horribly unprovisioned the thread gets at the lowest
> priority, that might actually be faster than polling and raising the
> prio whenever it does get ran.

So I am assuming you mean something like the below:

my_idle()
{
   local_irq_enable();
   /* Remove the setting of the polling flag */
   HMT_low();
   return index;
}

And then exit into the generic idle loop. But the issue I see here is
that the TS_POLLING/TIF_POLLING_NRFLAGS gets set immediately. So, if on
testing need_resched() immediately after this returns that the
TIF_NEED_RESCHED flag is set, the thread will exit at low priority right?
 We could raise the priority of the thread in arch_cpu_idle_exit() soon
after setting the polling flag but that would mean for cases where the
TIF_NEED_RESCHED flag is not set we unnecessarily raise the priority of
the thread.

Thanks

Regards
Preeti U Murthy

> 

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