linux-kernel - Re: [PATCH] cpuidle: Fix the CPU stuck at C0 for 2-3s after PM

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Date:	Thu, 14 Aug 2014 13:00:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Chuansheng Liu <chuansheng.liu@...el.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, changcheng.liu@...el.com,
	xiaoming.wang@...el.com, souvik.k.chakravarty@...el.com
Subject: Re: [PATCH] cpuidle: Fix the CPU stuck at C0 for 2-3s after PM_QOS
 back to DEFAULT

On Thu, Aug 14, 2014 at 12:29:32PM +0200, Daniel Lezcano wrote:
> Hi Chuansheng,
> 
> On 14 August 2014 04:11, Chuansheng Liu <chuansheng.liu@...el.com> wrote:
> 
> > We found sometimes even after we let PM_QOS back to DEFAULT,
> > the CPU still stuck at C0 for 2-3s, don't do the new suitable C-state
> > selection immediately after received the IPI interrupt.
> >
> > The code model is simply like below:
> > {
> >         pm_qos_update_request(&pm_qos, C1 - 1);
> >                 < == Here keep all cores at C0
> >         ...;
> >         pm_qos_update_request(&pm_qos, PM_QOS_DEFAULT_VALUE);
> >                 < == Here some cores still stuck at C0 for 2-3s
> > }
> >
> > The reason is when pm_qos come back to DEFAULT, there is IPI interrupt to
> > wake up the core, but when core is in poll idle state, the IPI interrupt
> > can not break the polling loop.

So seeing how you're from @intel.com I'm assuming you're using x86 here.

I'm not seeing how this can be possible, MWAIT is interrupted by IPIs
just fine, which means we'll fall out of the cpuidle_enter(), which
means we'll cpuidle_reflect(), and then leave cpuidle_idle_call().

It will indeed not leave the cpu_idle_loop() function and go right back
into cpuidle_idle_call(), but that will then call cpuidle_select() which
should pick a new C state.

So the interrupt _should_ work. If it doesn't you need to explain why.

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